JP2011221233A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of reducing first copy output time (FCOT) and precisely controlling density correction by reducing density unevenness of pattern images for density correction without increasing the circuit scale of a control system.SOLUTION: An image forming apparatus forms a latent image on a photoreceptor drum for carrying a toner image, develops the latent image to generate a toner image, and then transfers the toner image directly or indirectly onto a transfer medium to form an image. An exposure control unit 207 comprises a pattern storage memory 208 having stored therein a plurality of screen patterns in advance, a pattern generation circuit 209 that generates image patterns for density correction in accordance with the screen patterns, and a pulse width modulation (PWM) unit 210 that forms a latent image by exposing the photoreceptor drum to light in accordance with the image patterns for density correction. Density correction control is performed based on detected densities obtained by detecting the toner image formed in accordance with the image patterns for density correction as a pattern image for density correction.

Description

  The present invention relates to an image forming apparatus such as a copying machine using an electrophotographic process, and more particularly to an image forming apparatus capable of correcting color misregistration and the like due to a secular change in image density.

  In general, in an image forming apparatus such as a copying machine using an electrophotographic process, lighting of a laser beam is controlled according to image data. Then, this laser beam exposes the photosensitive drum, which is an image carrier, to form an electrostatic latent image on the photosensitive drum.

  Subsequently, the electrostatic latent image on the photosensitive drum is developed with a developer such as toner to form a toner image. The toner image is directly or indirectly transferred onto a transfer medium such as a recording sheet, and then a fixing process is performed to form an image on the transfer medium.

  When forming a color image, for example, for each color of yellow (Y), cyan (C), magenta (M), and black (K), an electrostatic latent image is formed on the photosensitive drum corresponding to each color. Form. Then, the electrostatic latent images on the respective photosensitive drums are developed with yellow (Y), cyan (C), magenta (M), and black (K) developers to obtain toner images of respective colors. Each color toner image is sequentially transferred to an intermediate transfer member such as an intermediate transfer belt to obtain a color toner image, and the color toner image is transferred onto a transfer medium to form a color image.

  By the way, in this type of image forming apparatus, the density or the like of the image formed on the transfer medium may change due to the environment such as the ambient temperature and humidity of the image forming apparatus and so-called aging degradation. Many.

  For this reason, various density correction controls are executed in order to improve the reproducibility of the image, that is, to avoid changes in the density of the image.

  As one method of density correction control, for example, a density toner is used to read a reference color toner image (hereinafter referred to as a reference color toner image) formed on an intermediate transfer body such as a photosensitive drum or an intermediate transfer belt. In some cases, a density signal indicating the shading is obtained.

  Then, here, for the reference color toner image, the reference density specified in advance and the density indicated by the density signal are compared, and according to the comparison result, the laser emission intensity, the charge amount of the photosensitive drum, and the development The density correction control is performed by selectively feedback-controlling the bias and the like (see, for example, Patent Document 1 or Patent Document 2).

  When performing the density correction control as described above, so-called irrational screen (also called irrational tangent screen) processing using a dither method is used as a method for expressing the halftone of the reference toner image. There are some (see, for example, Patent Document 3).

  The above-described method using irrational screen processing can bring the screen angle of each color close to the ideal angle, and is therefore effective in forming a high-quality image with high visibility and less moiré. It has been.

  Here, conventional print control and density correction control will be described. FIG. 7 is a control flowchart for explaining the operation of the control system used in the conventional image forming apparatus. FIG. 8 is a diagram showing an example of a density correction pattern image (that is, a reference color toner image) formed using an irrational screen in a conventional image forming apparatus.

  With reference to FIGS. 7 and 8, here, a case where density correction control is performed by controlling the light emission intensity of the laser will be described. Although not shown, the image forming apparatus includes a signal processing unit that processes image data (hereinafter also referred to as an image signal) and an exposure control unit. The exposure control unit controls the light emission intensity of the laser according to the processed image signal obtained as a result of processing the image data by the signal processing unit.

  First, for example, when the image forming apparatus is powered on, the image forming apparatus determines whether or not to perform density correction control (step S101). When the density correction control is not performed (NO in step S101), the image forming apparatus shifts to a printing operation (print mode).

  After shifting to the print mode, the image forming apparatus waits until the activation of the signal processing unit and the exposure control unit is completed (step S102). After the activation of the signal processing unit and the exposure control unit is completed, image data obtained as a result of reading the document is given to the signal processing unit (image data input: step S103).

  When receiving the image data, the signal processing unit corrects the image data (step S104). Here, for example, RGB signals are input to the signal processing unit as the image data, and the signal processing unit performs shading correction, color conversion processing, noise removal processing, and the like on the RGB signals. As a result, variations in the optical system used when reading the original are removed, and the characteristics are corrected.

  Thereafter, the signal processing unit performs conversion from RGB signals to YMCK signals, conversion processing for matching the color gamut of the image forming apparatus, correction of two-dimensional frequency characteristics, and the like.

  Further, the signal processing unit performs error diffusion, screen processing, and the like on the YMCK signal (gradation processing: step S105). In this gradation process, an optimal screen process is selected in accordance with the characteristics of the image, and gradation conversion is executed in accordance with the selected screen process.

  The YMCK signal (that is, the processed image signal) processed by the signal processing unit is given to the exposure control unit. The exposure control unit performs pulse width modulation (PWM modulation) on the YMCK signal (step S106), supplies the YMCK signal as a PWM signal to the laser driving unit, and drives the laser.

  On the other hand, if the image forming apparatus determines that density correction control is to be performed (YES in step S101), the image forming apparatus shifts to a density correction control mode.

  Even when the mode is shifted to the density correction control mode, the image forming apparatus waits until the activation of the signal processing unit and the exposure control unit is completed (step S107). After the activation of the signal processing unit and the exposure control unit is completed, the signal processing unit generates a density correction pattern (step S108).

  Then, the signal processing unit performs tone conversion on the density correction pattern by screen processing (step S109), and then gives it to the exposure control unit as a density correction image pattern (density correction signal). This density correction image pattern is one of the processed image signals.

  In the exposure control unit, the image pattern for density correction is subjected to pulse width modulation (step S110), and is supplied to the laser driving unit as a PWM signal to drive the laser.

  As a result, a color toner image corresponding to the density correction image pattern is formed on an intermediate transfer member such as an intermediate transfer belt as described above (hereinafter, this color toner image is referred to as a density correction image). Called pattern image).

  The density correction pattern image 800A has, for example, a plurality of toner images as shown in FIG. In the illustrated example, the density correction pattern image 800A includes, for example, first to third toner images 801 to 803 formed on the intermediate transfer member 800. As shown in an enlarged view in FIG. 8, the first toner image 801 is screen-processed by an irrational screen 804 using a dither method. Similarly, the second and third toner images 802 and 803 are screened by an irrational screen 804.

  A density correction pattern image 800A shown in FIG. 8 is read by a density sensor (not shown). Then, the density sensor gives a density signal indicating the density of the density correction pattern image 800A to the exposure control unit. Then, the exposure control unit compares the reference gray level determined in advance according to the density correction pattern with the gray level indicated by the density signal, and adjusts the light emission intensity of the laser according to the comparison result.

JP-A-5-333647 JP-A-5-333648 JP-A-61-107873

  By the way, when performing gradation processing as described above, if screen processing is performed using an irrational screen, it is difficult to make the shape of the irrational screen of each color constant, so unavoidably density correction. In some cases, density unevenness may occur in the pattern image for use.

  As described above, when performing gradation correction using an irrational screen when performing density correction control, that is, when screen processing is performed, density correction is accurately performed because density unevenness occurs in the density correction pattern image. I can't. For this reason, even if density correction control is performed, the density of the image becomes conspicuous.

  Furthermore, when performing screen processing using an irrational screen, it is generally necessary to perform screen processing in accordance with various settings from the user and the characteristics of the input image data. When screen processing is performed using an irrational screen, the operation is complicated and the circuit scale is inevitably increased.

  For this reason, conventionally, a function for performing screen processing using an irrational screen is provided only in the signal processing unit so that the circuit scale of the control system is somewhat reduced.

  This means that even when density correction control is performed, the signal processing unit needs to perform screen processing using an irrational screen. Therefore, when density correction control is performed when the image forming apparatus is powered on or returned from the sleep state, it is affected by the signal control unit that takes time to start.

  That is, when density correction control is performed, the time (FCOT) until the first print (printing) is completed after the print button provided in the image forming apparatus is pressed is extremely long. There is a problem.

  In order to reduce the FCOT, for example, it is assumed that a density correction pattern image is formed using only the exposure control unit. In this case, since the exposure control unit does not have a function for generating an irrational screen, the irrational screen is stored in advance in a non-volatile memory or the like, and the density correction image is obtained using the irrational screen. Need to get a pattern.

  However, dots are formed when a density correction pattern image is formed using a density correction image pattern obtained as a result of screen processing by selectively repeatedly using an irrational screen stored in a nonvolatile memory or the like. The phenomenon that the position is biased occurs. As a result, there is a problem that density unevenness due to an irrational screen is further emphasized.

  Accordingly, an object of the present invention is to make it possible to perform density correction control with high accuracy by reducing the FCOT and reducing the density unevenness of the density correction pattern image without increasing the circuit scale of the control system. It is to provide a forming apparatus.

  In order to solve the above problems, an image forming apparatus according to the present invention forms a latent image on an image carrier for carrying a toner image, develops the latent image to form the toner image, and the toner image. In an image forming apparatus that directly or indirectly transfers the image to a transfer medium to form an image, a pattern storage memory in which a plurality of screen patterns are stored in advance, and a density correction image pattern in accordance with the plurality of screen patterns A pattern generation circuit to be generated; exposure control means for exposing the image carrier in accordance with the density correction image pattern; and the toner image formed in accordance with the density correction image pattern as a density correction pattern image. It has density detecting means that detects and obtains the density as a detected density, and density correction control means that executes density correction control according to the detected density. .

  As described above, according to the present invention, after the density correction image pattern is generated according to the plurality of screen patterns stored in the pattern storage memory, the image carrier is exposed according to the density correction image pattern. It is carried out. A toner image formed according to the density correction image pattern is detected as a density correction pattern image, and density correction control is executed according to the detected density. Therefore, it is not necessary to perform gradation processing as in the printing operation during density correction control. Therefore, the FCOT can be reduced without increasing the circuit scale of the control system. Since the density unevenness of the density correction pattern image can be reduced, there is an effect that the density correction control can be performed with high accuracy.

1 is a diagram showing an example of a digital multi-function peripheral that is one of image forming apparatuses according to an embodiment of the present invention. FIG. 2 is a block diagram illustrating an example of a control circuit used in the digital multifunction peripheral illustrated in FIG. 1. 3 is a control flowchart for explaining the operation of the control circuit shown in FIG. FIG. 3 is a diagram illustrating an example of a density correction pattern image formed according to a density correction image pattern generated using one type of irrational screen in the pattern generation circuit shown in FIG. 2. FIG. 3 is a diagram showing an example of a density correction pattern image formed in accordance with density correction image patterns generated using a plurality of types of irrational screens in the pattern generation circuit shown in FIG. 2. FIG. 6 is a diagram illustrating another example of the density correction pattern image formed according to the density correction image pattern generated using a plurality of types of irrational screens in the pattern generation circuit shown in FIG. 2. 7 is a control flowchart for explaining the operation of a control system used in a conventional image forming apparatus. FIG. 8 is a diagram illustrating an example of a density correction pattern image formed using an irrational screen in a conventional image forming apparatus.

  Hereinafter, an image forming apparatus according to the present invention will be described with reference to the drawings, taking a digital multifunction peripheral as an example.

  FIG. 1 is a cutaway view showing an example of a digital multi-function peripheral which is one of image forming apparatuses according to an embodiment of the present invention.

  Referring to FIG. 1, the illustrated digital multifunction peripheral 100 </ b> A includes a document conveyance unit 130, a document reading unit 120, an image forming unit 100, a conveyance unit 190, and a deck sheet feeding stage 180. In the example shown in FIG. 1, a post-processing device 10 is connected to the digital multifunction peripheral 100A.

  The document transport unit 130 has a document table 131 on which a document or a bundle of documents to be printed (printed) such as a copy is placed. In the following description, it is assumed that a document bundle is placed on the document table 131.

  When a bundle of documents is set on the document table 131, the documents are conveyed one by one to the document reading position by the paper feed roller 132. In the document conveying section 130, a document conveying belt 137 is driven by a motor 136, and the document is arranged at the document reading position by the document conveying belt 137.

  Thereafter, the document reading unit 120 performs a document reading operation as described later. When the document reading operation ends, the flapper 135 changes the document transport path. Then, the motor 136 is driven in reverse, and the document is discharged to the discharge tray 138 via the discharge roller 134. Thereafter, a new document is conveyed to the document reading position, and the same operation is repeated.

  The document reading unit 120 includes, for example, an exposure lamp 122 such as a fluorescent lamp or a halogen lamp. The exposure lamp 122 irradiates the document located on the document placement glass (document table) 126 while moving in a direction orthogonal to the longitudinal direction (the direction extending from the front side to the back side of the drawing in the drawing).

  Reflected light (also referred to as scattered light) reflected from the original by irradiation from the exposure lamp 122 is reflected by the first and second mirror bases 121 and 123 and reaches the lens 124. The first and mirror tables 121 and 123 are moved by being driven by a motor 125. At this time, the second mirror table 123 moves at a half speed with respect to the first mirror table 121. As a result, the distance from the document surface irradiated by the exposure lamp 122 to the lens 124 is always kept constant.

  The reflected light that reaches the lens forms an image on, for example, a CCD (Charge Coupled Device) line sensor (not shown) in which thousands of light receiving elements are arranged in a line. The CCD line sensor sequentially photoelectrically converts the image formed on a line basis to generate an electric signal.

  The electrical signal is converted into a digital signal and then applied to a control circuit (not shown in FIG. 1) described later as a digital signal (that is, image data). The control circuit controls the image forming unit 100 according to the image data to perform image formation (that is, a printing operation).

  The image forming unit 100 includes a semiconductor laser 101. The control circuit drives and controls the semiconductor laser 101 based on the image data. The polygon mirror 102 is rotationally driven by a motor 103 in parallel with the axial direction of the photosensitive drum 107 (direction extending from the front side to the back side in the drawing). As a result, the light beam (laser beam) emitted from the semiconductor laser 101 is deflected and scanned by the polygon mirror 102. Then, the surface of the photosensitive drum (image carrier) 107 rotating at a constant speed is irradiated with a light beam.

  Before the light beam is irradiated, the remaining charge remaining on the surface of the photosensitive drum 107 is neutralized by a pre-exposure lamp (not shown). Thereafter, a primary charger (not shown) uniformly charges the surface of the photosensitive drum 107. Then, the surface of the photosensitive drum 107 is irradiated with a light beam.

  The photosensitive drum 107 receives the light beam while rotating, and thereby an electrostatic latent image (latent image) is formed on the surface of the photosensitive drum 107. The electrostatic latent image is visualized with a developer (toner) of a predetermined color by the developing device 104 to be a toner image.

  A developing bias is applied to the developing device 104, and the electrostatic latent image is visualized by the relationship between the developing bias and the surface potential of the photosensitive drum 107.

  The digital multi-function peripheral 100A includes a plurality of paper feed stages 140, 150, 160, 170, and 180 in addition to the deck paper feed stage 180 described above. For example, the paper feed stages 140, 150, 160, and 170 store transfer papers (transfer media) having different sizes. Then, the transfer paper is selectively conveyed from the paper feed stages 140, 150, 160, and 170 and the deck paper feed stage 180 to the image forming unit 100.

  The paper feed stages 140, 150, 160, and 170 have the same configuration, and the deck paper feed stage 180 can store a larger amount of transfer paper than the paper feed stages 140, 150, 160, and 170. .

  Here, paying attention to the paper feed stage 140, the paper feed stage 140 has a paper feed cassette (hereinafter simply referred to as a cassette) 141. A number of transfer sheets are stored and stored in the cassette 141. A bottom plate 142 is disposed on the bottom surface of the cassette 141, and the bottom plate 142 is moved up and down by a lift-up motor 143. Then, when the bottom plate 142 rises and reaches a predetermined standby height, the transfer paper enters a conveyance standby state.

  When the transfer paper enters the transport standby state, the transfer paper is picked up by the pickup roller 144 and is sent to the paper feed roller pair 145. Torque is applied to the feed roller pair 145 in the reverse rotation direction to the paper feed, and the feed roller pair 145 conveys the transfer paper one by one while preventing double feeding of the transfer paper. Send it to the path.

  Similarly, the paper feed stages 150, 160, and 170 include cassettes 151, 161, and 171, bottom plates 152, 162, and 172, lift-up motors 153, 163, and 173, pickup rollers 154, 164, and 174 and feed roller pairs 155, 165, and 175 are provided.

  In the conveyance path, conveyance rollers 146, 156, and 176 for conveying the transfer paper fed from the paper feed stages 140, 150, 160, and 170 are arranged.

  On the other hand, the deck paper feed stage 180 includes a paper storage 181 for storing and storing transfer paper. A bottom plate 182 is disposed on the bottom surface of the paper 181, and the bottom plate 182 is connected to a belt 183 a that is rotationally driven by a motor 183. Then, the bottom plate 182 is raised or lowered according to the movement of the belt 183a.

  When the bottom plate 182 rises and reaches the conveyance standby position, the transfer paper is picked up by the pickup roller 185 while being prevented from being double-fed and sent to the paper feed roller pair 184. Then, the transfer paper is transported to the transport path by the paper feed roller pair 184.

  The transfer paper transported from one of the paper feed stages 140, 150, 160, and 170 and the deck paper feed stage 180 is transported to the registration rollers 106 arranged in the transport path. When the transfer paper is detected by the paper feed sensor 105, the control circuit drives and controls the registration roller 106 to convey the transfer paper to the transfer position. At this time, the transfer paper is fed to the transfer position in synchronism with the timing of the leading edge of the toner image formed on the photosensitive drum 107 and the leading edge of the transfer paper.

  At the transfer position, the transfer charger 108 transfers the toner image on the photosensitive drum onto the transfer paper. After the toner image is transferred, residual toner remaining on the surface of the photosensitive drum 107 is removed by a cleaner (not shown).

  Since the transfer sheet after transfer has a large curvature of the photosensitive drum 107, it is easy to separate from the photosensitive drum 107. However, a voltage is applied to a static elimination needle (not shown) between the photosensitive drum 107 and the transfer sheet. Is weakened so that the transfer paper can be easily separated from the photosensitive drum 107. Thereafter, the transfer paper is sent to the fixing unit 109, and the toner image on the transfer paper is fixed.

  The fixing unit 109 includes a ceramic heater 110 and a film (fixing belt) 111. The fixing belt 111 is suspended and driven by two small rollers 111a and 111b. One large roller 111c is disposed facing the fixing belt 111, and the transfer paper passes through a path defined by the fixing belt 111 and the large roller 111c.

  At this time, heat from the ceramic heater 110 is efficiently transmitted to the transfer paper via the fixing belt 111, and the toner image on the transfer paper (that is, the toner image carried on the transfer paper) is fixed.

  A cooling roller (not shown) is disposed in the fixing unit 109 to radiate heat from the small rollers 111a and 111b and the large roller 111c. The large roller 111c and the small rollers 111a and 111b discharge the transfer paper from the fixing unit 109 and correct the curl of the transfer paper.

  A direction flapper 112 is disposed on the exit side of the fixing unit 109, and the discharge destination of the transfer paper is switched by the direction flapper 112. For example, in the first operation mode (discharge mode), the transfer paper is discharged to the discharge tray 114 by the direction flapper 112.

  On the other hand, in the second mode (post-processing mode), the transfer paper is transported to the transport unit 190 side by the direction flapper 112.

  As illustrated, the transport unit 190 includes a transport roller 191, and the transfer paper is transported to the post-processing apparatus 10 by the transport roller 191.

  The post-processing device 10 receives the transfer paper from the image forming unit 100 via the roller 11. Now, when the discharge tray 14 is selected as the output destination of the transfer paper, the transport direction is switched by the flapper 12. Then, the transfer paper is discharged to the tray 14 by the roller 13.

  This tray 14 is a temporary discharge tray that is used as a transfer paper discharge destination when another print process is interrupted during a normal print process. Note that trays 18 and 19 are used as normal discharge trays.

  When the tray 18 or 19 is used, the lower conveyance path is selected / switched by the flapper 12, and further, when the conveyance path on the roller 16 side is selected by the flapper 13, the transfer paper is discharged to the tray 18 or 19. be able to.

  On the other hand, a conveyance path extending downward in the vertical direction is selected by the flappers 13 and 14, and the transfer paper is fed into the conveyance path, and then the conveyance direction is reversed by the reverse roller 15. When the path on the roller 16 side is selected by the flapper 14, reverse paper discharge can be performed on the tray 18 or 19.

  When the transfer paper is discharged to the tray 18 or 19, the transfer paper bundle can be stapled using the stapler 17. The trays 18 and 19 can be moved in the vertical direction by the shift motor 20. The selection of whether the transfer paper or the transfer paper bundle is discharged to the tray 18 or 19 is performed by moving the trays 18 and 19 up and down by the shift motor 20.

  The tray 27 is a discharge tray used for bookbinding. When bookbinding is performed, the transfer paper is transported from the transport roller 11 to the roller 21 side via the reverse roller 15 by the flappers 12, 13, and 14. Further, the roller 22 side is selected by the flapper 25, and the transfer paper is conveyed to the primary accumulation unit 23 by the roller 22.

  When a predetermined amount of transfer paper is stored in the primary storage unit 23, that is, after the transfer paper has been stored, bookbinding is performed by the stapler 24. Thereafter, the direction of the flapper 25 is changed, and the roller 22 is rotated in the direction opposite to that when the transfer paper is accumulated. As a result, the bound transfer paper is discharged to the tray 27 via the roller 26.

  In the example shown in FIG. 1, only one photosensitive drum 107 is shown for convenience of description of the printing operation by the digital multifunction peripheral 100A. However, when performing color printing, for example, each color (yellow (Y ), Cyan (C), magenta (M), and black (K)), a photosensitive drum is provided. Then, each color toner image on each photosensitive drum is transferred to an intermediate transfer body (not shown) such as an intermediate transfer belt to form a color toner image. Thereafter, the color toner image on the intermediate transfer member is transferred to the transfer paper.

  In any case, the toner image formed on the photosensitive drum (image carrier) is directly or indirectly transferred to the transfer paper.

  FIG. 2 is a block diagram illustrating an example of a control circuit 200A used in the digital multifunction peripheral 100A illustrated in FIG.

  In the example shown in FIG. 2, only a function for processing image data (signal processing unit 201) and a function used for exposure control of the photosensitive drum 107 (exposure control unit 207) are shown. Is omitted.

  Referring to FIG. 2, the illustrated control circuit 200A includes a signal processing unit 201 and an exposure control unit (exposure control means) 207, and the exposure control unit 207 drives the semiconductor laser 101 (FIG. 1) described above. Control. As shown in the figure, a color sensor 200 is connected to the signal processing unit 201. The color sensor 200 is, for example, the CCD line sensor described with reference to FIG.

  The signal processing unit 201 includes a controller 202, a memory 203, an interface (I / F) 204, a write correction processing unit 205, and a gradation processing unit 206. The exposure control unit 207 includes a pattern storage memory 208, a pattern generation circuit 209, and a PWM modulation unit 210.

  In the signal processing unit 201, the controller 202 receives image data obtained as a result of document reading from the color sensor 200 as, for example, an RGB signal, and also receives image data from a computer such as a PC via the I / F 204. . Further, the controller 202 receives a facsimile image as image data from a modem (not shown) via the I / F 204.

  In any case, image data is input to the controller 202, and the controller 202 compresses the image data as necessary and temporarily stores it in the memory 203.

  The illustrated pattern storage memory 208 stores in advance a plurality of screen patterns representing a plurality of irrational screens (also called irrational tangent screens). Then, the pattern generation circuit 209 selectively uses the plurality of screen patterns to generate a density correction image pattern as will be described later.

  Next, the operation of the control circuit 200A shown in FIG. 2 will be described. FIG. 3 is a control flowchart for explaining the operation of the control circuit 200A shown in FIG.

  Referring to FIGS. 2 and 3, here, image forming apparatus 100A (FIG. 1) performs density correction control every time the number of prints reaches a predetermined number.

  The density correction control is also performed when the user gives a density correction control command from an operation panel (not shown).

  Now, when the image forming apparatus 100A (FIG. 1) is turned on, it is determined whether or not a predetermined number of sheets have been printed since the previous density correction control was performed in the image forming apparatus 100A. It is determined whether or not to perform density correction control according to this result (step S201).

  When the density correction control is not performed (NO in step S201), the image forming apparatus 100A shifts to the print operation mode (print mode).

  When the printing operation mode is entered, the image forming apparatus 100A waits until the activation of the signal processing unit 201 and the exposure control unit 207 is completed (step S202). After the activation of the signal processing unit 201 and the exposure control unit 207 is completed, the image data is given to the signal processing unit 201 as described above (step S203).

  Upon receiving the image data, the signal processing unit 201 corrects the image data (step S204). First, the controller 202 performs, for example, shading correction, color conversion processing, noise removal processing, and the like on the RGB signals that are image data. As a result, variations in the optical system including the color sensor 200 are removed, and the characteristics are corrected. The correction is performed on the image data obtained as a result of reading the document, and is not performed on the image data input to the controller 202 via the I / F 204.

  In this manner, the controller 202 corrects the image data as necessary, and then compresses the image data and temporarily stores it in the memory 203.

  When the controller 202 finishes storing the image data in the memory 203, the controller 202 then reads the image data from the memory 203 and gives the image data to the write correction processing unit 205. The write correction processing unit 205 converts RGB signals into YMCK signals, performs conversion processing to match the color gamut of the image forming apparatus 100A, correction of two-dimensional frequency characteristics, and the like.

  Then, the write correction processing unit 205 gives the YMCK signal to the gradation processing unit 206. The gradation processing unit 206 performs error diffusion, screen processing, and the like on this YMCK signal (gradation processing: step S205).

  In this gradation process, an optimal screen process is selected in accordance with the characteristics of the image data, and gradation conversion is executed in accordance with the selected screen process. For example, in the illustrated example, screen processing using an irrational number (irrational tangent) screen using a dither method is performed, and halftone correction is performed.

  After the gradation processing is performed, the YMCK signal is given to the exposure control unit 207 as a processed image signal. In the exposure control unit 207, the PWM modulation unit 210 performs pulse width modulation on the processed image signal (step S206), and supplies it as a PWM signal to a laser drive unit (not shown), and the semiconductor laser 101 (FIG. 1). To drive.

  As a result, the photosensitive drum 107 is exposed as described with reference to FIG. 1, and the printing operation is performed as described above.

  On the other hand, if it is determined that density correction control is to be performed (YES in step S201), image forming apparatus 100A shifts to a density correction control mode (density correction mode). In this density correction control mode, the image forming apparatus 100A waits until the activation of the exposure control unit 207 is completed regardless of the activation of the signal processing unit 201 (step S207).

  After the activation of the exposure control unit 207 is completed, a density correction control command is given to the pattern generation circuit 209, which is not shown in FIG. In response to the density correction control command, the pattern generation circuit 209 accesses the pattern storage memory 208 and reads a screen pattern indicating an irrational screen (step S208). Then, the pattern generation circuit 209 generates a density correction image pattern according to the read screen pattern (step 209).

  Here, it is assumed that a screen pattern representing one type of irrational screen is stored in the pattern storage memory 208. The pattern generation circuit 209 generates a density correction image pattern by using at least a part of the screen pattern. Then, the pattern generation circuit 209 gives this density correction image pattern to the PWM modulator 210 (FIG. 2) as a processed image signal.

  The PWM modulation unit 210 performs pulse width modulation on the image pattern for density correction (processed image signal) (step S210), applies it to the laser drive unit as a PWM pattern signal, and drives the semiconductor laser 101 (FIG. 1).

  Thereby, as described above, for example, a color toner image corresponding to the density correction pattern is formed as a density correction pattern image on an intermediate transfer member such as an intermediate transfer belt. Then, the density correction pattern image is transferred onto a transfer sheet.

  FIG. 4 is a diagram showing an example of a density correction pattern image formed in accordance with the density correction image pattern generated using one irrational screen in the pattern generation circuit 209 shown in FIG.

  As shown in FIG. 4, the density correction pattern image 400 </ b> A includes, for example, first to third pattern images 401 to 403 formed on the intermediate transfer body 400. As shown in an enlarged view in FIG. 4, the first pattern image 401 is formed using at least a part of one type of screen pattern. In the illustrated example, one screen pattern 404 is combined as a screen pattern to form a density correction image pattern 405.

  The first pattern image 401 is a toner image corresponding to the density correction image pattern 405. When the density correction image pattern 405 is formed using a part of one type of screen pattern, the first pattern image 401 is displayed. In 401, a phenomenon in which dot deviation occurs was confirmed.

  That is, it has been confirmed that when the density correction image pattern 405 is formed using a part of one type of screen pattern, density unevenness peculiar to an irrational screen occurs.

  Similarly, since the second and third pattern images 402 and 403 are also formed in accordance with the density correction image pattern 405, density unevenness peculiar to an irrational screen occurs.

  As a result, even if the density correction pattern image 400A is read by a density sensor (density detection means: not shown) and density correction control is performed in accordance with the density signal that is the reading result, the density correction is performed with high accuracy. Difficult to do.

  Therefore, in this embodiment, as described above, a plurality of types of screen patterns are stored in the pattern storage memory 208. Then, the pattern generation circuit 209 forms a pattern image according to each screen pattern.

  Now, it is assumed that the first to Nth screen patterns are stored in the pattern storage memory 208 (where N is an integer of 2 or more). When forming the density correction pattern image, the pattern generation circuit 209 reads the first to N-th screen patterns from the pattern storage memory 208 and corresponds to the first to N-th screen patterns, respectively. N image patterns are generated (that is, an image pattern is generated for each screen pattern). The pattern generation circuit 209 collectively sets the first to Nth image patterns as density correction image patterns.

  FIG. 5 is a diagram showing an example of a density correction pattern image formed in accordance with the density correction image pattern generated using a plurality of types of irrational screens in the pattern generation circuit 209 shown in FIG. The illustrated example shows a case where N = 3.

  2 and 5, in the illustrated example, the pattern storage memory 208 stores first to third screen patterns. The pattern generation circuit 209 reads the first to third screen patterns from the pattern storage memory 208. Then, the pattern generation circuit 209 generates a first image pattern 502 according to the first screen pattern 501. Further, the pattern generation circuit 209 generates a second image pattern 504 according to the second screen pattern 503.

  Similarly, the pattern generation circuit 209 generates a third image pattern according to the third screen pattern (the third screen pattern and the third image pattern are not shown in FIG. 5).

  As described above, the pattern generation circuit 209 supplies the first and second image patterns 502 and 504 and the third image pattern to the PWM modulation unit 210 as density correction image patterns. Then, the PWM modulation unit 210 performs pulse width modulation on the above-described density correction image pattern as a processed image signal to generate a PWM signal, and drives the semiconductor laser 101 (FIG. 1) according to the PWM pattern signal.

  As a result, a density correction pattern image 505 shown in FIG. 5 is formed on the intermediate transfer body 400. The density correction pattern image 505 includes first to third pattern images 506 to 508, and the first and second pattern images 506 and 507 are the first and second image patterns 502 and 507, respectively. 504. Similarly, the third pattern image 508 corresponds to the third image pattern.

  Such a density correction pattern 505 is read by a density sensor, and density signals corresponding to the first to third pattern images 506 to 508 are respectively obtained as first to third density signals (detected densities) (that is, The density is detected for each image pattern). Thereafter, the gray scale values (average density) are averaged by averaging the gray levels represented by the first to third density signals by an arithmetic circuit (not shown) or the like (for example, this arithmetic circuit is built in the exposure control unit 207). Get.

  Then, the exposure control unit 207 performs density correction control by adjusting the PWM modulation unit 210 in accordance with the grayscale average value. Thus, it was confirmed that the density correction control of the toner image can be performed with high accuracy when the density correction control is performed.

  FIG. 6 is a diagram showing another example of the density correction pattern image formed in accordance with the density correction image pattern generated using a plurality of types of irrational screens in the pattern generation circuit 209 shown in FIG. Here, it is assumed that the first and second screen patterns 601 and 602 are stored in the pattern storage memory 209.

  2 and 6, the pattern generation circuit 209 reads the first and second screen patterns 601 and 602 from the pattern storage memory 208. Then, the pattern generation circuit 209 generates the density correction image pattern 603 by combining the first and second screen patterns 601 and 602 in a predetermined order.

  The density correction image pattern 603 is provided as a processed image signal from the pattern generation circuit 209 to the PWM modulation unit 210. The PWM modulation unit 210 performs pulse width modulation on the density correction image pattern 603 to generate a PWM signal, and drives the semiconductor laser 101 (FIG. 1) in accordance with the PWM pattern signal.

  As a result, a density correction pattern image 604 shown in FIG. 6 is formed on the intermediate transfer body 400. The density correction pattern image 604 includes first to third pattern images 605 to 607, and the first to third pattern images 605 to 607 are toners corresponding to the density correction image pattern 603. It is a statue.

  As described above, when the density correction image pattern 603 is formed by combining the two types of screen patterns 601 and 602, and the density correction pattern image 604 is formed using the density correction image pattern 603, the first to first patterns. In the third pattern images 605 to 607, it was confirmed that the deviation of positions where dots are formed is reduced. As a result, it was confirmed that when the density correction pattern image 604 is detected by the density sensor and density correction control is performed as described above, the density correction control can be performed with high accuracy.

  As described above, in the image forming apparatus 100A according to the embodiment of the present invention, after the exposure control unit 207 is activated, the density correction control can be executed without waiting for the signal control unit 201 to be activated. It is possible to reduce the FCOT and perform density correction control in a short time.

  Furthermore, in the image forming apparatus 100A according to the embodiment of the present invention, a plurality of screen patterns representing irrational screens are stored in the pattern storage memory 208, and a density correction image pattern is generated using the plurality of screen patterns. Since it is generated, density unevenness peculiar to an irrational screen can be reduced. As a result, it is possible to execute density correction control with high accuracy.

  Further, in the image forming apparatus 100A according to the embodiment of the present invention, the density correction image pattern is simply formed using a plurality of screen patterns, so that an increase in circuit scale can be prevented as much as possible.

  As is apparent from the above description, here, the exposure control unit 207 functions as a density correction control unit. In the above-described embodiment, the example in which the exposure intensity is adjusted and the density correction control is performed according to the detected density detected by the density sensor has been described. However, the charge amount of the photosensitive drum 107 is adjusted. May be. Furthermore, the developing bias may be adjusted.

  In any case, if the density correction control is performed by controlling each component of the image forming apparatus 100A according to the detected density of the density correction image pattern formed as described above, the density is accurately controlled. Correction can be performed.

201 Signal Processing Unit 202 Controller 203 Memory 204 Interface (I / F)
205 Write Correction Processing Unit 206 Gradation Processing Unit 207 Exposure Control Unit 208 Pattern Storage Memory 209 Pattern Generation Circuit 210 PWM Modulation Unit

Claims (6)

A latent image is formed on an image carrier for supporting a toner image, the latent image is developed to form the toner image, and the toner image is directly or indirectly transferred to a transfer medium to form an image. In the image forming apparatus,
A pattern storage memory in which a plurality of screen patterns are stored in advance;
A pattern generation circuit for generating an image pattern for density correction according to the plurality of screen patterns;
Exposure control means for exposing the image carrier in accordance with the density correction image pattern;
Density detecting means for detecting the toner image formed according to the density correction image pattern as a density correction pattern image and obtaining the density as a detected density;
An image forming apparatus comprising: a density correction control unit that executes density correction control according to the detected density. The pattern generation circuit generates an image pattern for each of the plurality of screen patterns, and uses the image pattern as the density correction pattern.
The image according to claim 1, wherein the density correction control unit executes the density correction control according to an average density obtained by averaging the detected densities obtained from the density detection unit for each image pattern. Forming equipment.   The image forming apparatus according to claim 1, wherein the pattern generation circuit generates the density correction image pattern by combining the plurality of screen patterns.   The image forming apparatus according to claim 1, wherein a screen pattern representing a forcible tangent screen is used as the screen pattern. Signal processing means for receiving image data and performing gradation processing on at least the image data to obtain a processed image signal;
The exposure control means controls the exposure of the image carrier in accordance with the processed image signal in the printing mode to form the latent image on the image carrier, and the density correction in the density correction mode. 5. The image forming apparatus according to claim 1, wherein the latent image is formed by controlling the exposure of the image carrier in accordance with an image pattern for use.   6. The image forming apparatus according to claim 5, wherein the pattern storage memory and the pattern generation circuit are provided in the exposure control means.