Patent Publication Number: US-7212226-B2

Title: Image density control apparatus and image formation apparatus

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to an image density control apparatus that controls density of a toner image obtained from developing a latent image by toner, the latent image being formed on a photoconductor by an exposure unit. The invention also relates to an image formation apparatus having such an image density control apparatus. 
   2. Description of Related Art 
   As a solution to changes in density of toner images due to a deterioration of the photoconductor and/or varying environment conditions, image formation apparatuses using electrophotography technology (printer, facsimile apparatus, copier, etc.) often employ an image density controller that stabilizes the density at an appropriate level and controls image formation conditions such as intensity of laser beam. As an example of the image density control, a plurality of types of test patterns are used to control image formation conditions (Prior Art 1). Also, another invention proposes a control that especially focuses on line widths within an image (Prior Art 2).
         Prior Art 1: Japanese Patent Laid Open H03-279971 (FIGS. 3 and 4)   Prior Art 2: Japanese Patent Laid Open 2001-80113 (FIGS. 3, 4, and 5)       

   The above described image density controls using such test patterns is able to largely control the image density so that the density is stabilized at an appropriate level despite a deterioration of the photoconductor and/or varying environment conditions. However, the above controls do not satisfy the need of securing a clear and high quality image for various types of images provided. 
   For example, the above image density controls cannot offer a flexible control where sufficiently thick toner (high density) is required for an all black central region of the image area configured with black pixels, in order to avoid partially missed or low toner areas, while relatively low density is required for thin lines and small characters, in order to avoid over-expanded lines and distorting characters. 
   Further, such an image density control employing test patterns is required to calculate an appropriate light intensity when a sensor detects the density of toner image from a test pattern. Therefore, highly accurate sensor detection is needed for a successful control that utilizes the test patterns. In addition, a more accurate control is needed since the conventional method cannot provide accurate density detection, when a sensor output reaches a saturation point depending on the types of test patterns. 
   SUMMARY OF THE INVENTION 
   The present invention addresses the above-described problems of conventional technologies. The purpose of the invention is to provide an image density control apparatus and an image formation apparatus that can obtain an accurate and stabilized image density despite a deterioration of the photoconductor and/or varying environment conditions, and reproduce a clear and high quality image from various types of original images. Further, another purpose of the invention is to improve the accuracy of toner image density detection by employing test patterns, and provide an accurate density control even when the sensor output reaches a saturation point. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is further described in the detailed description which follows, with reference to the noted plurality of drawings by way of non- limiting examples of exemplary embodiments of the present invention, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein: 
       FIG. 1  is a schematic cross sectional view illustrating an image formation apparatus according to the invention; 
       FIG. 2  is a block diagram illustrating a general configuration of an image density controller of the image formation apparatus of  FIG. 1 ; 
       FIGS. 3A ,  3 B, and  3 C illustrate test patterns used by the image density controller of  FIG. 2 ; 
       FIG. 4  illustrates toner image formations according to differences in light intensities using checkered flag test patterns of  FIG. 3 ; 
       FIG. 5  is a perspective view of a schematic diagram illustrating how a test pattern is generated by the image density controller of  FIG. 2 ; 
       FIG. 6  illustrates a procedure of obtaining an optimum light intensity by the image density controller of  FIG. 2 ; 
       FIG. 7  illustrates a procedure of assessing image configuration by the image density controller of  FIG. 2 ; 
       FIG. 8  illustrates a procedure of assessing image configuration and determining light intensity by the image density controller of  FIG. 2 ; 
       FIG. 9  is a flowchart illustrating the procedure of  FIG. 8 ; 
       FIG. 10  illustrates another procedure of assessing image configuration and determining light intensity by the image density controller of  FIG. 2 ; 
       FIG. 11  is a flowchart illustrating the procedure of  FIG. 10 ; 
       FIG. 12  illustrates a procedure of the image density controller of  FIG. 2 , when the sensor output is saturated; 
       FIG. 13  illustrates another procedure of the image density controller of  FIG. 2 , when the sensor output is saturated; 
       FIG. 14  is a block diagram illustrating a procedure of binary error diffusion by the image density controller of  FIG. 2 ; 
       FIG. 15  is a flowchart illustrating operation timing to obtain an optimum light intensity by the image density controller of  FIG. 2 ; 
       FIGS. 16A and 16B  illustrate actual generated toner images; 
       FIGS. 17A and 17B  illustrate a procedure of improving the dynamic range of the checkered flag test patterns; 
       FIG. 18  illustrates differences in dynamic ranges of various checkered flag test patterns; 
       FIG. 19  illustrates a partially enlarged image obtained from a binary error diffusion of half-tone image data; 
       FIGS. 20A and 20B  illustrate an actual generated toner images; and 
       FIG. 21  illustrates changes in output image density of a half-tone data having 256 gradations. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The embodiments of the present invention are explained in the following, in reference to the above-described drawings. 
     FIG. 1  is a schematic cross sectional view illustrating an image formation apparatus according to the invention. The image formation apparatus includes photoconductor drum  1 , charge roller  2  that evenly charges an image forming surface on photoconductor drum  1 , LSU (laser scanning unit)  3  that forms a static latent image by running a flux of light for exposure on the image forming surface on photoconductor drum  1 , developer roller  4  that develops the static latent image on the image forming surface on photoconductor drum  1 , developer  5  having developer roller  4 , transfer roller  6  transfers the toner image (formed on the image forming surface on photoconductor drum  1 ) onto a recording paper, and cleaning blade  7  that cleans the image forming surface on photoconductor drum  1 . Further, when the recording paper from paper feeder  8  is delivered between photoconductor drum  1  and transfer roller  6 , the paper is then ejected to exit unit  10  via fuser unit  9 . In addition, the image formation apparatus also includes scanner  11  for copier and facsimile transmission functions. 
   The present image formation apparatus is provided with a power saver mode function that saves power consumption when the apparatus is idle. The image formation apparatus activates the power saver mode after a predetermined time period of no specific operation from operation display panel  12 . The power saver mode shuts off the power supply to components including image formation unit  13  having photoconductor drum  1 , LSU  3 , etc., but not to operation display panel  12 . 
     FIG. 2  is a block diagram illustrating a general configuration of an image density controller of the image formation apparatus of  FIG. 1 . The image density controller includes photo sensor  21  that detects density of toner image for each test pattern formed on photoconductor drum  1 , microcomputer  22  that obtains optimum light intensity for each test pattern based on the detection result of photo sensor  21 , and pattern detection and light intensity setting circuit  25  having image configuration assessment unit  23  that assesses image configuration per a predetermined assessment area unit of the processing image, and light intensity determination unit  24  that determines the light intensity for processing image based on the optimum light intensity of the test pattern chosen for the obtained image configuration. 
   The test pattern image data used during the process for obtaining optimum light intensity by microcomputer  22  is generated at test pattern generation circuit  26 . 
   Light intensity data in pixel unit, being output from pattern detection and light intensity setting circuit  25  is transmitted to LSU  3  via laser modulation circuit  27  and laser drive circuit  28 . Laser modulation circuit  27  controls pulse width modulation (PWM), in order to control lighting time of a light source per pixel unit according to the light intensity determined by pattern detection and light intensity setting circuit  25 . 
   The image apparatus of the invention also includes data converter  29  that performs a binarization of processing image data that is configured with multi-level data having half tones. Prior to the image configuration assessment process at image configuration assessment unit  23 , data converter  29  performs a binarization with the error diffusion method on the processing image data ( FIG. 14 ). 
     FIG. 3  illustrates test patterns used by the image density controller of  FIG. 2 .  FIG. 3(A)  shows an all black test pattern.  FIG. 3(B)  shows the first checkered flag test pattern having white and black regions alternatively, each having 4×4 pixels, that are regularly aligned.  FIG. 3(C)  shows the second checkered flag test pattern having each black pixel within a 4×4 pixel black region is converted into a white pixel. 
   The above mentioned test pattern is appropriate for evaluating toner image density, since the overall average density can be obtained from photo sensor  21 . Especially, the sensor output of the all black test pattern can indicate the density according to the thickness of the toner layer. In addition, the sensor output of the checkered flag test pattern is appropriate for evaluating the toner image widths, since the sensor output of the checkered flag test pattern indicates a level of the brightness of the over all test pattern according to the ratio between the exposure surface of the photoconductor and toner image. 
   The second checkered flag test pattern has a bigger ratio of white pixels compared to the one of the first checkered flag test pattern, thereby having a brighter test pattern. Microcomputer  22  can selectively uses the appropriate pattern from these checkered flag test patterns having different back-and-white ratios, according to the characteristics of photo sensor  21 . Therefore, it is possible to increase the dynamic range of photo sensor  21 , i.e., to accurately identify the width of the toner image based on a test pattern ( FIGS. 17 and 18 ). 
     FIG. 4  illustrates toner image formations according to differences in light intensities using checkered flag test patterns of  FIG. 3 . When the light intensity is too strong, the toner image becomes over-expanded. When the light intensity is optimum, the toner image can realistically materialize the black and white pixels. When the light intensity is too small, the toner image becomes under-expanded. By detecting the toner image of these checkered flag patterns using photo sensor  21 , it is possible to assess average width in the vertical, horizontal, and diagonal directions. 
     FIG. 5  is a perspective view of a schematic diagram illustrating how a test pattern is generated by the image density controller of  FIG. 2 . In this example, first photo sensor  21   a  (that detects density of an all black test pattern) and second photo sensor  21   b  (that detects density of a checkered flag test pattern) are provided on photoconductor drum  1 . Light-emitting elements of photo sensors  21   a  and  21   b  illuminate the test patterns on photoconductor drum  1 . Then, the reflected light is received by light-intercepting elements. 
   The test pattern is generated at predetermined times by changing light intensity in plurality of stages, where photo sensors  21   a  and  21   b  detect density of plurality of toner images having different light intensities for each test pattern. Microcomputer  22  compares the obtained sensor output value with a predetermined output target value, so that an optimum light intensity that can achieve the output target value is calculated for each test pattern. 
     FIG. 6  illustrates a procedure of obtaining an optimum light intensity by the image density controller of  FIG. 2 . In this example, a checkered flag test pattern is used. In the procedure of obtaining the optimum light intensity, firstly, three test patterns are generated having different light intensities in stages. For example, the testing light intensities are set at 175 (first time), 191 (second time), and 207 (third time). In this embodiment, the light intensities have 256 (0-255) multi-level data varieties. 
   When there is an output target value Vw REF  (e.g., 2.1V) within the range of the first—third output values Vw 175 , Vw 191 , and Vw 207 , the procedure is completed. Then, a light intensity (that can achieve output target value Vw REF ) is calculated, by a linear interpolation, from two of the output values (among Vw 175 , Vw 191 , and Vw 207 ) sandwiching output target value Vw REF , and output target value Vw REF . For example, as shown in “A” in the figure, when output target value Vw REF  is between Vw 191  and Vw 207 , light intensity (duty) is calculated as follows:
 
duty=191+16 *|Vw   191   −Vw   REF   |/|Vw   191   −Vw   207 |
 
   When there is no output target value Vw REF  within the range of the first—third output values Vw 175 , Vw 191 , and Vw 207 , the test pattern having a different light intensity is regenerated. When the light intensity that can achieve output target value Vw REF  is smaller than the first—third test light intensities, the following fourth—sixth test light intensities are set as 127, 143, and 159, respectively, for example. When the light intensity that can achieve output target value Vw REF  is greater than the first—third test light intensities, the following fourth—sixth test light intensities are set as 223, 239, and 255, respectively, for example. When there is output target value Vw REF  within the range of the three output values Vw 127 , Vw 143 , and Vw 159  (or, Vw 223 , Vw 239 , and Vw 255 ), the light intensity (that can achieve output target value Vw REF ) is calculated by the linear interpolation, similar to the above-described procedure. 
   In addition, as shown in “B” in the figure, when there is output target value Vw REF  within the range of the first—sixth output values Vw 175  and Vw 159 , the light intensity (duty) that can achieve output target value Vw REF  is calculated by the linear interpolation as follows:
 
duty=191+16 *|Vw   159   −Vw   REF   |/|Vw   159   −Vw   175 |
 
   In this embodiment, the test pattern is generated up to 6 times. When output target value Vw REF  is greater than the maximum output value Vw 127 , as shown in “C” in the figure, the maximum test light intensity 127 becomes the optimum light intensity. When output target value Vw REF  is smaller than the minimum output value Vw 255 , as shown in “D” in the figure, the minimum test light intensity 255 becomes the optimum light intensity. 
   Further, in case of using an all black test pattern, a light intensity that can achieve output target value Vw REF  (e.g., 1.8V) is calculated similar to the above example of the checkered flag test pattern. In this case, the first—third test light intensities are set as 79, 95, and 111, respectively, for example. When the light intensity that can achieve output target value Vw REF  is smaller than the first three test light intensities, the fourth—sixth test light intensities are set as 31, 47, and 63, respectively, for example. When the light intensity that can achieve output target value Vw REF  is greater than the first three test light intensities, the fourth—sixth test light intensities are set as 127, 143, and 159, respectively, for example. Additionally, the test light intensities of the all black test pattern are smaller than the test light intensities of the checkered flag test pattern, because a process for sensor output saturation is performed ( FIGS. 12 and 13 ). 
     FIG. 7  illustrates a procedure of assessing image configuration by the image density controller of  FIG. 2 . Image configuration assessment unit  23  of pattern detection and light intensity setting circuit  25  of  FIG. 2  assesses the image configuration per a predetermined assessment area within a processing image. In this embodiment, the image configuration is assessed per pixel, according to the appearance of surrounding black and white pixels within the assessment area. In particular, 3×3 pixels, i.e., a pixel for assessment (object pixel) plus 8 other surrounding adjacent pixels (above, below, right, left, and 4 diagonally faced ones) (total 9 pixels) are considered as the assessment area. 
   When the processing image data is input, image configuration assessment unit  23  assesses the image configuration per pixel, on one line of the main scanning direction. When the assessment for the line is completed, an adjacent line in the secondary scanning direction is assessed. This procedure is repeated until the entire processing image is assessed. 
     FIG. 8  illustrates a procedure of assessing image configuration and determining light intensity by the image density controller of  FIG. 2 . Image configuration assessment unit  23  assesses whether the pixels of the assessment area are all black or else (not all black). Light intensity determination unit  24  determines the light intensity for each pixel from the optimum light intensity based on the various test patterns. 
   Pixels corresponding to all black configuration (the first image configuration) are located at the center of image area comprising black pixels, and thus are required to have high density for toner image, in order to prevent a problem that a part of black pixels falls out or the density of the pixels becomes low by low toner adhesion. Therefore, when the object pixel configures an all black configuration, light intensity is determined according to optimum light intensity (duty 1 ) of the all black test pattern, which is suitable for assessing the toner image density. In this example, pixels “b” and “c” on the processing image have an all black configuration. 
   Pixels corresponding to “other configuration” (mixed with white pixels within the assessment area) (the second image configuration) are located at a border of the image area. In order to prevent over-expanded line and character distortion, widths of toner image need to be controlled. Therefore, when the object pixel is considered to have the “other configuration”, light intensity is determined according to optimum light intensity (duty 2 ) of the checkered flag test pattern, which is suitable for assessing the toner image widths. In this example, pixels “a” and “d” on the processing image have the “other configuration”. 
   Accordingly, the PWM controls the laser lighting time, so that the laser lighting time for pixels “b” and “c” within the image area, is longer to have a higher density having a thicker toner image. At the edge of the image area, the laser lighting time for pixels “a” and “d” is shorter to control the width of the toner image. 
     FIG. 9  is a flowchart illustrating the procedure of  FIG. 8 . At step  101 , the object pixel is switched to the next pixel. At the following step  102 , it is checked whether the pattern matches with the all black configuration. When it does not match, the control moves to step  103  to determine the optimum light intensity of the checkered flag test pattern as the light intensity of the object pixel. When it matches with the all black configuration, the control moves to step  104  to determine the optimum light intensity of the all black test pattern as the light intensity of the object pixel. 
     FIG. 10  illustrates another procedure of assessing image configuration and determining light intensity by the image density controller of  FIG. 2 . In this example, four types of configurations (all black, isolated point, isolated line, and other) are provided for the assessment. The isolated point comprises a single black pixel (in the center) and 8 other adjacent pixels are all while pixels. The isolated line comprises a series of black pixels in a line. Two opposing pixels sandwiching the center black pixel are black pixels, and the two opposing pixels are configured a vertical, horizontal, or diagonal line, together with the center black pixel. All other adjacent pixels (6 pixels surrounding the line) must be white pixels. 
   When the object pixel configures an isolated point or an isolated line, it is necessary to control the overall width of the toner image, in order to reduce distortion of points and lines. Therefore, the light intensity is determined based on optimum light intensity duty 2  according to checkered flag pattern, which is suitable for evaluating the widths of the toner image. Optimum light intensity duty 2  is also correctively increased by a predetermined corrective factor K. In particular, optimum light intensity duty 2  is multiplied by corrective factor K (=1.4) in order to calculate the light intensity for the object pixel as follows:
 
duty=duty 2 ×1.4
 
   Accordingly, when the object pixel configures an isolated point or an isolated line, it is possible to prevent an over-expanded width of the toner image that is due to a relatively long laser lighting time, or an under-expanded width of toner image that leads to missing and/or blur finish. 
     FIG. 11  is a flowchart illustrating the procedure of  FIG. 10 . In this example, at step  201 , the object pixel is switched to the next pixel. At the following step  202 , it is checked whether the pattern matches with the all black configuration. When it matches, the control moves to step  203  to determine the optimum light intensity of the all black test pattern as the light intensity of the object pixel. When it does not match, the control moves to step  204  to check whether the object pixel configures an isolated point or an isolated line pattern (steps  205 – 208 ). When none of the patterns matches with the object pixel, the control moves to step  209  to determine the optimum light intensity of the checkered flag test pattern as the light intensity of the object pixel. When one of the patterns matches with the object pixels, the controls moves to step  210  to correct the optimum light intensity of the checkered flag test pattern to calculate the light intensity of the object pixel. 
     FIGS. 12 and 13  illustrate a procedure of the image density controller of  FIG. 2 , when the sensor output is saturated. When an all black test pattern is used, the output of photo sensor  21   a  sometimes becomes saturated, hindering accurate density detection. Therefore, microcomputer  22  calculates an optimum light intensity by modifying the output target value of photo sensor  21  into a value outside of the saturation region. Then, light intensity determination unit  24  corrects the optimum light intensity (obtained by microcomputer  22 ) using a predetermined factor, and determines the desired light intensity. 
   In the example shown in  FIG. 12 , the output from photo sensor  21  becomes saturated at image density level 1.3. Since image density level 1.4 cannot be detected, it is impossible to perform an appropriate control at the level. Therefore, in order to calculate an optimum light intensity, the density target value is corrected into a level lower than the saturation region of the sensor output (e.g., into level 1.2). Then, the corresponding output value 1.8 becomes the output target value Vw REF . 
   Next, as shown in  FIG. 13 , the above-calculated optimum light intensity is modified so that the density of the all black test pattern becomes 1.4. In particular, the light intensity (duty) of the object pixel is calculated by multiplying the optimum light intensity duty 1  by corrective factor K=1.6 as follows:
 
duty=duty 1 ×1.6
 
     FIG. 14  is a block diagram illustrating a procedure of binary error diffusion by the image density controller of  FIG. 2 . At data converter  29 , filtering unit  41  filters input signal I xy  (comprising multilevel data of object pixel) and adder  42  adds an error weighted average A xy  (processed before the object pixel) to the signal in order to obtain multilevel signal I′ xy . Then, binarization unit  43  performs a binarization on I′ xy  by comparing with a predetermined binarization threshold value to obtain output signal P xy . Then, based on the binary signal P xy  and multilevel signal I′ xy  of the object pixel, subtracter  44  calculates a binarization error E xy , which will be multiplied by factor K b  by multiplier  45  and stored in error memory  46 . Weighting adder  47  refers to error memory  46  and obtains weighted average A xy  of peripheral pixel error E xy . Weighted average A xy  is multiplied by factor K a  by multiplier  48  and added by adder  42  for the next object pixel. 
     FIG. 15  is a flowchart illustrating operation timing to obtain an optimum light intensity by the image density controller of  FIG. 2 . Microcomputer  22  performs an optimum light intensity obtaining process using generated test pattern every predetermined time interval (e.g., every 8 hours), since the last optimum light intensity obtaining process. 
   First, at step  301 , an optimum light intensity obtaining process is performed. At step  302 , the performing time is stored in a nonvolatile memory. Then, at step  303 , the power is turned off, or an energy saver mode is activated. When the power is turned on or the energy saver mode is deactivated at step  304 , the current time is counted at step  305 . At the following step  306 , it is determined whether 8 hours or more have passed since the last optimum light intensity obtaining process. When 8 hours or more have passed, another optimum light intensity obtaining process is performed at step  307 , and control returns to step  302 . When 8 hours or more have not passed, the control returns to step  305 . 
   Accordingly, the process for obtaining optimum light intensity is performed at an appropriate timing in accordance with the changes in toner charge amount. Further, it is possible to prevent excess use of toner, since test patterns are not generated every time the power is supplied to the apparatus (e.g., when the energy saver mode is activated) and performs the process for obtaining optimum light intensity. 
     FIG. 16  illustrates an actually generated toner image. This is to examine whether a 2 dot line pairs (2 pixel width lines formed every 2 pixels interval) can be resolved as a line. As shown in (B), when the image density control of the present invention is not performed, the lines are distorted and it is difficult to identify them as lines. However, as shown in (A), when the image density control of the present invention is performed, the line distortion is appropriately controlled and the lines can be clearly identified. 
     FIG. 17  illustrates a procedure of improving the dynamic range of the checkered flag test patterns.  FIG. 18  illustrates differences in dynamic ranges of various checkered flag test patterns. In case of 2 lines (85 μm (600dpi) width) shown in  FIG. 17(A) , the sensor output is 1.3V for the first checkered flag test pattern, and 2.1 for the second checkered flag test pattern. Therefore, the second checkered flag test pattern has a higher value. In case of 3 lines shown in  FIG. 17(B) , the similar effect can be seen. 
   Accordingly, compared to the first checkered flag test pattern, the second checkered flag test pattern uses the sensor at a higher output value region. Thus, as shown in  FIG. 18 , compared to the first checkered flag test pattern, the second checkered flag test pattern has a wider dynamic range and raises the level of detection accuracy of the sensor. 
     FIG. 19  illustrates a partially enlarged image obtained from a binary error diffusion of half-tone image data.  FIG. 20  illustrates an actual generated toner images. In  FIG. 20(A) , toner widths are appropriately corrected by the image density control including the binary error diffusion of the present invention, showing that the original data pattern obtained by the binary error diffusion is realized accurately by the toner image. 
     FIG. 21  illustrates changes in output image density of a half-tone data having 256 gradations. When the image density control including the binary error diffusion is not performed (e.g., comparisons  1 ,  2 , and  3 ), γ curve is affected by the changes in environment and time. However, when the image density control including the binary error diffusion of the present invention is performed, the incline of the γ curve becomes smaller and has stable characteristics similar to linear, indicating that the present invention is superior in tone reproduction for half tone. 
   In addition, the present invention is not limited to monochrome images, but also can be applied to color images. In such a case, the above-mentioned black pixels can be replaced with color pixels that can apply toner of certain color. According to the light amount of each color, the toner application amount can be controlled. 
   It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to exemplary embodiments, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular structures, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims. 
   The present invention is not limited to the above-described embodiments, and various variations and modifications may be possible without departing from the scope of the present invention. 
   This application is based on the Japanese Patent Application No. 2003-082523 filed on Mar. 25, 2003, entire content of which is expressly incorporated by reference herein.