JP2008250146A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus that properly performs density correction of a developer image, using a grayscale representation by multi-valued dithering. <P>SOLUTION: The apparatus includes a correction image pattern generation unit for generating a correction image pattern that is data for forming a substantially uniform developer image, which is formed within a range of a predetermined density; an image forming unit for forming the developer image on the basis of the correction image pattern and the input image data; an image carrier for carrying the developer image, on the basis of the correction image pattern formed by the image forming unit; a density detecting unit for detecting a developer density of the developer image, on the basis of the correction image pattern carried by the image carrier; and a density correction unit for correcting the density of the developer image, based on the input image information data on the basis of detection results of the density detecting unit. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus.

  Conventionally, a pseudo halftone method such as a systematic dither method has been used as a method for expressing image gradation in an image forming apparatus such as a color electrophotographic printer. The systematic dither method is a method of performing gradation expression by a ratio of a plurality of pixels formed per unit area using a binary output system.

  The density value of the gradation image by the systematic dither method, which is a kind of pseudo halftone method, varies depending on the sensitivity of the photosensitive drum, the change in developer charging characteristics over time, the environment around the apparatus, and the like. Therefore, first, an image forming apparatus having an image gradation processing function based on a systematic dither method uses a developer image based on a pseudo gradation pattern in advance on a photosensitive drum or a conveyance belt before image formation on a recording medium. Form on top.

  Then, the image forming apparatus detects the density value of the developer image with a density detecting unit or the like, and stabilizes the gradation characteristics by adjusting the exposure energy and the development characteristics based on the detected developer density. Density correction is performed (for example, Patent Document 1).

JP 2004-258281 A

  On the other hand, in recent years, there has also been provided an image forming apparatus that forms a gradation image by a so-called multi-value dither method, which combines a multi-value recording method capable of expressing a plurality of gradations with one pixel and the above-described systematic dither method. It was.

  However, even if the density value of the developer image based on the gradation pattern is detected by a detection means or the like, and the exposure energy and development characteristics are adjusted based on the detected density, the multilevel dither method is used. Density correction in the case of tonal expression has a problem in that a stable gradation characteristic cannot be obtained because nonlinearity between the exposure energy input value in one pixel and the actual developer density value becomes a problem.

  The present invention has been made in view of such circumstances, and provides an image forming apparatus capable of appropriately correcting the density of a developer image using gradation expression by a multi-value dither method. Objective.

  In order to solve the above-described problem, the image forming apparatus according to the present invention generates a correction image pattern that is data for forming a substantially uniform developer image formed by pixels within a predetermined density range. An image pattern generation unit, an image forming unit that forms a developer image based on the correction image pattern and the input image data, and an image that carries the developer image based on the correction image pattern formed by the image forming unit Based on the input image information data based on the detection result of the carrier, the density detector for detecting the developer density of the developer image based on the correction image pattern carried by the image carrier, and the detection result of the density detector And a density correction unit that corrects the density of the developer image.

  According to the image forming apparatus of the present invention, by using the developer density detection result of the developer image based on the substantially uniform correction image pattern formed by the pixels within the predetermined density range, the multi-value dither method is used. It is possible to appropriately correct the density of the developer image using the gradation expression.

(First embodiment)
FIG. 1 is a cross-sectional view showing a configuration of an image forming apparatus 1 according to the present invention. An image forming apparatus 1 includes an image forming unit (image drum unit) which is an electrophotographic LED (Light-Emitting Diode) print mechanism for recording black K, yellow Y, magenta M, and cyan C images, and a recording medium. A conveyance mechanism for conveying, a paper feed mechanism for supplying the recording medium to the conveyance path, and a fixing mechanism for fixing the developer image on the recording medium are provided.

  FIG. 2 is a schematic cross-sectional view of the color misregistration detection sensor 24. The color misregistration detection sensor 24 includes an infrared LED 101, a specular reflection light receiving phototransistor 102, a diffuse reflection light receiving phototransistor 103, and the like. The specular reflection light receiving phototransistor 102 and the diffuse reflection light receiving phototransistor 103 are driven by a circuit (not shown) and generate a current proportional to the amount of received reflected light. The generated current is converted into a voltage by a circuit (not shown) and transmitted to a mechanism control unit described later.

  FIG. 3 is a block diagram of a control circuit of the image forming apparatus 1 according to the present invention. In FIG. 3, symbols K, Y, M, and C correspond to image forming units that form black, yellow, magenta, and cyan developer images, respectively.

  FIG. 4 is a block diagram of the LED head driving unit. The LED head drive unit includes an LED head interface unit 52, an LED array driver 91, an energy level storage unit 92, and an LED array 93.

  The image forming unit is arranged in the order of the image forming unit 2K, the image forming unit 2Y, the image forming unit 2M, and the image forming unit 2C from the upstream side along the conveyance path from the recording medium insertion side to the discharge side of the image forming apparatus 1. Each is arranged independently.

  Since the image forming unit 2K, the image forming unit 2Y, the image forming unit 2M, and the image forming unit 2C all have the same configuration, the following description is based on the image forming unit 2K.

  The image forming unit 2K includes a photosensitive drum 6K on which an electrostatic latent image is formed, an LED head 3K that exposes the photosensitive drum 6K based on image data to form an electrostatic latent image on the photosensitive drum 6K, and a photosensitive drum 6K. A charging roller 5K for charging, a developing roller 7K for developing an electrostatic latent image with toner, a developing blade 8K, a sponge roller 9K, and a developer image on the toner cartridge 10K and the photosensitive drum 6K are transferred to a recording medium. Transfer roller 4K.

  The toner cartridge 10K contains black toner. The toner cartridge 10Y, the toner cartridge 10M, and the toner cartridge 10C in the other image forming unit 2Y, image forming unit 2M, and image forming unit 2C are respectively yellow. , Magenta and cyan toners are accommodated.

  The transport mechanism includes a transport belt 12, a driving roller 13, a driven roller 14, and a suction roller 15. The conveyor belt 12 is made of a high-resistance semiconductive plastic film and is formed into a seamless endless shape. Further, the surface of the conveyor belt 12 is glossy.

  The drive roller 13 is connected to a belt motor (not shown) and rotates counterclockwise. The conveyor belt 12 is stretched between the drive roller 13 and the driven roller 14, and the conveyor belt 12 is interposed between the photosensitive drums 6K to 6C and the transfer rollers 4K to 4C of each image forming unit. It is arranged. Further, the suction roller 15 is provided at a position facing the driven roller 14 with the conveyance belt 12 interposed therebetween.

  The paper feed mechanism includes a hopping roller 16, a registration roller 17, a pinch roller 18, a recording medium accommodation cassette 19, and sensors 21, 22, and 23. The recording medium accommodation cassette 19 is positioned at the lowermost part of the image forming apparatus 1, and the hopping roller 16 is disposed on the upper part of the recording medium accommodation cassette 19. The pinch roller 18 is disposed so as to sandwich and convey the recording medium together with the registration roller 17.

  The recording media stored in the recording medium storage cassette 19 are separated one by one by a separating means (not shown), and taken out from the recording medium storage cassette 19 by the hopping roller 16.

  The recording medium taken out from the recording medium accommodating cassette 19 is guided by the guide 20 and reaches the registration roller 17. When the recording medium is obliquely fed, the skew of the recording medium due to the pinch roller 18 facing the registration roller 17 is corrected. Next, the recording medium conveyed between the adsorbing roller 15 and the conveying belt 12 from the registration roller 17 is charged by the static electricity supplied from the adsorbing roller 15 and electrostatically adsorbed on the upper surface of the conveying belt 12.

  The sensors 21 and 22 are for detecting a recording medium, and are respectively disposed before and after the registration roller 17 along the recording medium conveyance path. The sensor 23 is provided in the vicinity of the recording medium conveyance outlet side of the conveyance belt 12. The sensor 23 checks the recording medium that has failed to be separated from the transport belt 12 or detects the rear end position of the recording medium.

  The fixing mechanism includes a heat roller 25, a pressure roller 26 for nipping and conveying a recording medium, a thermistor 28, a discharge sensor 27, a guide 29, and a stacker 30. The heat roller 25 is driven by a driving force supplied from a heat roller motor described later, and the pressure roller 26 rotates in synchronization with the heat roller 25.

  The developer image formed on the recording medium conveyed from the conveying mechanism is fixed on the recording medium by being heated and pressurized between the heat roller 25 and the pressure roller 26.

  The thermistor 28 is disposed near the surface of the heat roller 25 and detects the temperature of the heat roller 25. The discharge sensor 27 is provided on the recording medium discharge port side of the heat roller 25, and detects jamming in the fixing mechanism and winding of the recording medium around the heat roller 25. The recording medium discharged from the heat roller 25 is conveyed along the guide 29 and discharged to the stacker 30.

  A cleaning blade 32 and a waste toner tank 33 are provided on the surface side of the transport belt 12 facing the recording medium accommodation cassette 19 side. The cleaning blade 32 is made of a flexible rubber material or plastic material, and is formed so as to be pressed against the driven roller 14 via the transport belt 12. The cleaning blade 32 can scrape off the toner adhering and remaining on the surface of the conveyor belt 12 as the conveyor belt 12 is driven into the waste toner tank 33.

  The color misregistration detection sensor 24 that detects the toner density is provided directly below the drive roller 13. The color misregistration detection sensor 24 is a reflective sensor of one light emission system and two light reception systems, and measures the reflection intensity of the developer image based on the correction image pattern formed on the surface of the conveyance belt 12. The measurement result is used for the operation of detecting the print density of the image forming apparatus 1.

  The image forming apparatus 1 includes a host interface unit 50, a command / image processing unit 51, an LED head interface unit 52, a mechanism control unit 53, a heater 59, and a high voltage control unit 60.

  The host interface unit 50 is a physical interface that connects the host computer and the image forming apparatus 1 and includes a connector, a communication chip, and the like.

  The command / image processing unit 51 processes a command from the host computer side and expands the image data into bitmap data. A microprocessor, a RAM (Random Access Memory), and special hardware for expanding image data into bitmap data are configured.

  Further, the command / image processing unit 51 can process image data as binary data when multi-step density data is not assigned to each pixel of the image data. When multi-level density data is given to each pixel, it can be processed as multi-value data.

  The LED head interface unit 52 includes a semi-custom LSI (Large Scale Integration), a RAM, and the like, and the image data expanded in the bitmap from the command / image processing unit 51 is an interface of the LED heads 3K, 3Y, 3M, 3C. Process according to.

  When the data transmitted from the command / image processing unit 51 is binary data, the LED head interface unit 52 exposes each LED element of the LED heads 3K, 3Y, 3M, and 3C with a constant energy. To control. On the other hand, when the data is multi-value data, the exposure energy is controlled to increase / decrease for each lighting.

  When the mechanism control unit 53 receives an instruction from the command / image processing unit 51, based on the detection result from each sensor, the hopping motor 54, the registration motor 55, the belt motor 56, the heat roller motor 57, the drum motors K, Y , M, and C58 are driven. Further, the mechanism control unit 53 controls the heater 59 to control the printing mechanism unit and the high-pressure control unit 60.

  Further, the storage means 80 of the mechanism control unit 53 is suitable for the motor rotation speed of each of the hopping motor 54, the registration motor 55, the belt motor 56, the heat roller motor 57, the drum motors K, Y, M, and C58, and the printing operation. It stores information related to various fixing temperatures. The storage unit 80 stores image data transmitted from the host computer.

  The heater 59 is formed by heating means such as a halogen lamp disposed in the heat roller 25. The surface temperature of the heat roller 25 heated by the heater 59 is detected by the thermistor 28 disposed near the surface of the heat roller 25.

  The high voltage control unit 60 is configured by a microprocessor, a custom LSI, or the like. The high voltage controller 60 generates a charge voltage, a developing bias, a transfer voltage, and the like for each image forming unit.

  The CH generation unit 61 supplies a charge voltage to each image forming unit, and the DB generation unit 62 controls application of a developing bias to each image forming unit.

  The TR generator 63 supplies a transfer voltage to the transfer roller of each image forming unit. Further, the TR generator 63 has a current / voltage detection circuit for supplying power to the transfer roller of each image forming unit with a constant current or a constant voltage.

  The LED array driver 91 drives the LED array 93 corresponding to the data transmitted from the LED head interface unit 52. In addition, each LED element of the LED array 93 in a present Example is arranged in the same direction as the axial direction of the photosensitive drum 6 (6K-6C) with a 600 dpi pitch.

  The LED array driver 91 causes the LED elements constituting one pixel having a resolution of 600 dpi in the recording medium conveyance direction to emit light in eight stages. When multi-gradation data is transmitted from the LED head interface unit 52 to the LED array driver 91, the LED array driver 91 writes a multi-gradation latent image with a combination of eight levels of light emission.

When the light emission of the LED element 8 stages, LED array driver 91, ^1/2 the emission time of the LED element with respect to the reference emission time ^{T 0, 1/2 1,} 1/2 2, 1/2 3, controls 1/2 ^{4, 1/2 5, 1/2} 6, ^{1/2 7} obtained by multiplying causing time light emission. By controlling the LED elements in this way, the dot size changes as shown in FIG. The LED element line corresponding to this light emission time is called a subline with respect to the 600 dpi 1 line.

  FIG. 5 is a diagram for explaining gradation dot expression by subline control. Here, in order to express halftone dots, the LED array driver 91 performs 256 gradation exposures per 600 dpi 600 dpi in the total lighting time by changing the combination of actually blinking sublines. And the energy level memory | storage means 92 shall store the combination of arbitrary 32 types of exposure patterns among 256 types of exposure patterns for every LED element.

Next, the operation of the image forming apparatus 1 will be described. First, a normal printing operation will be described.
First, the image forming apparatus 1 receives image data sent from an external device such as a host computer via the host interface unit 50.

  After receiving the image data, the command / image processing unit 51 issues an initial processing command to the mechanism control unit 53. The mechanism control unit 53 that has received the command turns on the heater 59. Specifically, the mechanism control unit 53 refers to the environmental temperature sensor 34, reads a suitable fixing temperature in the printing operation stored in the storage unit 80, and maintains the fixing temperature by ON / OFF control of the heater 59. . The image forming apparatus 1 stores the image data of each color for one page to be printed on the recording medium in the memory of the command / image processing unit 51, and performs the printing operation when the fixing temperature reaches a suitable temperature. Start.

  When the initial operation ends, the command / image processing unit 51 supplies a command to the mechanism control unit 53. The mechanism control unit 53 that has received a command from the command / image processing unit 51 drives the belt motor 56 and the drum motors K, Y, M, and C58. As a result, the mechanism control unit 53 starts driving the belt driving roller 13, the transport belt 12, and the image forming units 2K, 2Y, 2M, and 2C.

  At this time, the color toners in the image forming units 2K, 2Y, 2M, and 2C are charged by electrostatic force generated by friction between the sponge rollers 9K, 9Y, 9M, and 9C and the developing rollers 7K, 7Y, 7M, and 7C. .

  Next, the mechanism control unit 53 drives the hopping motor 54 to rotate the paper feed roller 16, and separates each recording medium in the recording medium accommodation cassette 19 and sends it to the guide 20. When the leading end of the recording medium reaches between the registration roller 17 and the pinch roller 18, the mechanism control unit 53 detects this by the sensor 21 and stops the hopping motor 54.

  Next, the mechanism control unit 53 rotates the registration roller 17 and the pinch roller 18 respectively. When the recording medium is conveyed by the registration roller 17 and the leading end reaches between the adsorption roller 15 and the conveyance belt 12, at the same time, the mechanism control unit 53 turns on an adsorption charging power source (not shown) to draw the adsorption roller. 15 is supplied with voltage.

  The leading edge of the recording medium is attracted to the conveyor belt 12 by the electrostatic force between the attracting roller 15 and the driven roller 14. Further, when the registration roller 17 rotates, the recording medium is conveyed in the recording medium conveying direction while being sucked by the conveying belt 12.

  Simultaneously with the recording medium conveyance operation, the mechanism control unit 53 applies voltages to the charging rollers 5K, 5Y, 5M, and 5C and the developing rollers 7K, 7Y, 7M, and 7C of the image forming units 2K, 2Y, 2M, and 2C. In order to supply, a command is supplied to the high voltage control unit 56. As a result, the CH generation unit 61 energizes the charge voltage to each image forming unit, and the DB generation unit 62 controls the application of the developing bias to each image forming unit. The surfaces of the photosensitive drums 6K, 6Y, 6M, and 6C of the image forming units 2K, 2Y, 2M, and 2C are uniformly charged, and the developing rollers 7K, 7Y, 7M, and 7C are charged to a predetermined voltage. The toner adheres to the developing rollers 7K, 7Y, 7M, and 7C. Then, it is leveled to a uniform thickness by the developing blades 8K, 8Y, 8M, and 8C.

  Simultaneously with these processes, the command / image processing unit 51 develops the black image data for one line from the storage unit 80 in which the black image data is stored into the bitmap format, and further to the LED head 3K. It converts into the format which can be transmitted, and transmits to LED head 3K. At this time, when the image data received from the host computer is multi-value data that is multi-gradation data per pixel, the command / image processing unit 51 transmits the multi-value data to the LED head 3K.

  The LED head 3K controls the light emission of the LED based on the transmitted black toner image data, and forms an electrostatic latent image for one line corresponding to the image data transmitted to the surface of the charged photosensitive drum 6K. To do. In this way, the black image data transmitted from the command / image processing unit 51 for each line is successively converted into an electrostatic latent image on the surface of the photosensitive drum 6K. Then, black toner is attached from the charged developing roller 7K to the surface of the photosensitive drum 6K on which the electrostatic latent image corresponding to the black image data transmitted from the command / image processing unit 51 is formed. The electrostatic latent image is successively developed with black toner by the rotation of the photosensitive drum 6K, and a toner image corresponding to the black image data transmitted from the command / image processing unit 51 is formed. When the leading edge of the recording medium reaches between the photosensitive drum 6K and the transfer roller 4K, the mechanism control unit 53 issues a transfer command to the high pressure control unit 56, and the high pressure control unit 56 generates black TR. The part 63 is turned ON.

  The developer image on the surface of the photosensitive drum 6K is electrically transferred onto the recording medium by the transfer roller 4K. By the rotation of the photosensitive drum 6K, the developer images are successively transferred onto the recording medium, and a black image for one page is transferred onto the recording medium. With the above operation, the formation of the black developer image on the recording medium by the image forming unit 2K is completed. When the image formation with the black toner is completed, the recording medium moves from the position of the image forming unit 2K to the position of the image forming unit 2Y, and then the developer by the image forming unit 2Y that forms a yellow developer image. An image is formed. Further, a developer image is formed continuously with magenta and cyan.

  The recording medium on which the developer image has been formed by all of the image forming units 2K, 2Y, 2M, and 2C is conveyed to the fixing mechanism. When the recording medium reaches the heat roller 25 heated to a fixing temperature, the developer image is pressed and heated by the heat roller 25 and the pressure roller 26 and fixed on the recording medium. When the fixing is completed, the recording medium is discharged to the discharge stacker 30.

  The sensor 27 detects the trailing edge of the recording medium, and notifies the mechanism control unit 53 that the recording medium has been discharged to the discharge stacker 30. When the discharge of the recording medium is completed, the mechanism control unit 53 stops all the motors.

  When the transfer of the developer image to the recording medium in each of the image forming units of the image forming unit 2K, the image forming unit 2Y, the image forming unit 2M, and the image forming unit 2C is completed, the mechanism control unit 53 63 is turned OFF, and the CH generator 61 and the DB generator 62 are turned OFF when the drum rotation is stopped.

  Next, a halftone dot density detection operation and a halftone dot density adjustment operation will be described.

  FIG. 6 is a diagram illustrating an example of the relationship between the exposure energy of the LED and the developing toner density.

  As shown in the figure, in the prior art, when the latent image written on the surface of the photosensitive drum is developed on the recording medium by selecting the 256 energy levels, the developer actually formed on the recording medium is developed. The toner density of an image generally has a non-linear characteristic with respect to the exposure energy level.

  If the non-linear characteristic between the exposure energy of the LED and the developing toner density shown in FIG. 6 remains as it is, it is difficult to express gradation that requires a linear characteristic. Therefore, as shown in FIG. 7, the image forming apparatus 1 uses a combination of 32 levels of energy values obtained by dividing the 256 levels of energy into 31 equal parts between the maximum developed toner density value and the zero point developed toner density value. Assume that each LED element of the head 3 (3K to 3C) is individually selected.

  This combination of 32 levels of energy values is selected as an initial value when the LED head is manufactured, and the combination is stored in the energy level storage means 92 as a table as shown in FIG. E0, E1,..., E31 represent 32 levels of initial energy values, and D0, D1,..., D31 represent development toner density values corresponding to the respective initial energy values.

  D0, D1,..., D31 are target developer toner density values that exhibit completely linear characteristics when E0, E1,.

  By using the equally differentiated energy values, each LED head element can perform linear 32-level latent image writing. However, even if an energy level having a linear relationship with the developing toner concentration is selected as the initial value, the developing toner concentration is a late change such as an environmental change or a change with time, mounting conditions of the LED head 3 (3K to 3C), etc. For this reason, the initial development toner concentration set when the LED head 3 (3K to 3C) is manufactured may not be realized.

  In an image forming apparatus having a conventional multi-value printing mode, density gradation expression is performed by a multi-value dither method as shown in FIG. FIG. 9 shows an example of multi-value dither. In this configuration, a maximum of one halftone dot is included in the 3 × 3 matrix, and the density expression of 31 × 9 + 1 = 280 gradations is possible with the 3 × 3 dither matrix alone.

  Here, as shown in FIG. 10, if the linearity of halftone dots is maintained in the gradation expression, the developing toner density gradation curve shows a smooth linear characteristic. However, when the linearity of the halftone dots is impaired, as shown in FIG. 11, the developing toner density gradation curve does not show a smooth linear characteristic, and the image quality is deteriorated.

  Therefore, since a means for correcting the nonlinearity of the developing toner density with respect to the initial energy value generated due to a subsequent reason such as an environmental change or a change with time is required, the means will be described below.

  When the mechanism control unit 53 receives the signal for detecting the halftone dot density, the mechanism control unit 53 prints the halftone density detection pattern 111, which is a developer image based on the correction image pattern shown in FIG. / A command is supplied to the image processing unit 51. The halftone density detection pattern 111 formed on the surface of the conveyance belt 12 is configured by eight sets arranged in the order of yellow, magenta, cyan, and black (YMCK) from the downstream side in the e direction that is the conveyance belt 12 driving direction shown in FIG. It has become. Each set is a pattern corresponding to energy levels of values E3, E7, E11, E15, E19, E23, E27, and E31 in order from the downstream side in the recording medium conveyance direction.

  Further, as shown in FIG. 13, each pattern is a pattern that is uniformly filled with a developing toner density corresponding to each energy value of values E3, E7, E11, E15, E19, E23, E27, and E31. is there.

  The halftone density detection pattern 111 formed on the surface of the transport belt 12 is transported by the transport belt 12 and reaches the position of the color misregistration detection sensor 24. The mechanism control unit 53 irradiates the infrared LED 101 of the color misregistration detection sensor 24 with predetermined light emission energy. Infrared light emitted from the infrared LED 101 of the color misregistration detection sensor 24 is reflected on the halftone density detection pattern 111 and the surface of the transport belt 12, and the reflected light is reflected by the phototransistor 102 for receiving specular reflection light, diffusely reflected, and the like. Light is received by the light receiving phototransistor 103.

  When the reflection pattern corresponding to the halftone density detection pattern 111 is derived from the halftone density detection pattern 111 of yellow, magenta, and cyan, the mechanism control unit 53 reads the output voltage of the diffuse reflected light receiving phototransistor 103, When the reflection pattern corresponding to the halftone density detection pattern 111 corresponds to the halftone density detection pattern 111 of black, the output voltage of the phototransistor 102 for receiving the specular reflection light is read.

  The mechanism control unit 53 refers to the voltage-developing toner density conversion coefficient table shown in FIG. 14, and outputs the output voltage and voltage-developing toner density conversion coefficient of the specular reflection light receiving phototransistor 102 and the diffuse reflection light receiving phototransistor 103. With reference to the table, the detected voltage is converted as the actually measured developing toner density based on the conversion formula: developer density value = a (constant) × detected voltage value + b (constant).

  The values of the constant a and the constant b in the conversion formula are selected from the energy level of the halftone density detection pattern 111 and the voltage-developing toner density conversion coefficient table corresponding to each color. The numerical values in the voltage-development toner density conversion coefficient table are the voltages when the halftone density detection pattern 111 is read by the color misregistration detection sensor 24 and the actual values when the halftone density detection pattern 111 is printed on the recording medium. It is a value obtained experimentally from the relationship with the concentration measurement result.

  From the above conversion equation, the mechanism control unit 53 determines that the value D3 is an actual detected developing toner density value corresponding to the values E3, E7, E11, E15, E19, E23, E27, and E31 of each color of yellow, magenta, cyan, and black. ', D7', D11 ', D15', D19 ', D23', D27 ', D31' are calculated. Then, the mechanism control unit 53 uses the detected developing toner density values D3 ′, D7 ′, D11 ′, D15 ′, D19 ′, D23 ′, D27 ′, and D31 ′ as target developing toner density values D3, D7, D11, D15, Values E3 ', E7', E11 ', E15', E19 ', E23', E27 ', E31', which are combinations of energy values to be corrected to D19, D23, D27, D31, are calculated. The method will be described below.

  FIG. 15 shows an approximate target developing toner density curve obtained by connecting values D0 (= 0), D3, D7, D11, D15, D19, D23, D27, and D31, which are target developing toner density values, and a value D0. (= 0), D3 ′, D7 ′, D11 ′, D15 ′, D19 ′, D23 ′, D27 ′, D31 ′ are approximated detected developing toner density curves which are connected by straight lines. Then, the mechanism control unit 53 calculates an energy value for realizing the target development toner density value D3 from the detected development toner density curve. This energy value corresponds to the corrected energy value E3 ′.

  FIG. 16 is a diagram for explaining the energy level correction method in more detail. As shown in FIG. 16, a new energy value for correcting the deviation of the detected developed toner density value from the target developed toner density value is a correction energy value (corresponding to the target developed toner density value (D3)) by the mechanism control unit 53. E3 ′) can be uniquely determined by reading from the detected development toner density curve.

  Since the correction energy value takes a jump value of 256 gradations, the mechanism control unit 53 selects the value E3 ′ closest to the calculated energy level from 256 energy levels.

  In the same manner, the mechanism control unit 53 performs values E7 ', E11', E15 ', E19', E23 ', E27' for realizing the target developing toner density values D7, D11, D15, D19, D23, D27, D31. , E31 'and associate with the corresponding input level. Thus, the correction of the values E3, E7, E11, E15, E19, E23, E27, and E31 for realizing the target developing toner density curve by the mechanism control unit 53 is completed.

  At this time, the value D31 ′ may be different from the value D31. In this case, the mechanism control unit 53 multiplies the target developing toner density value by (D31 ′ / D31) and normalizes the target. Halftone dot density values Dn0, Dn3, Dn7, Dn11, Dn15, Dn19, Dn23, Dn27, and Dn31 are calculated in advance, and a corrected energy value is obtained using the normalized curve as a target developing toner density curve. Therefore, the maximum gradation level E31 is not corrected.

  FIG. 17 summarizes the newly obtained corrected exposure energy values and target development toner density values as a table. The corrected energy value is stored in the energy level storage unit 92. This is used when printing. In image formation based on image data, each LED element of the LED head 3 (3K to 3C) emits light based on the correction energy value, and an electrostatic latent image is formed on the surface of the photosensitive drum.

  Next, the procedure in which the mechanism control unit 53 corrects the remaining energy values from the eight correction energy values and the zero point density value D0 will be described. Since nine points of correction energy values including the zero point and the maximum toner density value have already been obtained, the mechanism control unit 53 obtains the remaining energy values by linear interpolation from the nine point energy levels.

  For example, the values E1 ′ and E2 ′ are the value E1 ′ that has a lower gradation value obtained by dividing the gradation between the values E0 ′ and E3 ′ into three equal parts, and the value E2 ′ that has a higher gradation value. Assume. In this case, the mechanism control unit 53 selects the closest energy from the 256 level energy to the calculated energy level.

  Similarly, regarding the remaining energy values other than the values E3 ′, E7 ′, E11 ′, E15 ′, E19 ′, E23 ′, E27 ′, E31 ′, the mechanism control unit 53 obtains the values by interpolation. The energy value is corrected individually for each LED element from the result of developing toner density detection.

  A detected developed toner density curve with 32 energy levels selected from a combination of 256 levels of energy is corrected to approximate the target developed toner density curve. As a result, even when unevenness occurs in the gradation density of the multi-value dither pattern as shown in FIG. 11, for example, the image forming apparatus 1 can detect appropriately by energy value correction as shown in FIG. It becomes possible to correct the developing toner density curve.

  For the density gradation correction of halftone dots, a detection pattern that is uniformly painted, such as the halftone density detection pattern 111, is ideal. However, as shown in FIG. 18, for example, it is also possible to arrange a predetermined amount of halftone dots of different gradation levels in the pattern.

  According to the experiment, as shown in FIG. 19, in the halftone density detection pattern filled with the same density dots on the entire surface, when the mixing ratio of blank dots becomes about 25% or more, the density detection of the halftone density detection pattern is performed. The concentration drops rapidly. The decrease in detection density is because interference between halftone dots is hindered.

  Therefore, in order to correct the density gradation of halftone dots, the halftone density detection pattern is good if the mixing ratio of blank dots is 25% or less, that is, the same halftone dots occupy 75% or more. is there.

  The same applies to the case where the dots to be mixed are not blank, and the ratio of the halftone dots to be detected in the halftone density detection pattern occupies 75% or more of the range read by the color misregistration detection sensor 24. If there is, it is good.

  As described above, according to the image forming apparatus 1, it is possible to appropriately correct the density of the halftone dots by using the result detected using the halftone density detection pattern uniformly filled with the halftone dots. is there.

  In the image forming apparatus 1, halftone dot density correction can be executed with the LED head mounted on the apparatus. In addition, it is possible to correct variations in halftone dots due to environmental changes, temporal changes, and the like that occur after the image forming apparatus is manufactured. Therefore, by using the image forming apparatus 1 according to the present invention, it is possible to obtain a smooth gradation image even if a variation in halftone dots due to an environmental change, a change with time, or the like occurs.

(Second embodiment)
In addition to the effects of the image forming apparatus 1, the apparatus according to the second embodiment can shorten the time required for correcting the density of the image forming apparatus or reduce the amount of consumed developer. That is, the apparatus of the second embodiment generates not only the halftone density detection pattern but also the number of shortened halftone density detection patterns 121 smaller than the number of halftone density detection patterns as shown in FIG. A determination table for determining whether to execute the halftone dot density correction operation is provided.

  The image forming apparatus also includes a determination table 122 that provides a determination criterion for determining whether or not to execute the halftone dot density correction operation from the detection result of the shortened halftone density detection pattern 121.

  When the mechanism control unit 53 receives a signal for detecting halftone dot density, it prints a shortened halftone density detection pattern 121 shown in FIG. 20 on the surface of the conveyance belt 12. The shortened halftone density detection pattern 121 has a configuration in which three sets of yellow, magenta, cyan, and black are arranged in this order from the downstream side in the recording medium conveyance direction, and each set is E11, in order from the downstream side in the recording medium conveyance direction. It is a pattern corresponding to the energy levels of E23 and E31. Further, as shown in FIG. 20, each pattern is a pattern that is uniformly painted with a developing toner density corresponding to the energy values of E11, E23, and E31.

  The shortened halftone density detection pattern 121 formed on the surface of the transport belt 12 is transported by the transport belt 12 and reaches the position of the color misregistration detection sensor 24. As in the first embodiment, the mechanism control unit 53 drives the color misregistration detection sensor 24 to calculate D11 ′, D23 ′, and D31 ′ that are the densities of the shortened halftone density detection pattern 121.

  Next, the mechanism control unit 53 refers to the determination table 122 shown in FIG. The determination table 122 is information on the maximum allowable error (positive value) between the target development toner density values D11, D23, and D31 and the detected development toner densities D11 ′, D23 ′, and D31 ′ for yellow, magenta, cyan, and black. including. Then, the mechanism control unit 53 calculates the difference between the calculated detected developer density D11 ′, D23 ′, D31 ′ and the target developed toner density D11, D23, D31, respectively. This calculation is performed for all colors of yellow, magenta, cyan, and black.

  The mechanism control unit 53 compares the absolute value of each calculated value with the allowable maximum value of the determination table 122, and if any comparison result exceeds the allowable value, the halftone dot density correction operation Is started immediately.

  On the other hand, when all the calculated values are below the allowable maximum value of the determination table 122, the mechanism control unit 53 determines that the halftone dot density correction operation is unnecessary, and ends the series of operations.

  The shortened halftone density detection pattern 121 is not necessarily a combination of E11, E23, and E31. Any number of patterns may be used as long as it is less than the number of halftone density detection patterns 111 used for halftone dot density correction described in the first embodiment. Further, halftone dot density correction may be executed only for colors whose calculation results exceed the allowable value in the determination result.

  As described above, the image forming apparatus according to the second embodiment has density detection patterns and halftones that are smaller in number than the halftone density detection patterns used in the halftone dot density correction operation shown in the first embodiment. A dot density error allowable range determination table is provided, and determination can be made based on this table.

  Therefore, the image forming apparatus according to the second embodiment executes the halftone dot density correction operation when the deviation of the halftone dot density from the target value is large, so the density of the halftone dots is sufficiently accurate. The level can be kept close to the target value. On the contrary, when the deviation of the halftone dot density from the target value is small, the image forming apparatus does not execute the halftone dot density correction operation, so that it is possible to save the time and the developer required for the halftone dot density correction. it can.

  The present embodiment is a preferred embodiment of the present invention, but the present invention is not limited to this, and each configuration can be changed as appropriate without departing from the spirit of the present invention.

  In the above-described embodiment, the sub-line control is exemplified as the halftone dot correction unit, and the detailed description has been given. However, other LED driving time control or a correction unit based on a current value may be used. In this embodiment, the sub-line control is exemplified by the 8-sub-line control. However, the number of lines may be any number as long as it is 2 lines or more, and is not limited to 8 lines. The halftone dot density correction means may be a combination of exposure time and current, and is not limited to this embodiment.

  In this embodiment, an LED head is used as the electrostatic latent image forming means. However, a laser light source or the like may be used, and the present invention is not limited to this embodiment. In the present embodiment, the medium on which the electrostatic latent image is written is the photosensitive drum. However, the photosensitive belt may be a photosensitive belt in which a photosensitive agent is applied to a belt-like member, and is not limited to the present embodiment.

  In this embodiment, the case where the order of black, yellow, magenta, and cyan from the upstream in the recording medium conveyance direction has been described. However, the image forming apparatus includes a multicolor developer and includes a plurality of image forming units. In this case, the order of arrangement of the image forming units may be upstream from cyan, for example, and is not limited to this embodiment.

  In this embodiment, the description has been given of the case where the number of image forming units is four. However, the number is not limited to the number of image forming units, and the number of image forming units is not limited to four. The present invention is also applicable to an image forming apparatus having only an image forming unit, for example, black.

  Further, the combination and number of energy levels of the halftone dot density detection pattern given in this embodiment are merely examples, and the present invention is not limited to this.

  In this embodiment, an electrophotographic image forming apparatus is used as an example, but an ink jet printer or the like may be used as long as the image forming apparatus generates halftone dots.

1 is a cross-sectional view illustrating a configuration of an image forming apparatus according to the present invention. It is a cross-sectional schematic diagram of a density sensor. 2 is a block diagram of a control circuit of the image forming apparatus according to the present invention. FIG. It is a block diagram of the LED head drive part concerning this invention. It is a figure explaining the gradation dot expression by subline control. It is a figure which shows an example of the relationship between exposure energy and a developing developer density | concentration. It is a figure explaining selection of the energy level which implement | achieves a linear 32 level density gradation. It is a table containing exposure energy values (initial values) and halftone dot density values. It is an example of multi-value dither. It is a figure explaining the multi-value dither gradation characteristic when the density correction of halftone dots is appropriate. It is a figure explaining the multi-value dither gradation characteristic when the density correction of halftone dots is not appropriate. It is a whole figure of a halftone density detection pattern. It is a figure explaining the whole surface uniform filling pattern for each halftone dot density detection. It is a figure explaining an example of a density sensor output voltage-on-paper density conversion coefficient table. It is a figure explaining an approximate halftone dot density curve. It is a figure explaining the energy level correction method. 6 is a table storing corrected exposure energy values and target halftone dot density values. It is a figure explaining the example of a non-uniform halftone dot density detection pattern. It is a figure explaining the relationship between the blank dot mixing ratio of a halftone dot detection pattern, and a density | concentration. It is a whole figure of a shortened halftone density detection pattern. It is an example of a halftone dot density correction execution determination table.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 2k, 2Y, 2M, 2C Image forming part 3k, 3Y, 3M, 3C LED head 4k, 4Y, 4M, 4C Transfer roller 5k, 5Y, 5M, 5C Charging roller 6k, 6Y, 6M, 6C Photosensitive drum 7k, 7Y, 7M, 7C Developing rollers 8k, 8Y, 8M, 8C Developing blades 9k, 9Y, 9M, 9C Sponge rollers 10k, 10Y, 10M, 10C Developer cartridge 12 Conveying belt 13 Drive roller 14 Drive roller 14 Drive roller 15 Adsorption roller 16 Hopping roller 17 Registration roller 18 Pinch roller 19 Recording medium storage cassette 20 Guide 21, 22, 23 Sensor 24 Density sensor 25 Heat roller 26 Pressure roller 27 Discharge sensor 28 Thermistor 29 Guide 30 Stacker 32 Cleaning blade 33 Waste developer tank 34 Environment Temperature sensor 0 Host interface unit 51 Command / image processing unit 52 LED head interface unit 53 Mechanism control unit 54 Hopping motor 55 Registration motor 56 Belt motor 57 Heat roller motor 58 Drum motor 59 Heater 60 High pressure control unit 61 CH generation unit 62 DB generation unit 63 TR generator 80 Storage means 91 LED array driver 92 Energy level storage means 93 LED array 101 Infrared LED
102 Phototransistor 103 for receiving specular reflection light 103 Phototransistor 111 for receiving diffuse reflection light Halftone density detection pattern 121 Abbreviated halftone density detection pattern 122 Determination table

Claims (6)

A correction image pattern generation unit that generates a correction image pattern that is data for forming a substantially uniform developer image formed by pixels within a predetermined density range;
An image forming unit for forming a developer image based on the image pattern for correction and input image data;
An image carrier that carries a developer image based on the correction image pattern formed by the image forming unit;
A density detector for detecting a developer density of a developer image based on the correction image pattern carried by the image carrier;
An image forming apparatus comprising: a density correction unit that corrects the density of a developer image based on the input image information data based on a detection result of the density detection unit.   The image forming apparatus according to claim 1, wherein the developer image based on the correction image pattern is a developer image including 75% or more of pixels within a predetermined density range.   The image forming apparatus according to claim 1, wherein the density correction unit performs density correction of a developer image based on the input image information by adjusting a gradation level of each pixel.   The image forming apparatus according to claim 1, further comprising an image pattern generation unit configured to generate a determination image pattern together with the correction image pattern.   5. The image forming apparatus according to claim 4, wherein the number of density detection pattern combinations used in the determination image pattern is smaller than the number of density detection pattern combinations used in the correction image pattern.   5. The image according to claim 4, further comprising a determination unit that determines whether or not to operate the density correction unit based on a detection result of the developer image based on the determination image pattern by the density detection unit. Forming equipment.