JP2019008092A - Image formation device - Google Patents

Abstract

To enable a setting of proper density by selecting output conversion data to be used for converting a detection voltage of a density sensor into a print density value on the basis of an amount of adjustment of an exposure time.SOLUTION: An image formation device has: a density sensor (136) that detects light reflected in an area of a density detection-purpose pattern serving as a developer image transferred on a transferred member (133) to thereby output a detection voltage corresponding to density of the density detection-purpose pattern; an adjustment unit (14) that adjusts an amount of exposure energy of an exposure unit (190K); and a storage unit (12) that stores a plurality of kinds of output conversion data to be used for converting the detection voltage into a print density value, and a predetermined target density value. Processing control units (10 and 20) are configured to select any output conversion data of the plurality of kinds of output conversion data on the basis of an amount of adjustment of the amount of exposure energy; and conduct processing of converting the detection value into the print density value, using the selected output conversion data.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an image forming apparatus having a function of performing density correction processing.

  Conventionally, density adjustment in an electrophotographic image forming apparatus is performed by means for changing a developing voltage applied to a developing roller and means for changing an exposure light amount (that is, an exposure energy amount) on a photosensitive member. Here, the exposure light quantity is an amount corresponding to an LED driving time (exposure time) in an LED (Light Emitting Diode) head that is an exposure means for exposing the photosensitive member.

  In the density correction processing of the image forming apparatus described in Patent Document 1, (1) the density of a density detection patch, which is a developer image formed on a transfer target member (such as a conveyor belt), is optically detected by a density sensor. And (2) using output conversion data for converting the detection voltage output from the density sensor into a density value on the recording medium (paper), that is, a print density value (also referred to as “on-paper density value”), A print density value corresponding to the detected voltage is acquired, and (3) control is performed to bring the acquired print density value close to the target density value stored in advance in the apparatus. In Patent Document 1, output conversion data corresponds to a relational expression of monotonic increase or monotonic decrease (relational expression in which the detection voltage of the density sensor and the print density value correspond one-to-one), and the physical data of the print engine unit. Even if the development voltage and exposure time, which are characteristics, are changed, it is determined that they do not change.

JP 2008-233369 A (paragraphs 0005, 0057 to 0063, FIGS. 1 and 8)

  However, when the LED drive time (exposure time) in the LED head is changed, the relationship between the detection voltage output from the density sensor and the print density value is one-to-one in preset output conversion data. May deviate from the relationship. As a result, an error occurs in the print density value obtained from the detected voltage using the output conversion data, and the correction is performed so that the print density value including the error approaches the target density value (that is, the development voltage of the print engine unit or the LED driving time). Is changed), the density of the image formed on the recording medium after the density correction process does not become an appropriate density.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an appropriate density for an image formed on a recording medium even when the exposure time by the exposure unit is changed in the density correction process. It is an object of the present invention to provide an image forming apparatus that can be used.

  An image forming apparatus according to an aspect of the present invention includes a photoconductor, a charging unit that charges the surface of the photoconductor, an exposure unit that forms an electrostatic latent image by exposing the charged surface, and the static A developing unit that forms a developer image on the surface by developing an electrostatic latent image, a transfer unit that transfers the developer image onto a transfer member, and the developer that is transferred onto the transfer member A density sensor that outputs a detection voltage corresponding to the density of the density detection pattern by detecting light reflected by a density detection pattern area that is an image, and an adjustment unit that adjusts the exposure energy amount of the exposure unit A storage unit that stores a plurality of types of output conversion data used to convert the detected voltage into a print density value, a predetermined target density value, and a process control unit that controls the operation of the entire apparatus. Have The processing control unit selects one of the plurality of types of output conversion data based on the adjustment amount of the exposure energy amount, and uses the selected output conversion data to print the detection voltage. A process of converting to a density value is performed.

  According to the present invention, since the output conversion data used for converting the detection voltage of the density sensor into the print density value is stored for each exposure time adjustment amount, the exposure time by the exposure unit in the density correction processing. Even if it is changed, the density of the image formed on the recording medium can be corrected to an appropriate density by selecting the output conversion data based on the adjustment amount of the exposure time by the exposure unit. In other words, even when correction processing for changing the exposure time by the exposure unit that exposes the photosensitive member is performed, by using the output conversion data selected based on the adjustment amount of the exposure time, the density correction processing is performed. Accuracy can be improved.

1 is a side view schematically showing an internal configuration of an image forming apparatus according to an embodiment of the present invention. (A) to (c) are cross-sectional views schematically showing the configuration of the concentration sensor. 2 is a block diagram schematically showing a configuration of a control system of the image forming apparatus according to the present embodiment. FIG. FIG. 4 is a diagram illustrating an example of a “target density data table” stored in an image forming apparatus of a comparative example and the image forming apparatus according to the present embodiment. 6 is a diagram illustrating an example of a “sensor detection voltage-density value conversion coefficient table” stored in an image forming apparatus according to a comparative example. FIG. FIG. 6 is a diagram illustrating an example of a “development voltage value adjustment amount table” stored in an image forming apparatus of a comparative example and the image forming apparatus according to the present embodiment. It is a figure which shows an example of the "LED drive time adjustment amount table" stored in the image forming apparatus of a comparative example and the image forming apparatus which concerns on this Embodiment. 6 is a flowchart schematically illustrating density correction processing in an image forming apparatus according to a comparative example. FIG. 6 is a plan view illustrating an example of a density detection pattern (including a plurality of density detection areas) used by the image forming apparatus of the comparative example and the image forming apparatus according to the present embodiment. FIG. 10 is a plan view showing another example of a density detection pattern (including a plurality of density detection areas) used by the image forming apparatus of the comparative example and the image forming apparatus according to the present embodiment. (A) to (c) are plan views showing examples of a dither pattern with a duty of 30%, a duty of 70%, and a duty of 100%. (A) is a figure which shows the relationship (yellow, magenta, cyan developer image) of the sensor detection voltage-printing density value of a comparative example with an approximate straight line (linear function), and (b) is a sensor of a comparative example. It is a figure which shows the relationship (black developer image) of a detection voltage-printing density value with a cubic approximation curve (cubic function). 6 is a diagram illustrating an example of a “sensor detection voltage-density value conversion coefficient table” stored in an image forming apparatus according to a comparative example and the image forming apparatus according to the present embodiment; FIG. (A) shows an example of a “standard target tone characteristic table” stored in the storage unit of the image forming apparatus according to the present embodiment, and (b) shows an example of a “tone correction value table”. FIG. It is explanatory drawing which shows the gradation correction process of the image forming apparatus which concerns on this Embodiment. 3 is a flowchart schematically showing density correction processing of the image forming apparatus according to the present embodiment. It is a figure which shows the relationship of the sensor detection voltage-printing density value in the image forming apparatus which concerns on this Embodiment. 6 is a diagram illustrating an example of a “sensor detection voltage-density value conversion coefficient table” stored in a storage unit of the image forming apparatus according to the present embodiment. FIG. It is a figure which shows the other example of the "sensor detection voltage-concentration value conversion coefficient table" stored in the memory | storage part of the image forming apparatus which concerns on this Embodiment. It is a figure which shows the other example of the "sensor detection voltage-concentration value conversion coefficient table" stored in the memory | storage part of the image forming apparatus which concerns on this Embodiment. It is a side view which shows roughly the internal structure of the other example of the image forming apparatus which concerns on embodiment of this invention.

  Hereinafter, an image forming apparatus according to an embodiment of the present invention will be described with reference to the accompanying drawings. The following embodiments are merely examples, and various modifications can be made within the scope of the present invention.

<< 1 >> Configuration of Embodiment FIG. 1 is a side view schematically showing an internal configuration of an image forming apparatus 1 according to an embodiment of the present invention. As shown in FIG. 1, the image forming apparatus 1 includes image forming units (also referred to as “image drum units”) 110K and 110Y that are four independent printing mechanisms that form a developer image (toner image) by electrophotography. , 110M, 110C are arranged along a conveyance path from the recording medium (paper) P supply side (right side in FIG. 1) to the discharge side (left side in FIG. 1).

  The image forming unit 110K forms a black (K) developer image, the image forming unit 110Y forms a yellow (Y) developer image, and the image forming unit 110M forms a magenta (M) developer image. The image forming unit 110C forms a cyan (C) developer image. The image forming units 110K, 110Y, 110M, and 110C are charged to uniformly charge the surfaces of the photosensitive drums 111K, 111Y, 111M, and 111C, which are photosensitive members as image carriers, and the photosensitive drums 111K, 111Y, 111M, and 111C. And charging rollers 112K, 112Y, 112M, and 112C as sections (charging means). The surfaces of the photosensitive drums 111K, 111Y, 111M, and 111C are exposed by LED heads 190K, 190Y, 190M, and 190C, which are light emitting element heads as exposure units (exposure means), and electrostatic latent images are formed.

  The image forming units 110K, 110Y, 110M, and 110C supply a developer (toner) to the electrostatic latent images formed on the surfaces of the photosensitive drums 111K, 111Y, 111M, and 111C, thereby generating an electrostatic latent image. It has a developing section (developing means) for developing. The developing unit includes developing rollers 113K, 113Y, 113M, and 113C that supply the developer to the photosensitive drums 111K, 111Y, 111M, and 111C, and supply rollers 114K and 114Y that supply the developer to the developing rollers 113K, 113Y, 113M, and 113C. , 114M, 114C and developing blades 115K, 115Y, 115M, 115C for regulating the thickness of the developer layer (toner layer) on the surface of the developing rollers 113K, 113Y, 113M, 113C. Developer cartridges (toner cartridges) 116K, 116Y, 116M, and 116C that respectively store black (K), yellow (Y), magenta (M), and cyan (C) developers are mounted on the developing unit.

  In addition, the image forming units 110K, 110Y, 110M, and 110C include neutralizing units 117K, 117Y, 117M, and 117C that perform static neutralization on the surfaces of the photosensitive drums 111K, 111Y, 111M, and 111C.

  The black (K) developer in the developer cartridge 116K is supplied onto the surface of the developing roller 113K via the supply roller 114K, and the developer on the surface of the developing roller 113K is thinned by the developing blade 115K to be photosensitive. It reaches the contact surface with the drum 111K. The developer is rubbed strongly by the developing roller 113K and the supply roller 114K and is frictionally charged. The developing blade 115K limits the amount of developer conveyed from the developing roller 113K to the photosensitive drum 111K to an appropriate amount. The structure of the developing portion of the yellow (Y), magenta (M), and cyan (C) developer is the same as that of the developing portion of the black (K) developer.

  The LED heads 190K, 190Y, 190M, and 190C are an LED array disposed facing the photosensitive drums 111K, 111Y, 111M, and 111C, a driver IC (Integrated Circuit) that drives the LED array, and a register group that holds data. And a self-occ lens array that is an erecting equal-magnification imaging lens array that condenses the light emitted from the LED array. The LED heads 190K, 190Y, 190M, and 190C change the LED array according to a black image signal, a yellow image signal, a magenta image signal, and a cyan image signal that are image data signals input from the LED head interface unit 32 (FIG. 3). The LED which comprises is light-emitted.

  The surface of the photosensitive drum 111K is exposed by the light emission of the LED head 190K, and an electrostatic latent image is formed on the surface of the photosensitive drum 111K. Black (K) developer on the surface of the developing roller 113K is supplied to the electrostatic latent image on the surface of the photosensitive drum 111K by electrostatic force, and adheres to the surface of the photosensitive drum 111K to form a developer image. The development process using the yellow (Y), magenta (M), and cyan (C) developer is the same as the development process using the black (K) developer.

  Transfer rollers 140K, 140Y, 140M, and 140C as transfer portions (transfer means) are disposed at positions facing the photosensitive drums 111K, 111Y, 111M, and 111C with the conveyance belt (transfer belt) 133 interposed therebetween. The conveyor belt 133 is made of a high resistance semiconductive plastic film formed in a seamless endless shape. The conveyor belt 133 has a glossy surface. The driving roller 131 is connected to a belt motor 53 (FIG. 3), and the belt motor 53 rotates the driving roller 131 in the direction of arrow F. The upper surface of the conveyor belt 133 is stretched between the photosensitive drums 111K, 111Y, 111M, and 111C and the transfer rollers 140K, 140Y, 140M, and 140C.

  As shown in FIG. 1, the image forming apparatus 1 includes a paper feed mechanism as a medium supply unit (paper feed unit) for supplying a recording medium to the conveyance path. The paper feed mechanism includes a paper feed roller 122, a registration roller 124, and a medium storage cassette 121. The recording medium (paper) P stored in the medium storage cassette 121 is taken out one by one by the paper feed roller 122, and the taken out recording medium P is guided by the guide 126 and reaches the registration roller 124. When a skew that is an oblique feed of the recording medium occurs, the skew of the recording medium P is corrected by the pinch roller 125 facing the registration roller 124.

  Sensors 127 and 128 for detecting the position of the recording medium P are disposed in front of and behind the registration roller 124, respectively. A sensor 161 is provided on the downstream side of the conveying belt 133 on the drive roller 131 side to check the recording medium that has failed to be separated from the conveying belt 133 or to detect the trailing end position of the recording medium that has passed. Yes.

  The recording medium separated from the conveyance belt 133 is guided to a fixing unit (fixing mechanism) 150 including a heat roller 151 and a pressure roller 152 pressed against the heat roller 151. The heat roller 151 is driven by the heater motor 54 (FIG. 3), and the pressure roller 152 rotates around the heat roller 151. As shown in FIG. 1, the fixing unit 150 is disposed further downstream in the medium conveyance direction of the sensor 161 provided on the drive roller 131 side of the conveyance belt 133, and heats and pressurizes the developer image on the recording medium. Then, the developer image is fixed on the recording medium. A thermistor 154 is disposed near the surface of the heat roller 151 to monitor the temperature of the heat roller 151. Further, a discharge sensor 162 is provided further downstream of the heat roller 151 to monitor jamming in the fixing unit 150 and winding of the recording medium around the heat roller 151. A guide 163 for conveying the recording medium to the stacker 164 at the upper part of the casing of the image forming apparatus 1 is provided on the downstream side of the discharge sensor 162, and the printed recording medium is discharged to the stacker 164.

  In addition, a cleaning mechanism including a cleaning blade 134 and a waste toner tank 135 is provided on the lower surface of the transport belt 133. The driven roller 132 and the cleaning blade 134 are provided at positions facing each other so as to sandwich the lower surface portion of the conveyor belt 133. The cleaning blade 134 scrapes off the developer remaining on the surface of the upper half of the conveyor belt 133 and stores it in the waste toner tank 135.

  2A to 2C are schematic cross-sectional views showing the configuration of the concentration sensor 136 shown in FIG. As shown in FIG. 1, a density detection unit (density detection unit) that optically detects the density of a developer image is provided below the transport belt 133 at a position facing the transport belt 133 as a transfer target member. Concentration sensor 136 is arranged. The density sensor 136 is a reflection type optical sensor, and measures the intensity of reflected light from the developer image of the density detection pattern (FIGS. 9 and 10) printed on the conveyor belt 133. The image forming apparatus 1 detects the print density value from the detection voltage output from the density sensor 136.

  As shown in FIG. 2A, the density sensor 136 includes an infrared LED 1361 that irradiates infrared light onto a region to which the developer has adhered, and a specular reflected light receiving phototransistor (first photodetector) 1362. And a diffusely reflected light receiving phototransistor (second photodetector) 1363. Both the density of yellow, magenta, and cyan and the density of black can be detected.

  As shown in FIG. 2B, when detecting the density of yellow, magenta, and cyan density detection patterns, the density detection pattern emitted from the infrared LED 1361 and printed on the conveyor belt 133 is used. The light diffused and reflected by the yellow developer, magenta developer, or cyan developer that forms 73 is received by the diffuse reflected light receiving phototransistor 1363, and the diffuse reflected light receiving phototransistor 1363 responds to the amount of light. Generate the detected voltage. The larger the amount of yellow developer, magenta developer, or cyan developer that forms the density detection pattern 73 (that is, the higher the density), the more the diffuse reflection received by the diffuse reflected light receiving phototransistor 1363 is received. The amount of light increases and the detection voltage increases.

  In addition, as shown in FIG. 2C, when performing density detection of a black density detection pattern, a density detection pattern 74 emitted from the infrared LED 1361 and printed on the transport belt 133 is formed. The specularly reflected light by the conveyor belt 133 is received by the specular reflected light receiving phototransistor 1362 through the black developer, and the specular reflected light receiving phototransistor 1362 generates a detection voltage corresponding to the amount of light. The black developer that forms the density detection pattern 74 absorbs outgoing light from the infrared LED 1361. Therefore, the smaller the black developer (that is, the lower the density), the light is received by the specular reflection light receiving phototransistor 1362. The specular reflection light by the conveyor belt 133 increases, and the more black developer (that is, the higher the density), the less the specular reflection light. Therefore, in order to increase the detection accuracy of the specular reflection light, the functions of the transport belt 133 are required to be sufficiently glossy and have a high specular reflectivity and to have a uniform specular reflectivity without unevenness.

  A cover 137 for the density sensor 136 is disposed between the density sensor 136 and the conveyor belt 133. The cover 137 is on the density sensor 136 except during the density correction processing operation, and covers the density sensor 136 with a developer, paper dust, and the like so as not to get dirty. During the density correction processing operation, it moves from above the density sensor 136 by a drive mechanism (not shown). Further, since the cover 137 is used as a reference reflector for adjusting the light emission current of an infrared LED 1361 of the density sensor 136 described later, it has a diffuse reflection characteristic that is a predetermined reference.

  FIG. 3 is a block diagram schematically showing the configuration of the control system of the image forming apparatus 1 according to the present embodiment. In FIG. 3, a host interface unit 31 is a part responsible for a physical layer interface with a host computer as a host device, and has a connector and a communication chip.

  The command / image processing unit 20 is a part that performs processing for interpreting commands from the host computer side, processing for developing image data into a bitmap, and the like. The command / image processing unit 20 includes, for example, a microprocessor, a RAM (Random Access Memory), and hardware for development.

  The LED head interface unit 32 includes a semi-custom LSI (Large-Scale Integrated circuit), a RAM, and the like, and converts the image data developed into a bitmap by the command / image processing unit 20 into LED heads 190K, 190Y, 190M, and 190C. Process the data into a format compatible with the interface.

  The mechanism control unit 10 receives input from each sensor in accordance with a command from the command / image processing unit 20, and controls driving of the hopping motor 51, registration motor 52, belt motor 53, heater motor 54, drum motor 55, heater Drive control of 153, control of the high pressure control unit 41, control of the mechanism unit of the printing system, and control of the high pressure control unit 41 are performed. A hopping motor 51, a registration motor 52, a belt motor 53, a heater motor 54, and a drum motor 55 operate a paper feed roller 122, a registration roller 124, a driving roller 131, a heat roller 151, and photosensitive drums 111K, 111Y, 111M, and 111C. . The mechanism control unit 10 has a driver for driving the hopping motor 51, registration motor 52, belt motor 53, heater motor 54, and drum motor 55.

  The command / image processing unit 20 and the mechanism control unit 10 constitute a processing control unit that is a control unit that controls the overall operation of the image forming apparatus 1. The whole or a part of the processing control unit may include a processor as an information processing unit and a memory as a storage unit that stores a program executed by the processor. In this case, the control operation of the processing control unit is performed by the processor executing the program or by a combination of the execution of the program and the operation of the control circuit. The mechanism control unit 10 of the processing control unit includes an adjustment unit 14 as an exposure energy amount adjustment unit that adjusts the exposure energy amount (the amount corresponding to the LED driving time) of the LED of the LED head that is the exposure unit.

  The heater 153 is, for example, a halogen lamp disposed in the heat roller 151. A thermistor 154 is disposed on the heat roller 151, and the driving of the halogen lamp is controlled based on the detection value of the thermistor 154, so that the temperature of the heat roller 151 is controlled to an appropriate value.

  The density correction control unit 11 of the mechanism control unit 10 determines whether or not to perform density correction processing based on preset execution conditions for density correction processing such as when the power is turned on and every predetermined number of sheets are printed. The density correction control unit 11 of the mechanism control unit 10 causes the development voltage and the LED to make the print density value (on-paper density value) converted from the detection voltage output from the density sensor 136 close to the target density value. The amount of light emission (drive time) of the heads 190K, 190Y, 190M, and 190C should be increased or decreased, that is, adjustment amounts (for example, development voltage value adjustment amount, LED drive time adjustment amount) are calculated.

  In the storage unit 12, various control parameters used for the density correction process include a “target density value data table” 61 shown in FIG. 4, a “sensor detection voltage-density value conversion coefficient table” 62 shown in FIG. The “development voltage value adjustment amount table” 63 shown in FIG. 7, the “LED drive time adjustment amount table” 64 shown in FIG. 7, and the density detection pattern 71 (FIG. 9) used for density correction processing are stored in advance.

  The high voltage control unit 41 includes a microprocessor or a custom LSI, and controls generation of charging voltage, developing voltage, supply voltage, and transfer voltage for the image forming units 110K, 110Y, 110M, and 110C.

  The charging voltage generator 42 generates and stops the charging voltage in the image forming units 110K, 110Y, 110M, and 110C. The development voltage generator 43 generates and stops the development voltage in the image forming units 110K, 110Y, 110M, and 110C. The supply voltage generation unit 44 generates and stops supply voltages in the image forming units 110K, 110Y, 110M, and 110C. The transfer voltage generator 45 generates and stops transfer voltages in the transfer rollers 140K, 140Y, 140M, and 140C.

<< 2 >> Printing Operation of Embodiment First, the printing operation will be described. When the command / image processing unit 20 receives image data output from the host computer, which is an external device, via the host interface unit 31, the command / image processing unit 20 instructs the mechanism control unit 10 to warm up the heater 153 of the heat roller 151. And bitmap data corresponding to each color is generated by performing image data expansion processing. The mechanism control unit 10 that has received an instruction to warm up the heater 153 from the command / image processing unit 20 drives the heater motor 54, drives the heat roller 151, and controls the lighting of the halogen lamp, thereby controlling the heat roller 151. Adjust the fixing temperature.

  When the image data of each color for one page to be printed on the recording medium is stored in the storage unit 22 which is the memory of the command / image processing unit 20 and the fixing temperature of the heat roller 151 reaches the set temperature, the print preparation is completed. Complete. When the printable conditions are met, the command / image processing unit 20 issues a print start command to the mechanism control unit 10.

  The mechanism control unit 10 drives the belt motor 53 to rotate the driving roller 131 to move the conveyance belt 133 and drives the drum motor 55 to operate the image forming units 110K, 110Y, 110M, and 110C. Let Thereafter, the mechanism control unit 10 drives the hopping motor 51 to rotate the paper feed roller 122 and sends only one recording medium in the medium storage cassette 121 to the guide 126. When the leading edge of the recording medium reaches between the registration roller 124 and the pinch roller 125, the hopping motor stops.

  Next, the mechanism control unit 10 drives the registration motor 52 to rotate the registration roller 124 and rotate the heat roller 151. Further, when the registration roller 124 rotates, the recording medium is transported by the transport belt 133.

  At this time, an image forming operation is performed in the image forming units 110K, 110Y, 110M, and 110C. First, the mechanism control unit 10 controls the high voltage control unit 41 and the charging voltage generation unit 42 to apply a voltage to the charging rollers 112K, 112Y, 112M, and 112C, so that the surfaces of the photosensitive drums 111K, 111Y, 111M, and 111C are applied. Charge uniformly.

  Next, the command / image processing unit 20 transmits image data for one line to the LED head interface unit 32 from the storage unit 22 which is a memory storing image data. The LED head interface unit 32 converts the received data into a format that can be transmitted to the LED head, and transmits the data to the LED heads 190K, 190Y, 190M, and 190C. The LED heads 190K, 190Y, 190M, and 190C turn on the LEDs corresponding to the received image data, and the electrostatic latent image for one line corresponding to the image data on the surface of the charged photosensitive drums 111K, 111Y, 111M, and 111C. Form. In this way, one line at a time is placed on the surface of the photosensitive drums 111K, 111Y, 111M, and 111C sequentially until the length of one page is reached based on the image data sent from the storage unit 22 for each line. An electrostatic latent image is formed. The electrostatic latent images formed on the photosensitive drums 111K, 111Y, 111M, and 111C reach the contact portions of the developing rollers 113K, 113Y, 113M, and 113C as the photosensitive drums 111K, 111Y, 111M, and 111C rotate.

  Negative voltages are applied to the developing rollers 113K, 113Y, 113M, and 113C and the supply rollers 114K, 114Y, 114M, and 114C. As a result, the developer supplied from the developer cartridges 116K, 116Y, 116M, and 116C is frictionally charged to a negative polarity by the developing rollers 113K, 113Y, 113M, and 113C and the supply rollers 114K, 114Y, 114M, and 114C. The developer charged to negative polarity adheres to the developing rollers 113K, 113Y, 113M, and 113C due to the potential difference between the developing rollers 113K, 113Y, 113M, and 113C and the supply rollers 114K, 114Y, 114M, and 114C. The attached developer is made uniform by the developing blades 115K, 115Y, 115M, and 115C to form a developer layer. The developer layer formed on the developing rollers 113K, 113Y, 113M, and 113C is conveyed to contact portions with the photosensitive drums 111K, 111Y, 111M, and 111C by the rotation of the developing rollers 113K, 113Y, 113M, and 113C. In the exposure area on the photosensitive drums 111K, 111Y, 111M, and 111C between the developing rollers 113K, 113Y, 113M, and 113C and the photosensitive drums 111K, 111Y, 111M, and 111C, the developing rollers from the photosensitive drums 111K, 111Y, 111M, and 111C Electric fields in the direction toward 113K, 113Y, 113M, and 113C are formed. Therefore, on the surface of the photosensitive drums 111K, 111Y, 111M, and 111C, the developer charged to the negative polarity on the developing rollers 113K, 113Y, 113M, and 113C adheres to the exposure area, and a developer image is formed.

  Print density adjustment processing in a density correction operation described later is performed in this image forming operation. First, when the mechanism controller 10 sends an instruction to increase or decrease the voltage applied to the developing rollers 113K, 113Y, 113M, and 113C to the developing voltage generator 43, the developer layer that adheres to the developing rollers 113K, 113Y, 113M, and 113C. Since the thickness of the ink changes, the print density is adjusted. Further, the print density in the halftone is adjusted by the LED lighting time (LED driving time) of the LED head. When the mechanism control unit 10 sends an instruction to increase / decrease the LED drive time of the LED heads 190K, 190Y, 190M, and 190C to the LED head interface unit 32, the dots exposed on the photosensitive drums 111K, 111Y, 111M, and 111C Therefore, the halftone print density is adjusted. In other words, the print density is adjusted by changing the exposure light amount (exposure energy amount) of light applied to the surfaces of the photosensitive drums 111K, 111Y, 111M, and 111C by the LED heads 190K, 190Y, 190M, and 190C. The amount of exposure light is an amount proportional to the driving time of the LED head, that is, the LED lighting time per dot.

  Returning to the description of the printing operation, when the leading end of the recording medium reaches between the photosensitive drum 111K and the transfer roller 140K, the mechanism control unit 10 issues a command to the high-pressure control unit 41 and corresponds to the transfer roller 140K. Turn on the transfer power supply. As a result, the developer image on the surface of the photosensitive drum 111K is electrically transferred onto the recording medium by the transfer roller 140K. By the rotation of the photosensitive drum 111K, the developer images are sequentially transferred onto the recording medium, and an image for one page is transferred onto the recording medium.

  The conveyor belt 133 continuously moves, and the recording medium advances in order from the image forming unit 110K to the image forming units 110Y, 110M, and 110C, and the developer images are sequentially transferred by the image forming units 110Y, 110M, and 110C. . As described above, the four color developer images are transferred onto the recording medium so as to form a color developer image. Thereafter, the recording medium is guided to the fixing unit 150 by the conveyance belt 133.

  When the recording medium reaches the fixing unit 150, the developer image is fixed to the recording medium by the heat roller 151 that has already reached a fixing temperature and the pressure roller 152 that is in pressure contact with the heat roller 151. When the fixing is completed, the recording medium is discharged to the stacker 164. The mechanism control unit 10 can know the completion of discharge when the discharge sensor 162 detects the trailing edge of the recording medium.

  When the discharging is completed, the mechanism control unit 10 stops all driving of the motor. In the image forming units 110K, 110Y, 110M, and 110C, when the transfer of the developer image to the recording medium is completed, the transfer power supply is turned off, and the charging power supply and DB (development bias) power supply are photosensitive. Turns off when drum rotation stops. As described above, a color image is recorded on the recording medium fed out from the paper feeding mechanism.

<< 3 >> Density Correction Processing of Comparative Example Next, density correction processing in the (ordinary) image forming apparatus of the comparative example will be described with reference to FIGS. The image forming apparatus of the comparative example is different from the image forming apparatus 1 of the embodiment with respect to the processing content performed by the processing control unit, but is the same as the image forming apparatus 1 of the embodiment in other respects.

FIG. 8 is a flowchart showing density correction processing in the image forming apparatus of the comparative example. As shown in FIG. 8, in the image forming apparatus of the comparative example, in step S1, the density correction control unit 11 of the mechanism control unit 10 sets the development voltage value to a predetermined initial value DB ₀ [V]. The LED driving time (exposure time) is set to a predetermined initial value DK ₀ [s]. At this time, the correction value of the development voltage value (development voltage value adjustment amount) is 0, and the exposure light amount correction value (LED drive time adjustment amount) is 0.

  In the next step S2, the density correction control unit 11 of the mechanism control unit 10 performs a data table as output conversion data stored in the storage unit 12, that is, a “target density value data table” 61 (FIG. 4), “ The “sensor detection voltage-concentration value conversion coefficient table” 62 (FIG. 5), “development voltage value adjustment amount table” 63 (FIG. 6), and “LED drive time adjustment amount table” 64 (FIG. 7) are read.

  In the next step S3, when the density correction control unit 11 of the mechanism control unit 10 receives the density detection execution instruction, the density detection pattern 71 (see FIG. 5) based on the density detection pattern information stored in the storage unit 12 in advance. 9) Printing on the conveyor belt 133 is started.

  The density detection pattern 71 includes a dither pattern in which the fill duty (also referred to as “duty printing rate” or “print duty”) is Duty 30%, Duty 70%, and Duty 100%. The density detection pattern 71 includes black (K) duty 30%, yellow (Y) duty 30%, magenta (M) duty 30%, cyan (C) duty 30%, black duty 70%, yellow duty 70%, and magenta duty 70 from the downstream side in the transport direction. %, Cyan duty 70%, black duty 100%, yellow duty 100%, magenta duty 100%, cyan duty 100%. In each set, the developer development area ratio (the ratio of the developer developed on the conveyance belt 133 in a predetermined area from the downstream side in the conveyance direction, which is expressed as “Duty”) is Duty 30%. , Duty 70% and Duty 100%. In other words, 100% printing is the area ratio when the entire surface is printed in the printable range of the recording medium is called 100% duty, and printing corresponding to an area of 1% with respect to the 100% duty is called 1% duty.

  The dot arrangement of the dither pattern is as shown in FIGS. 11A to 11C, and is a 45 ° halftone dither. Further, each pattern shown in FIG. 9 has a length Lp [mm], and is printed so as to be arranged in the transport direction without leaving an interval. The pattern used for density detection is not limited to the pattern shown in FIG. 9, and the dither pattern type and configuration, the duty combination, the color arrangement order, the length and interval of each pattern, etc. are required. It can be changed according to the conditions.

  The mechanism control unit 10 causes the infrared LED 1361 of the density sensor 136 to emit light according to the color of the density detection pattern, and irradiates the density detection pattern 71 with infrared light. The specular reflection light receiving phototransistor 1362 and the diffuse reflection light receiving phototransistor 1363 are driven by a drive circuit, and flow a current corresponding to (for example, proportional to) the light reception energy. This current is converted into a voltage by a conversion circuit, and this voltage is read by the mechanism control unit 10 as a detection voltage. When the pattern to be detected is a yellow, magenta, or cyan pattern, the mechanism control unit 10 reads the detection voltage of the diffuse reflected light receiving phototransistor 1363. When the detection target pattern is a black pattern, the mechanism control unit 10 reads the detection voltage of the specular reflection light receiving phototransistor 1362.

  Since the pattern detected first is a black (K) duty 30% pattern, the detection voltage of the specular reflection light receiving phototransistor 1362 is read. Next, the conveyance belt 133 is driven to move the density detection pattern length Lp [mm], so that the center of the yellow (Y) duty pattern and the detection position of the density sensor 136 are aligned, and the diffuse reflected light receiving photo is obtained. The detection voltage of the transistor 1363 is read. Thereafter, similarly, the detection voltages are sequentially read from all the 12 density detection regions of the density detection pattern 71 shown in FIG.

  The mechanism control unit 10 converts the read detection voltage into a density value using a “sensor detection voltage-concentration value conversion coefficient table” 62 shown in FIG. The table value of the “sensor detection voltage-concentration value conversion coefficient table” 62 is obtained by preliminarily calculating coefficient A and coefficient B, which are coefficients of a primary approximation expression indicating the relationship between the sensor detection voltage and the density value shown in FIG. This is the optimum value obtained experimentally.

  Hereinafter, the process of calculating the density value of black (K) will be described. The process of calculating the density values of yellow (Y), magenta (M), and cyan (C) is the same as that for black (K).

The sensor detection voltage when the density detection pattern 71 to be detected is a density detection pattern of Duty 30% is expressed as KV _30, and the density value at that time is expressed as KOD ₃₀ . A sensor detection voltage when the density detection pattern 71 to be detected is a density detection pattern of Duty 70% is expressed as KV _70, and the density value at that time is expressed as KOD ₇₀ . The sensor detection voltage when the density detection pattern 71 to be detected is a density detection pattern with a duty of 100% is expressed as KV _100, and the density value at that time is expressed as KOD ₁₀₀ . At this time, the density value _{KOD 30, KOD 70, KOD 100} is, _K 30 is a coefficient A as shown in FIG. _{5 (A), K 70 (} A), the coefficient _{K 30} is a coefficient B and _{K 100} (A) ( It is calculated by the following formulas (1) to (3) using B), K ₇₀ (B), K ₁₀₀ (B).

In the next step S4, the mechanism controller 10 determines the density values KOD ₃₀ , KOD ₇₀ , and KOD ₁₀₀ acquired in step S3, and the target density value KOD _T30 , “target density value data table” 61 shown in FIG. Compare KOD _T70 and KOD _T100 . The mechanism control unit 10 determines the voltage adjustment amount (the voltage addition amount to be added to the development voltage value or the development from the difference (KOD ₃₀ -KOD _T30 ), (KOD ₇₀ -KOD _T70 ), and (KOD ₁₀₀ -KOD _T100 )). The voltage subtraction amount to be subtracted from the voltage value is calculated. For this calculation, the table value of the “development voltage value adjustment amount table” 63 stored in the storage unit 12 shown in FIG. 6 is used. The table values ΔKDB ₃₀ , ΔKDB ₇₀ , ΔKDB ₁₀₀ ,... In the “development voltage value adjustment amount table” 63 are change amounts of density values when the development voltage value changes by 1 [V]. When the development voltage value is changed, the layer thickness of the developer image formed by development can be changed. Utilizing this property, it is possible to increase / decrease the density from the low duty area where the duty is low to the high duty area where the duty is high.

  In the above processing, the detected voltage value adjustment amount (development voltage correction value) is calculated for three types of duty for each color. For example, for black, the detection voltage value adjustment amount for the following three types of duty is calculated.

  However, since the detection voltage value adjustment amount is determined not for each duty but for each color, the detection voltage value adjustment amount KDB (A), which is the average value of the detection voltage value adjustment amounts for the three types of duties, is (4). However, a detection voltage value adjustment amount other than the average value (for example, another representative value such as a weighted average value or an intermediate value of one value) may be used.

  The detection voltage value adjustment amount YDB (A) for yellow, the detection voltage value adjustment amount MDB (A) for magenta, and the detection voltage value adjustment amount CDB (A) for cyan are the same as the detection voltage value adjustment amount KDB (A) for black. It is calculated by the method of Further, the detection voltage value adjustment amounts KDB (A), YDB (A), MDB (A), and CDB (A) for each color are also expressed as DB (A).

The mechanism control unit 10 instructs the high voltage control unit 41 to increase or decrease the development voltage value from the development voltage value adjustment amount DB (A) for each color obtained in step S4. The developing voltage generation unit 43 supplies the developing voltage value DB ₁ [V] obtained by adding the developing voltage value adjustment amount DB (A) to the developing voltage initial value DB ₀ during the printing operation to the image forming units 110K, 110Y, 110M, and 110C. Supply. The development voltage value after the development voltage correction is obtained by the following equation (5).

  In the next step S5, as in step S3, when the mechanism control unit 10 receives an instruction to execute density detection, the density detection pattern 71 is printed on the transport belt 133, and the detection voltage of each color pattern is generated by the density sensor 136. The reading / mechanism control unit 10 converts the detection voltage into a density value by using the coefficient of the “sensor detection voltage-concentration value conversion coefficient table” 62 shown in FIG.

The sensor detection voltage when the density detection pattern 71 to be detected is a density detection pattern Duty30% _'is denoted by _30, the density value of that time _KOD' KV denoted _30. The concentration detection pattern 71 to be detected _'is denoted by _70, the density value of that time _KOD' KV sensor detection voltage when a a duty of 70% of the density detection pattern is denoted by _70. Further, the sensor detection voltage when the density detection pattern 71 to be detected is a density detection pattern of duty of 100% _'is denoted by _100, the density value of that time _{KOD' KV} denoted _100. At this time, the density values KOD ′ ₃₀ , KOD ′ ₇₀ , and KOD ′ ₁₀₀ are the coefficients A ₃₀ , K ₇₀ (A), and K ₁₀₀ (A) that are the coefficients A shown in FIG. K _{30 (B),} is calculated by K 70 _(B), the following equation using _{K 100} a (B) (6) ~ ( 8).

In the next step S6, the mechanism controller 10 determines the density values KOD ′ ₃₀ , KOD ′ ₇₀ , KOD ′ ₁₀₀ acquired in step S5, and the target print density in the “target density value data table” 61 shown in FIG. KOD _T30 , KOD _T70 , and KOD _T100 are compared. The mechanism control unit 10 determines each of the LED heads 190K, 190Y, 190M, and 190C for each color from these differences (KOD ′ ₃₀ −KOD _T30 ), (KOD ′ ₇₀ −KOD _T70 ), and (KOD ′ ₁₀₀ −KOD _T100 ). How much the LED drive time should be increased or decreased, that is, the LED drive time adjustment amount is calculated.

For this calculation, an “LED drive time adjustment amount table” 64 stored in the storage unit 12 shown in FIG. 7 is used. The table values ΔKDK ₃₀ , ΔKDK ₇₀ , ΔKDK ₁₀₀ ,... In the “LED driving time adjustment amount table” 64 are the amount of change in density value when the LED driving time changes by 1 [%]. When the LED driving time is changed, the density from the low duty portion where the duty is low to the high duty portion where the duty is high can be increased or decreased.

  The LED drive time adjustment amount (LED drive time correction value) is calculated for three types of duty for each color, but only one LED drive time adjustment amount for each color is determined for each color, not for each duty. For this reason, for example, the average value of the three calculated values can be determined as the LED drive time adjustment amount KDK (A), and can be obtained by the following equation (9).

  In the above processing, the LED drive time adjustment amount is calculated for each of the three types of duty for each color. For example, for black, the LED driving time adjustment amount in the following three types of duty is calculated.

  However, since the LED drive time adjustment amount is determined not for each duty but for each color, the LED drive time adjustment amount KDK (A), which is the average value of the LED drive time adjustment amounts for the three types of duty, is as follows. (9) However, an LED driving time adjustment amount other than the average value (for example, another representative value such as a weighted average value or an intermediate value of one value) may be used.

  The LED drive time adjustment amount YDK (A) for yellow, the LED drive time adjustment amount MDK (A) for magenta, and the LED drive time adjustment amount CDK (A) for cyan are the same as the LED drive time adjustment amount KDK (A) for black. It is calculated by the method of Further, the LED driving time adjustment amounts KDK (A), YDK (A), MDK (A), and CDK (A) for each color are also expressed as DK (A).

The mechanism control unit 10 instructs the LED head interface unit 32 to increase or decrease the driving time of each LED head 190K, 190Y, 190M, 190C from the LED driving time adjustment amount DK (A) obtained in step S6. LED head interface unit 32, LED driving time to the initial value DK ₀ of LED driving time during the printing operation the adjustment amount DK (A) the LED driving time for each LED heads 190K plus, 190Y, 190M, exposing the 190C. When the LED drive time adjustment amount DK (A) indicates the increase or decrease by an increase rate (for example, + 30%) or a decrease rate (for example, −30%), the LED drive time is corrected. The LED driving time DK ₁ [s] is obtained by the following equation (10).

  In the density correction process, the print density is stabilized by adjusting the development voltage and the LED driving time, which are physical characteristics of the print engine unit of the image forming apparatus 1, by the series of processes as described above.

In the next step S <b> 7, when the mechanism control unit 10 receives a density detection execution instruction, the mechanism control unit 10 starts printing on the transport belt 133 of the density detection pattern 72 shown in FIG. 10 stored in advance in the storage unit 12. To do. At this time, the density detection pattern 72 has the development voltage value DB ₁ [V] and the LED driving time DK ₁ [s] determined in Step S4 (Expression (5)) and Step S6 (Expression (10)). Generated.

  Six sets of density detection patterns 72 are arranged in the order of black, yellow, magenta, and cyan from the downstream side in the transport direction, and each set is Duty 15%, Duty 30%, Duty 50%, Duty 70%, and Duty 85 from the downstream side in the transport direction. %, Duty 100%.

  As will be described later, since the density value between each duty is approximately obtained from the print density data, the accuracy increases as the number of samples increases. Therefore, six combinations of Duty, that is, Duty 15%, Duty 30%, Duty 50%, Duty 70%, Duty 85%, and Duty 100% were set. The density detection pattern is not limited to the pattern shown in FIG. 10, and the color arrangement order and the duty combination may be changed according to various conditions required for the apparatus.

As in step S5, the mechanism control unit 10 detects the density detection pattern 72 printed on the conveyor belt 133 by the density sensor 136, reads the detection voltage of the density detection pattern, and the mechanism control unit 10 reads the detected voltage. The detected voltage is converted into a concentration value from the “sensor detection voltage-concentration value conversion coefficient table” 62. The sensor detection voltages of the patterns of the density detection pattern 72 to be detected are Duty 15%, Duty 30%, Duty 50%, Duty 70%, Duty 85%, Duty 100%, respectively, KV ″ ₁₅ , KV ″ ₃₀ , KV ″ ₅₀ , KV ″ ₇₀ , If KV " ₈₅ , KV" ₁₀₀ and the density values are KOD " ₁₅ , KOD" ₃₀ , KOD " ₅₀ , KOD" ₇₀ , KOD " ₈₅ , KOD" ₁₀₀ , respectively, the density values KOD " ₁₅ , KOD" ₃₀ , KOD “ ₅₀ , KOD” ₇₀ , KOD ” ₈₅ , and KOD” ₁₀₀ are calculated by the following equations (11) to (16).

  In some cases, the sensor detection voltage-concentration value correlation is approximated by a cubic equation. For example, the relationship between the sensor detection voltage value and the density value of the specular reflection light of the black developer may be obtained in a form as shown in FIG. As described above, when the approximate line cannot be approximated with sufficient accuracy, a quadratic or cubic approximate curve is used instead of the approximate line. In this case, the calculation of the density value uses the “sensor detection voltage-concentration value conversion coefficient table” 62a shown in FIG. 13 instead of the “sensor detection voltage-concentration value conversion coefficient table” 62 shown in FIG. To do. In this case, the above formulas (11) to (16) are replaced with the following formulas (17) to (22). Note that some of the coefficients shown in FIG. 13 may be zero.

In the next step S8, the gradation correction control unit 21 of the command / image processing unit 20 detects the print density data KOD " ₁₅ , KOD" ₃₀ , KOD " ₅₀ , KOD" ₇₀ , KOD " ₈₅ , KOD" _{100 to be} detected. The tone is corrected based on the received density value so that printing can be performed with the specified color density, so that the print density is stabilized and the color can be faithfully reproduced. The gradation correction is executed by the command / image processing unit 20 and changes the density by increasing or decreasing the developer development area ratio (Duty) described above, that is, the number of dots in the dither pattern.

  The gradation correction control unit 21 approximately calculates a density value corresponding to 256 gradation levels from the received print density data. For example, when expressing Duty 0% to 100% with 256 gradation levels from 0 to 255, Duty 15% is 38 gradation levels, Duty 30% is 76 gradation levels, Duty 50% is 128 gradation levels, Duty 70% is The 179 gradation level, Duty 85% is the 217 gradation level, and Duty 100% is the 255 gradation level.

  The storage unit 12 stores a “standard target gradation characteristic table” 81. FIG. 14A is a diagram showing a “standard target gradation characteristic table” 81, and the table values of the “standard target gradation characteristic table” 81 are experimentally preliminarily so that an ideal continuous gradation can be reproduced. It has been demanded.

  Next, the gradation correction control unit 21 compares the print density characteristic with the standard target gradation characteristic. At this time, if the print density characteristic and the standard target tone characteristic match, an ideal continuous tone can be reproduced. However, in actuality, as shown in FIG. Deviation may occur. As can be seen from FIG. 15, for example, the density values in the print density characteristic and the standard target gradation characteristic at the gradation level 179 are 1.20 in the print density characteristic and 1.05 in the standard target gradation characteristic.

  The density value 1.05 of the standard target gradation characteristic corresponds to the gradation level 148 of the standard target gradation characteristic. Here, the gradation correction control unit 21 stores the output gradation level 148 of the input gradation level 179 of the “gradation correction value table” 82 stored in the storage unit 12 as 148. The “gradation correction value table” 82 is a table for converting an input gradation level into an output gradation level. Similarly, for all 256 gradation levels, the input gradation level and the output gradation level are combined and stored in the “gradation correction value table” 82.

  In the density correction processing, adjustment of physical characteristics (developing voltage, exposure time, etc.) of the engine unit of the image forming apparatus and tone correction by the tone correction control unit 21 of the command / image processing unit 20 are performed by the series of processes as described above. By doing so, the printing density is stabilized.

  The above is the description of the density correction process of the comparative example. Here, a problem in the density correction process of the comparative example will be described. The coefficients in the “sensor detection voltage-concentration value conversion coefficient table” 62 shown in FIG. 5 are values determined experimentally in advance.

  First, as shown in the density detection pattern 72 shown in FIG. 10, the density detection areas of each color (KYMC) of Duty 15%, Duty 30%, Duty 50%, Duty 70%, Duty 85%, and Duty 100% are set as one set, and the development voltage is set. And the LED driving time are fixed, one set is printed on the conveyor belt 133, and the other set is printed on the recording medium under the same conditions.

  The density detection area printed on the conveyance belt 133 is detected by the density sensor 136, the detection voltage is read, and the density detection area printed on the recording medium is measured by a density measuring device (not shown). Thus, an approximate expression can be obtained by associating the sensor detection voltage value with the density value on the recording medium on a one-to-one basis. From this approximate expression, a summary of the coefficients is the “sensor detection voltage-concentration value conversion coefficient table” 62 shown in FIG. Actually, after adjusting the development voltage and the LED driving time to various conditions so that the density value can be more finely covered from the value 0 to a value larger than 1.5, the sensor detection voltage value and the density value on the paper are adjusted. By obtaining a one-to-one correlation between the two by experiments, a more accurate approximate expression is obtained.

  However, in the correlation obtained by such a method, the correlation between the sensor detection voltage value and the on-paper density value may not be determined one-to-one depending on the characteristics of the developer and the difference in the development method. For example, it is assumed that the density value on the paper is 1.0 when the development voltage and the LED driving time are initial values at a duty ratio of 70% for yellow (Y). From here, in order to change the on-paper density value to 1.1, measurement of the density of the density detection area printed on the paper is made when only the development voltage is changed and when only the LED driving time is changed. Even when the values are the same, the detection voltage of the density sensor 136 on the conveyor belt 133 may be different. As a result, the one-to-one relationship between the sensor detection voltage and the concentration value as shown in FIG.

<< 4 >> Density Correction Processing of Embodiment In the image forming apparatus 1 according to this embodiment, the relationship between the sensor detection voltage and the density value is not uniquely determined, but as shown in FIGS. 16 and 17. In addition, an approximate expression is created for each adjustment amount (correction value) of the LED driving time. That is, the “sensor detection voltage-concentration value conversion coefficient table” 91 for each adjustment amount (correction value) of the LED driving time is determined by experiment and stored in the image forming apparatus 1. That is, the processing control unit (10, 20) is one of a plurality of types of output conversion data (for example, three types of output conversion data indicated by three straight lines in FIG. 17) based on the adjustment amount of the exposure energy amount. Output conversion data is selected, and the detected voltage is converted into the print density value by using the selected output conversion data. Another example of a plurality of types of output conversion data is shown in FIG. In FIG. 18, a plurality of types of output conversion data are prepared in accordance with the adjustment amount of the exposure energy amount (correction value of the LED drive time of the LED head) for the area of 70% duty of the black developer.

  FIG. 16 is a flowchart showing density correction processing in the image forming apparatus 1 according to the present embodiment. In FIG. 16, steps S11 to S16 are the same as steps S1 to S6 shown in FIG.

  In step S21, the “sensor detection voltage-concentration value conversion coefficient table” 91 in FIG. 18 corresponding to the value of the LED drive time adjustment amount DK (A) is read. As a result, the conversion formula for converting the sensor detection voltage into the density value is replaced.

  The processing control unit (10, 20) decreases the print density value corresponding to the detection voltage when the adjustment amount of the exposure energy amount is a positive value that increases the exposure energy amount (when the density is reduced in black). Output to select the output conversion data to be output, and to increase the print density value corresponding to the detection voltage when the adjustment amount of the exposure energy amount is a negative value that decreases the exposure energy amount (when increasing the density in black) Select conversion data.

  Further, the processing control unit (10, 20) responds to the detection voltage when the adjustment amount of the exposure energy amount is a positive value that increases the exposure energy amount (in the case of increasing the density in yellow, magenta, and cyan). When the output conversion data that increases the print density value to be selected is selected, and the adjustment amount of the exposure energy amount is a negative value that decreases the exposure energy amount (when the density is reduced in yellow, magenta, and cyan), the detection voltage Output conversion data for decreasing the print density value corresponding to is selected.

In the next step S <b> 17, when the mechanism control unit 10 receives an instruction to execute density detection, the mechanism control unit 10 prints the density detection pattern 72 stored in advance in the storage unit 12 on the conveyance belt 133 shown in FIG. 10. Start. At this time, the density detection pattern 72 is generated under the conditions of the development voltage value DB ₁ [V] and the LED driving time DK ₁ [s] determined in steps S14 and S16.

  As in step S15, the mechanism control unit 10 detects the density detection pattern 72 printed on the transport belt 133 by the density sensor 136, reads the detection voltage of each color pattern, and the mechanism control unit 10 reads the detected detection. The voltage is converted into a print density value using a “sensor detection voltage-density value conversion coefficient table” 62a shown in FIG.

Duty15% of the detection target of the density detection pattern 72, Duty30%, Duty50%, Duty70%, Duty85%, KV respectively sensor detection voltage duty of 100% pattern _{"15, KV" 30, KV} "50, KV" 70, KV _Assuming that “ ₈₅ , KV” ₁₀₀ , the concentration values KOD ” ₁₅ , KOD” ₃₀ , KOD ” ₅₀ , KOD” ₇₀ , KOD ” ₈₅ , KOD” ₁₀₀ are calculated by the following equations (23) to (28). .

  Here, the superscript “X” means the LED driving time adjustment amount (LED driving time correction value) DK (A). Further, in the case where the correlation between the sensor detection voltage and the concentration value is approximated by a cubic equation, the calculation of the concentration value is shown in FIG. 19 instead of the “sensor detection voltage-concentration value conversion coefficient table” 91 shown in FIG. The “sensor detection voltage-concentration value conversion coefficient table” 82 is used. In this case, the above formulas (23) to (28) are replaced with the following formulas (29) to (34).

Another example of the “sensor detection voltage-concentration value conversion coefficient table” 91 is a “sensor detection voltage-concentration value conversion coefficient table” 93 shown in FIG. The “sensor detection voltage-concentration value conversion coefficient table” 93 is used to calculate the concentration value by using a coefficient for the voltage instead of having a sensor detection voltage-concentration value conversion table for each LED driving time adjustment amount. This is considered to depend on the drive time adjustment amount. That is, in the equations (23) to (28), the coefficient K _D (A) and the coefficient K _D (B) are replaced as those depending on the LED driving time adjustment amount. At this time, the coefficient K _D (A) and the coefficient K _D (B) are calculated by the following equations (35) and (36).

  Accordingly, the density value when the “sensor detection voltage-concentration value conversion coefficient table” 92 shown in FIG. 19 is used is obtained by substituting the expressions (35) and (36) into the expressions (23) to (28). Obtained by the following formulas (37) to (42).

  Step S18 in FIG. 16 is the same as step S8 in FIG.

  In the present embodiment, as shown in FIG. 1, the case of the image forming apparatus 1 in which the developer image is directly transferred from the photosensitive drum to the recording medium at the time of printing has been described. However, the density adjustment processing described in this embodiment may be applied to an image forming apparatus provided with an intermediate transfer belt 133a as shown in FIG. In FIG. 21, the same reference numerals as those shown in FIG. 1 are given to the same or corresponding elements as those shown in FIG. In FIG. 21, the image forming apparatus 1a includes a roller 138 that points to the intermediate transfer belt 133a, a point that includes a secondary transfer roller 170, a point that includes discharge rollers 165 to 167, and a cleaning member 118K that cleans the surface of the photosensitive drum. , 118Y, 118M, 118C is different from that shown in FIG. In other respects, the image forming apparatus 1a is the same as the apparatus shown in FIG.

<< 5 >> Effects of the Embodiment As described above, according to the image forming apparatuses 1 and 1a according to the present embodiment, the accuracy when the sensor detection voltage is converted into the density value in step S17 of FIG. Since the decrease can be prevented, the accuracy of gradation correction in step S18 in FIG. 16 is improved.

  Looking at the relationship between the sensor detection voltage and the density value in FIG. 15, the output voltage of the diffuse reflected light receiving phototransistor 1363 on the transfer belt 133 when the print density value is 1.0 in step S18 is the output voltage in step S17. When the LED driving time adjustment amount is determined to be + 30% (positive value), it is 0.94V, whereas when the LED driving time adjustment amount is determined to be -30% (negative value) in step S17. Is 1.12 V, and the detection voltage of the sensor is about 1.20 times due to the difference in the adjustment amount of the LED driving time.

  As described above, since the output voltage of the density sensor 136 varies depending on the LED driving time adjustment amount, a difference in the LED driving time adjustment amount is obtained by having a correction value for the output voltage of the density sensor 136 for each LED driving time adjustment amount. It is possible to prevent a decrease in detection accuracy of the concentration sensor caused by the above. Therefore, in the present embodiment, tone correction is performed with a more accurate detection value, and it becomes possible to improve the stability of print density.

<< 6 >> Usage Form In the above embodiment, the image forming apparatuses 1 and 1a are printers. However, the present invention is a multi-function apparatus having various functions such as a print function, a copy function, and a fax function. The present invention can also be applied to certain image forming apparatuses.

  In the above-described embodiment, the image forming apparatuses 1 and 1a including four image forming units have been described. However, the number of image forming units is not limited to four. The present invention can be applied to an image forming apparatus having two or more image forming units.

  In the above embodiment, the image forming apparatus in which the image forming units 110K, 110Y, 110M, and 110C for black, yellow, magenta, and cyan are arranged in order from the upstream in the conveyance direction of the recording medium has been described. The order of the arrangement is not limited to this example.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 10 Mechanism control part 11 Density correction control part 12 Storage part 14 Adjustment part 20 Command / image processing part 21 Gradation correction control part
110K, 110Y, 110M, 110C Image forming unit (printing mechanism, image drum unit), 111K, 111Y, 111M, 111C Photosensitive drum (photoconductor), 112K, 112Y, 112M, 112C Charging roller (charging unit), 113K, 113Y, 113M, 113C developing roller, 114K, 114Y, 114M, 114C supply roller, 115K, 115Y, 115M, 115C developing blade, 116K, 116Y, 116M, 116C developer cartridge, 117K, 117Y, 117M, 117C discharging unit, 133 conveying belt (Transfer belt), 133a intermediate transfer belt, 136 density sensor, 140K, 140Y, 140M, 140C transfer roller, 150 fixing unit, 190K, 190Y, 190M, 190C LED head (Exposed portion).

Claims (6)

A photoreceptor,
A charging unit for charging the surface of the photoreceptor;
An exposure unit that forms an electrostatic latent image by exposing the charged surface;
A developing unit that forms a developer image on the surface by developing the electrostatic latent image;
A transfer portion for transferring the developer image onto the transfer member;
A density sensor that outputs a detection voltage corresponding to the density of the density detection pattern by detecting light reflected from a density detection pattern area that is the developer image transferred onto the transfer member;
An adjustment unit for adjusting the amount of exposure energy of the exposure unit;
A storage unit for storing a plurality of types of output conversion data used for converting the detected voltage into a print density value, and a predetermined target density value;
A processing control unit for controlling the operation of the entire apparatus;
Have
The processing control unit selects any one of the plurality of types of output conversion data based on the adjustment amount of the exposure energy amount, and uses the selected output conversion data to print the detection voltage. An image forming apparatus characterized by performing a process of converting to a density value. The processing control unit
Based on a first detection voltage output from the density sensor when the first developer image is transferred onto the transfer member and the first developer image is detected by the density sensor. Determine the adjustment amount of the exposure energy amount by the adjustment unit,
After the adjustment of the exposure energy amount, when the second developer image is transferred onto the transfer member and the second developer image is detected by the density sensor, the second output is output from the density sensor. The output conversion data is selected based on the detection voltage of 2, and the process of converting the second detection voltage into the print density value is performed using the selected output conversion data. Item 2. The image forming apparatus according to Item 1. The processing control unit
Converting image data transmitted from the host to exposure data handled by the exposure unit;
The image forming apparatus according to claim 1, wherein gradation correction processing is performed to bring the print density value closer to the target density value. The density detection pattern has a plurality of density detection regions with different print duties for each color developer,
The output conversion data used when performing the process of converting the detection voltage into the print density value includes the output conversion data for each color of the developer and for each print duty. Item 4. The image forming apparatus according to any one of Items 1 to 3. The processing control unit
When the adjustment amount of the exposure energy amount is a positive value that increases the exposure energy amount, select output conversion data that decreases the print density value corresponding to the detection voltage,
The output conversion data for increasing the print density value corresponding to the detection voltage is selected when the adjustment amount of the exposure energy amount is a negative value for decreasing the exposure energy amount. The image forming apparatus according to any one of 1 to 4. The processing control unit
When the adjustment amount of the exposure energy amount is a positive value that increases the exposure energy amount, select output conversion data that increases the print density value corresponding to the detection voltage,
The output conversion data for decreasing the print density value corresponding to the detection voltage is selected when the adjustment amount of the exposure energy amount is a negative value for decreasing the exposure energy amount. The image forming apparatus according to any one of 1 to 4.

Images