JP2024060774A - Image forming device - Google Patents

Abstract

[Problem] When adjusting the print density to a predetermined density by adjusting the development bias voltage, the accuracy of density correction may be deteriorated if the density of the first patch formed is high. [Solution] The device includes a development roller 16, a transfer belt 26 that conveys a recording paper 40, a transfer roller 17 that transfers a toner image formed on a photosensitive drum 13, a density sensor 36 that detects the toner image transferred onto the transfer belt 26, and a mechanism control unit 64 that prints a density correction pattern on the transfer belt 26 with a development voltage initial value YDB'0, and obtains a development voltage value control amount YDB(A) that corrects the development voltage initial value YDB'0 from the detected density of the printed density correction pattern and a stored target density, and the mechanism control unit 64 makes the absolute value of the development voltage initial value YDB'0 smaller than the absolute value of the development voltage value YDB1 corrected by the development voltage value control amount YDB(A). [Selected Figure] Fig. 3

Description

The present invention relates to an image forming apparatus that forms an image on a recording medium using an electrophotographic method, and in particular to density correction processing.

In the electrophotographic method, it is common to execute density correction control each time in order to match the density when printed on paper to a specified density. In density correction control, first, toner is developed on a photosensitive drum using a specified development voltage, the developed toner is transferred to a transfer belt, and the toner density on the transfer belt is detected by a density sensor. In addition, the electrophotographic device stores the sensitivity of the development voltage to the toner density (hereinafter referred to as the development voltage sensitivity), and calculates the amount of correction of the development voltage from the development voltage sensitivity so that the toner density detected on the transfer belt becomes the specified toner density, and feeds this back to the development voltage, thereby bringing the density on paper closer to the target density (see, for example, Patent Document 1).

JP 2021-39161 A (pages 15-19, FIG. 12)

When the density of the first patch formed is high, the accuracy of density correction may be poor (the density after density correction may shift toward the high density side from the target density). This is because the development voltage sensitivity is not always constant. For this reason, if the development voltage correction amount is calculated to approach a specified toner density using the development voltage sensitivity stored in the device, the appropriate correction amount may not be obtained.

The image forming apparatus according to the present invention includes a developing means for developing an electrostatic latent image carried by an image carrier with a developer, a transfer belt facing the image carrier and transporting a medium, a transfer section for transferring the developer image formed on the image carrier, a density detection means for detecting the developer image transferred onto the transfer belt, and a density correction means for printing a density correction pattern of a predetermined duty on the transfer belt with an initial development bias and determining a correction value for correcting the initial development bias from the detected density of the printed density correction pattern and a stored target density, and the density correction means is characterized in that the absolute value of the initial development bias is made smaller than the absolute value of the corrected development bias corrected with the correction value.

Another image forming apparatus according to the present invention includes a developing means for developing an electrostatic latent image carried by an image carrier with a developer, a transfer belt facing the image carrier and transporting a medium, a transfer section for transferring the developer image formed on the image carrier, a density detection means for detecting the developer image transferred onto the transfer belt, and a density correction means for printing a density correction pattern of a predetermined duty on the transfer belt and determining a development bias from the detected density of the printed density correction pattern and a stored target density, and the density correction means prints the density correction pattern multiple times with different densities, compares the detected density of each density correction pattern with the target density, and determines the development bias voltage value when a density detection pattern that is lower than the target density and closest to the target density is printed as the development bias.

According to the present invention, the absolute value of the initial development bias in the density correction process is smaller than the absolute value of the corrected development bias corrected by the correction value, so that the density correction process can be performed appropriately.

1 is a schematic diagram illustrating a configuration of a main part of an image forming apparatus according to a first embodiment of the present invention; 1A is a cross-sectional view showing a schematic configuration of a density sensor, FIG. 1B is a diagram explaining density detection of yellow (Y), magenta (M), and cyan (C), and FIG. 1C is a diagram explaining density detection of black (K). FIG. 2 is a functional block diagram illustrating a configuration of a control system of the image forming apparatus. 5 is a diagram showing an example of a target gradation density value table used by the image forming apparatus; 5 is a diagram illustrating an example of a tone correction value table used by the image forming apparatus. 5 is a diagram illustrating an example of a sensor detection voltage/density value conversion table used by the image forming apparatus. 6 is a diagram illustrating an example of a target print density value data table used by the image forming apparatus. 5 is a diagram showing an example of a development voltage value/density sensitivity table used by the image forming apparatus; 11 is a diagram illustrating an example of an LED drive time/density sensitivity table used by the image forming apparatus. 4 is a flowchart showing the flow of a printing operation of the image forming apparatus. 11 is a flowchart showing a flow of density correction processing as a comparative example of the density correction processing of the image forming apparatus. 2 is a diagram showing an example of a density detection pattern PAT1 used by the image forming apparatus; 11 is a diagram showing an example of a density detection pattern PAT2 used by the image forming apparatus; 13 is a diagram showing an example of a density detection pattern PAT3 used by the image forming apparatus 11. FIG. 5A to 5F are diagrams showing examples of dither patterns P15, P30, P50, P70, P85, and P100 with a duty of 15%, 30%, 50%, 70%, 85%, and 100%, respectively. FIG. 1 is a diagram (graph) for explaining a problem to be solved by the present invention. FIG. 17 is a diagram (grab) for explaining the mechanism of the phenomenon described in FIG. 16 . 4 is a flowchart showing a flow of a density correction process according to the first embodiment of the present invention. 10 is a flowchart showing a flow of a density correction process according to a second embodiment of the present invention. 13 is a flowchart showing a flow of a density correction process according to a third embodiment of the present invention.

Embodiment 1.
FIG. 1 is a schematic diagram showing the main configuration of an image forming apparatus 11 according to a first embodiment of the present invention.

Image forming device 11 has a configuration as, for example, an electrophotographic color printer, and comprises image forming section, of four independent image forming units, image drum units (hereinafter referred to as ID units) 12K, 12C, 12M, 12Y (sometimes simply referred to as ID unit 12 when no particular distinction is required), which are detachably arranged in order from the upstream side along the transport direction (direction of arrow A) of recording paper 40 as a medium. ID unit 12K forms a black (K) image, ID unit 12C forms a cyan (C) image, ID unit 12M forms a magenta (M) image, and ID unit 12Y forms a yellow (Y) image.

In this embodiment, the configurations of these ID units 12K, 12C, 12M, and 12Y are the same, and only the color of the toner they contain is different. Therefore, the internal structure of the black (K) ID unit 12K will be described below as an example.

In the ID unit 12K, there are arranged a photoconductor drum 13K (sometimes simply referred to as photoconductor drum 13 when there is no particular need to distinguish between them) as an image carrier, a charging roller 14K (sometimes simply referred to as charging roller 14 when there is no particular need to distinguish between them) that uniformly charges the surface of the photoconductor drum 13K, a developing roller 16K (sometimes simply referred to as developing roller 16 when there is no particular need to distinguish between them) that adheres black toner (not shown) as a developer to the electrostatic latent image formed on the surface of the photoconductor drum 13K to form a toner image, and a toner supply roller 18K (sometimes simply referred to as toner supply roller 18 when there is no particular need to distinguish between them) that is pressed against the developing roller 16K.

The toner supply roller 18K is a roller that supplies black toner (not shown) contained in a toner cartridge 20K (sometimes simply referred to as toner cartridge 20 when no particular distinction is required) that is detachably attached to the main body of the ID unit 12K to the corresponding developing roller 16K. A developing blade 19K (sometimes simply referred to as developing blade 19 when no particular distinction is required) is pressed against the developing roller 16K. The developing blade 19K forms a thin layer of black toner supplied from the toner supply roller 18K on the developing roller 16K.

The developing roller 16 is a member that carries on its surface toner that develops an electrostatic latent image, and is arranged so as to be in contact with the surface (circumferential surface) of the photosensitive drum 13. This developing roller 16 has, for example, a metal shaft and a semiconductive urethane rubber layer that covers its outer periphery (surface). In addition, such a developing roller 16 is configured to rotate at a predetermined peripheral speed, for example, in the opposite direction to the photosensitive drum 13.

The cleaning blade 27K (sometimes simply referred to as cleaning blade 27 when no particular distinction is required) that is pressed against the surface of the photoreceptor drum 13K scrapes off the black toner (residual toner) remaining on the photoreceptor drum 13K after transfer, which will be described later, and is made of, for example, a flexible rubber or plastic material.

An LED head 15K (sometimes simply referred to as LED head 15 when no particular distinction is required) serving as an exposure means is disposed above photoconductor drum 13K, facing photoconductor drum 13K. Similarly, corresponding LED heads 15C, 15M, and 15Y are disposed on the other photoconductor drums 13C, 13M, and 13Y, respectively. LED head 15 selectively exposes photoconductor drum 13 according to image data of the corresponding color, forming an electrostatic latent image on its surface (surface portion).

The LED head 15 as an exposure head has an LED array, a board on which a drive IC for driving the LED array and a group of registers for holding data are mounted, and an erect life-size imaging lens array (e.g., a SELFOC (registered trademark) lens array) that focuses the light emitted from the LED array. The LED head 15 causes the LED array to emit light in response to image data signals input from a communication unit 61 (Figure 3) as an interface unit. The exposure head is not limited to the LED head 15, and may be a light-emitting thyristor head using a light-emitting thyristor array, or a laser scanning optical system equipped with a semiconductor laser, etc.

Transfer unit 21 is disposed below each photoconductor drum 13 of the four ID units 12 as a transfer section. Transfer unit 21 includes transfer rollers 17K, 17C, 17M, and 17Y (which may simply be referred to as transfer rollers 17 when no distinction is required), transfer belt drive roller 21a, transfer belt driven roller 21b, and transfer belt 26 that is stretched and arranged to be able to run in the direction of arrow A in FIG. 1 by transfer belt drive roller 21a and transfer belt driven roller 21b.

The transfer belt 26 has a glossy surface and is also used as a reference reflector for adjusting the light emission current of the infrared LED 36a of the concentration sensor 36 serving as a concentration detection means described later. A belt thermistor 38 is disposed below the transfer belt 26 to monitor the temperature of the transfer belt 26. Residual toner adhering to the transfer belt 26 is scraped off by a belt cleaning blade 34 and stored in a belt cleaner container 35.

Each transfer roller 17 is arranged in pressure contact with the corresponding photosensitive drum 13 via the transfer belt 26, and in the nip portion formed by the pressure contact, the recording paper 40 transported by the transfer belt 26 is charged to the polarity opposite to that of the toner, and the toner images of each color formed on the corresponding photosensitive drum 13 are sequentially transferred onto the recording paper 40 in a superimposed manner. A transfer discharge sensor 22 is provided downstream of the transfer belt 26 to detect a state in which the recording paper 40 has not been properly separated from the transfer belt 26, or to detect the rear end position of the recording paper 40 that has passed.

A paper feed mechanism for feeding recording paper 40 to the transfer belt 26 is disposed below the transfer unit 21 of the image forming device 11. The paper feed mechanism includes a paper feed tray 24 that stores the recording paper 40, a hopping roller 43 that removes the recording paper 40 from the paper feed tray 24, and a pair of registration rollers 45 that transport the recording paper 40 along the paper guide section 41 and correct skew. Paper detection sensors 44, 49 disposed before and after the pair of registration rollers 45 detect the passage of the paper with or without contact. The detection signal of the paper detection sensor 44 is the basis for the drive timing of the pair of registration rollers 45, and the detection signal of the paper detection sensor 49 is the basis for the light emission timing of the LED head 15 and the timing for applying high voltage to the transfer roller 17.

Furthermore, on the discharge side of the recording paper 40 by the transfer belt 26, a fixing unit 28 is detachably provided on the main body of the image forming device 11. The fixing unit 28 has a heating roller 29 equipped with a heater 31 as an internal heat source, a pressure roller 30 that is biased against the heating roller 29 by a compression spring (not shown), and a fixing thermistor 32 that detects the temperature of the heating roller 29, and fixes the toner image transferred to the recording paper 40 by heat and pressure.

On the discharge side of the fixing unit 28, a fixing discharge sensor 23 that detects the recording paper 40 is provided, and monitors whether a jam occurs in the fixing unit 28 or whether the recording paper 40 wraps around the heating roller 29. On the downstream side of the fixing discharge sensor 23, a paper guide unit 42 and a paper stacker unit 48 are provided.

A density sensor 36 is disposed below the transfer belt 26, facing the transfer belt 26, via a sensor cover 37. The density sensor 36 is used to detect the density of the density detection pattern by measuring the intensity of reflected light of the density detection pattern transferred onto the transfer belt 26, as described below. In addition, the image forming device 11 has an environmental sensor 25 disposed at a position where the temperature and humidity of the outside air can be measured. The environmental sensor 25 includes, for example, a temperature sensor and a humidity sensor.

In FIG. 1, the X, Y, and Z directions are the X direction, which is the transport direction of the recording paper 40 as it passes through the ID units 12K, 12C, 12M, and 12Y, the Y direction, which is the direction of the rotation axis of the photosensitive drum 13, and the Z direction, which is perpendicular to both of these directions. In this example, the Z direction is set to be approximately vertical.

The basic printing operation of the image forming device 11 configured as above will be outlined below. First, the recording paper 40 in the paper feed tray 24 is fed by the hopping roller 43, sent to the registration roller pair 45 where skew is corrected, and then sent to the transfer belt 26, which transports the paper sequentially to the ID units 12K, 12C, 12M, and 12Y as the transfer belt 26 moves. A paper detection sensor 49 is disposed after the registration roller pair 45, which detects the passage of the recording paper 40 by contact or non-contact, and outputs a detection signal to the mechanism control unit 64 (FIG. 3) described later.

Meanwhile, in the ID unit 12, the surface of the photoconductor drum 13 is charged by the charging roller 14 and then exposed by the corresponding LED head 15, and an electrostatic latent image is formed on the surface by this exposure. A thin layer of toner on the developing roller 16 is electrostatically attached to the part where the electrostatic latent image is formed, forming a toner image of the corresponding color. The toner images formed on each photoconductor drum 13 are transferred in succession to the recording paper 40 by the corresponding transfer roller 17, forming a color toner image on the recording paper. After transfer, any toner remaining on each photoconductor drum 13 is removed by the respective cleaning blades 27.

The recording paper 40 on which the color toner image has been formed is sent to the fixing device 28. In this fixing device 28, the color toner image is fixed to the recording paper 40, forming a color image. The recording paper 40 on which the color image has been formed is transported along the paper guide section 42 and discharged to the paper stacker section 48. Through the above process, a color image is formed on the recording paper 40.

The configuration of the density sensor 36 will now be described further with reference to FIG. 2. FIG. 2(a) is a cross-sectional view showing a schematic configuration of the density sensor 36, FIG. 2(b) is a diagram explaining the detection of the densities of yellow (Y), magenta (M), and cyan (C), and FIG. 2(c) is a diagram explaining the detection of the density of black (K).

As shown in the figure, the density sensor 36 has an infrared LED 36a as a light-emitting unit, a phototransistor 36b for receiving specular reflected light as a first light-receiving unit, and a phototransistor 36c for receiving diffuse reflected light as a second light-receiving unit. The density of each of the yellow (Y) toner image, magenta (M) toner image, and cyan (C) toner image is detected by the phototransistor 36c, which mainly receives the diffuse reflected light from the toner image, and the density of the black (K) toner image is detected by the phototransistor 36b, which mainly receives the specular reflected light from the surface of the transfer belt 26.

As shown in FIG. 2(b), for example, when detecting the density of a yellow (Y) toner image, the light emitted from the infrared LED 36a and diffusely reflected by the yellow (Y) toner image of the density detection pattern PY printed on the transfer belt 26 is received by the phototransistor 36c, and the phototransistor 36c outputs a voltage according to the amount of light. Therefore, the more yellow toner that forms the density detection pattern PY (i.e., the higher the density), the more diffusely reflected light is received by the phototransistor 36c (i.e., the higher the output voltage). The density detection of the magenta (M) density detection pattern PM and the cyan (C) density detection pattern PC is similar to the density detection of the yellow (Y) density detection pattern PY.

As shown in FIG. 2(c), when detecting the density of the black (K) density detection pattern PK, the light emitted from the infrared LED 36a passes through the black (K) toner that forms the density detection pattern PK printed on the transfer belt 26, and is specularly reflected by the transfer belt 26. The phototransistor 36b receives the light, and the phototransistor 36b outputs a voltage according to the amount of light. The black toner that forms the density detection pattern PK absorbs the light emitted from the infrared LED 36a, so the more black toner there is (i.e., the higher the density), the less specularly reflected light is received by the phototransistor 36b (i.e., the lower the output voltage).

A movable sensor cover 37 (Figure 1) that covers the density sensor 36 is disposed between the density sensor 36 and the transfer belt 26. The sensor cover 37 is moved by a drive means to a closed position that covers the density sensor 36 except during density detection, and during density detection, moves from the closed position over the density sensor 36 to an open position that opens the density sensor 36 (the density sensor 36 faces the transfer belt 26). The sensor cover 37 is also used as a reference diffuse reflector for calibrating the infrared LED 36a of the density sensor 36.

Figure 3 is a functional block diagram showing the configuration of the control system of the image forming device 11.

In the figure, the communication unit 61 is the part that handles the physical layer interface with the host computer, and is composed of, for example, a chip equipped with a connector and a processing circuit for communication. The command/image processing unit 62 has a processing circuit for interpreting commands and image data from the host side or expanding them into a bitmap. Specifically, the command/image processing unit 62 has a microprocessor as an information processing unit, a memory such as a RAM (Random Access Memory) as a storage unit, and special hardware for expanding programs or data. The command/image processing unit 62 controls the overall operation of the image forming device 11.

The density tone correction control unit 62a of the command/image processing unit 62 corrects the tone values output when the bitmap is expanded based on the density values detected by the density sensor 36 so that the print density of the input tone values becomes the target value.

Figure 4 is a diagram showing an example of a target gradation density value table 111 used by the image forming device 11. The target gradation density value table 111, which is various control parameters used in the density gradation correction process, is stored in advance in the memory unit 62b. Figure 5 is a diagram showing an example of a gradation correction value table 112 used by the image forming device 11. The gradation correction value table 112, which is made up of gradation correction values determined by the density gradation correction control, is updated by an update process and stored in the memory unit 62b.

In FIG. 3, the LED head interface unit 63 is composed of a semi-custom LSI and RAM (not shown), and processes the image data expanded into a bitmap from the command/image processing unit 62 to match the interface of the LED head 15.

The mechanism control unit 64, which serves as a density correction means, follows instructions from the command/image processing unit 62 and drives the hopping motor 81 that drives the hopping roller 43, the registration motor 82 that drives the registration roller pair 45, the belt motor 83 that drives the transfer belt drive roller 21a, the heat motor 84 that drives the heating roller 29, and the drum motor 85 that drives the photosensitive drum 13, etc., based on input from each sensor.

The mechanism control unit 64 also controls the heater 31 and the high voltage control unit 71. The heater 31 is a halogen lamp disposed inside the heating roller 29, and temperature control is performed based on the temperature detected by the fixing thermistor 32. The mechanism control unit 64 also measures the temperature and humidity (i.e., temperature or humidity or both) outside the device using the environmental sensor 25, and measures the surface temperature of the transfer belt 26 using the belt thermistor 38 disposed in a position where the surface temperature of the transfer belt 26 can be detected.

The density correction process execution determination unit 64a of the mechanism control unit 64 performs the density correction process, for example, when the image forming device 11 is turned on or after a predetermined number of sheets have been printed. Alternatively, the density correction process execution determination unit 64a of the mechanism control unit 64 performs the density correction process when the change in the temperature and humidity of the outside air detected by the environmental sensor 25 or the change in the temperature of the transfer belt 26 detected by the belt thermistor 38 meets the preset density correction process execution conditions.

Based on the density value detected by the density sensor 36, the density correction control unit 64b of the mechanism control unit 64 calculates how much to increase or decrease the development voltage applied to the development roller 16 or the amount of light emitted by the LED head 15 (driving time) so that the density becomes the target value. In addition, based on the density value detected by the density sensor 36, the density correction control unit 64b calculates the density value to be used in the density gradation correction process.

The memory unit 64c stores various control parameters used in the density correction process, namely, a sensor detection voltage/density value conversion table 113, a target print density value data table 114, a development voltage value/density sensitivity table 115, and an LED drive time/density sensitivity table 116.

Figure 6 shows an example of a sensor detection voltage/density value conversion table 113 used by the image forming device 11, Figure 7 shows an example of a target print density value data table 114 used by the image forming device 11, Figure 8 shows an example of a development voltage value/density sensitivity table 115 used by the image forming device 11, and Figure 9 shows an example of an LED drive time/density sensitivity table 116 used by the image forming device 11.

The memory unit 64c also stores in advance density detection patterns PAT1, PAT2, and PAT3 (shown in Figures 12 to 14 described below) as density correction patterns used in the density correction process, and stores the development voltage correction value and LED drive time correction value determined by the density correction control. When the density correction operation is completed, the memory unit 64c updates and stores the surface temperature of the transfer belt 26 measured by the belt thermistor 38 and the temperature and humidity outside the device measured by the environmental sensor 25.

In FIG. 3, the concentration sensor light emission amount adjustment unit 64d of the mechanism control unit 64 adjusts the light emission current of the infrared LED 36a so that the output voltage of the phototransistor 36b and the phototransistor 36c becomes a preset value for an arbitrary reference reflector. In this embodiment, the adjustment of the light emission current of the infrared LED 36a is called concentration sensor calibration.

The density correction history storage unit 64e, which serves as a history storage unit of the mechanism control unit 64, stores the development voltage correction values calculated by the development voltage correction described below for each of the previously set density correction processing execution conditions, such as the outside air environment detected by the environmental sensor 25, for example, changes in temperature and humidity, up to the past several times.

The high voltage control unit 71 has a high voltage control circuit composed of a microprocessor or custom LSI, etc., and controls the generation of the charging voltage, developing voltage, supply voltage, and transfer voltage applied to the transfer roller 17 in each ID unit 12. The charging voltage generating unit 72 applies and stops the application of the charging voltage to the charging roller 14, the developing voltage generating unit 73 applies the developing voltage to the developing roller 16, the supply voltage generating unit 74 applies the supply voltage to the toner supply roller 18, and the transfer voltage generating unit 75 applies and stops the application of the transfer voltage to the transfer roller 17.

The memory unit 71a of the high voltage control unit 71 stores the set voltage values of the charging voltage generating unit 72, the developing voltage generating unit 73, the supply voltage generating unit 74, and the transfer voltage generating unit 75.

In the above configuration, the printing operation performed by the image forming device 11 will be described with reference to the flowchart in FIG. 10. FIG. 10 is a flowchart showing the flow of the printing operation of the image forming device 11.

<Step S101> The image forming device 11 receives image data sent from an external device, i.e., a host computer, via the communication unit 61.

<Step S102> The command/image processing unit 62 issues an instruction to the mechanism control unit 64 to warm up the heater 31, and performs image data expansion processing based on the values of the gradation correction value table 112 (FIG. 5) stored in the memory unit 62b, to generate bitmap data of images of each color. The mechanism control unit 64, which has received the warm-up instruction from the command/image processing unit 62, controls the heat motor 84 to drive the heating roller 29, and controls the heater 31 (halogen lamp) to adjust the fixing temperature.

When the conditions for printing are met, that is, when the image data for each color for one page to be printed on the medium is stored in the memory that is the storage unit 62b of the command/image processing unit 62, and the fixing temperature has reached the optimal temperature, the command/image processing unit 62 issues a command to the mechanism control unit 64 to start printing.

<Step S103> Having received a command to start printing, the mechanism control unit 64 controls the belt motor 83 and drum motor 85 to drive the transfer belt drive roller 21a and the photosensitive drum 13. The high voltage control unit 71, which has received a command to output high voltage from the mechanism control unit 64, reads out the set voltage values of the charging voltage, developing voltage, and supply voltage stored in the memory unit 71a according to the temperature and humidity measured by the environmental sensor 25, generates and supplies the charging voltage to the charging roller 14 from the charging voltage generation unit 72, the developing voltage to the developing roller 16 from the developing voltage generation unit 73, and the supply voltage to the toner supply roller 18 from the supply voltage generation unit 74, and further reads out the developing voltage correction value stored in the memory unit 64c to correct the developing voltage.

Here, we will explain the operation of ID unit 12. Note that the image formation processes of ID units 12K, 12C, 12M, and 12Y for black (K), cyan (C), magenta (M), and yellow (Y) are the same.

When the charging voltage, developing voltage, and supply voltage are supplied by the high voltage control unit 71, a charging voltage of -1100 [V] is supplied to the charging roller 14, charging the surface of the photosensitive drum 13 to approximately -600 [V]. A developing voltage of -200 [V] is supplied to the developing roller 16, and a supply voltage of -250 [V] is supplied to the toner supply roller 18, and an electric field is formed in the direction from the developing roller 16 to the toner supply roller 18 near the contact area between the developing roller 16 and the toner supply roller 18.

The toner cartridge 20 contains toner, and the toner supplied from the toner cartridge 20 to the developing unit is rubbed strongly in the contact area between the developing roller 16 and the toner supply roller 18, causing it to become triboelectrically charged. In this embodiment, the toner is triboelectrically charged to a negative polarity due to the characteristics of the developing roller 16 and the toner supply roller 18.

The negatively charged toner adheres to the developing roller 16 by the Coulomb force of the electric field oriented from the developing roller 16 toward the toner supply roller 18 near the contact area between the developing roller 16 and the toner supply roller 18. As the developing roller 16 rotates, the adhered toner is carried to the contact area between the developing roller 16 and the developing blade 19, where it is smoothed to a uniform thickness by the developing blade 19 to form a toner layer. The developing roller 16 continues to rotate, carrying the toner layer to the contact area with the photoconductor drum 13.

Meanwhile, the command/image processing unit 62 sends bitmap data for each page to the LED head interface unit 63. The LED head interface unit 63 blinks the LEDs of the LED head 15 in accordance with the received bitmap data, exposes the photoconductor drum 13, which is charged to -600V, and discharges it to -60V, thereby writing an electrostatic latent image. At this time, it reads out the LED drive time correction value stored in the memory unit 64c and corrects the LED drive time.

As the photoconductor drum 13 rotates, the electrostatic latent image written on the surface of the photoconductor drum 13 reaches the contact area with the developing roller 16. Between the developing roller 16 and the photoconductor drum 13, an electric field is formed in the exposed area, which is an area that has been discharged to -60V, from the photoconductor drum 13 toward the developing roller 16, while an electric field in the opposite direction is formed in the unexposed area, which is an area that has not been discharged and remains at -600V. As a result, toner from the negatively charged toner layer on the developing roller 16 selectively adheres only to the exposed area, forming a toner image based on the electrostatic latent image. In other words, the electrostatic latent image is developed.

<Step S104> At the same time that the ID unit 12 starts to be driven, the mechanism control unit 64 drives the hopping motor 81 to rotate the hopping roller 43 and send only one sheet of recording paper 40 from the paper feed tray 24 to the paper guide unit 41. The mechanism control unit 64 monitors the output of the paper detection sensor 44, and stops the hopping motor 81 when it detects that the leading edge of the recording paper 40 has reached between the rollers of the registration roller pair 45.

Next, the mechanism control unit 64 drives the registration motor 82 to rotate the registration roller pair 45 to transport the recording paper 40, monitors the output of the paper detection sensor 49, and stops the registration motor 82 when it detects that the rear end of the recording paper 40 has reached the transfer belt 26. Next, the mechanism control unit 64 drives the belt motor 83 to rotate the transfer belt drive roller 21a, and sends the recording paper 40 electrostatically attracted to the transfer belt 26 to the transfer nip, which is the contact area between the photosensitive drum 13 and the transfer belt 26.

In accordance with the timing at which the leading edge of the recording paper 40 transported by the transfer belt 26 sequentially reaches the contact area between the photosensitive drum 13 and the transfer belt 26, the high-voltage control unit 71, which has received a high-voltage output command from the mechanism control unit 64, generates and supplies a transfer voltage to the transfer roller 17 from the transfer voltage generation unit 75. A transfer voltage of 3000V is supplied to the transfer roller 17, and an electric field is formed in the direction from the transfer belt 26 to the photosensitive drum 13, so that the toner image formed on the photosensitive drum 13 is transferred onto the medium, that is, onto the recording paper 40 transported by the transfer belt 26, or onto the transfer belt 26.

The recording paper 40 onto which the toner image has been transferred continues to be transported by the transfer belt 26 and sent to the fixing device 28. The mechanism control unit 64 monitors the output of the transfer discharge sensor 22 to check for any recording paper 40 that has failed to separate from the transfer belt 26, and when it detects that the rear end of the recording paper 40 has reached the fixing device 28, it stops the belt motor 83 and drum motor 85.

When the high voltage control unit 71 receives an instruction from the mechanism control unit 64 to stop high voltage output, it stops the supply of charging voltage to the charging roller 14 by the charging voltage generating unit 72, the supply of developing voltage to the developing roller 16 by the developing voltage generating unit 73, the supply of supply voltage to the toner supply roller 18 by the supply voltage generating unit 74, and the supply of transfer voltage to the transfer roller 17 by the transfer voltage generating unit 75. In addition, while the belt motor 83 is driving, the toner remaining on the surface of the upper half of the transfer belt 26 is scraped off by the belt cleaning blade 34 and stored in the belt cleaner container 35.

<Step S105> When the recording paper 40 reaches the fixing device 28, the fixing device 28 conveys the recording paper 40 between the heating roller 29, which has already reached the fixing temperature, and the pressure roller 30, which is in contact with the heating roller 29, and heats and presses the toner image on the recording paper 40 to fix the toner image to the recording paper 40. The recording paper 40 with the fixed toner image is conveyed along the paper guide section 42 and discharged onto the paper stacker section 48. The mechanism control section 64 monitors the output of the fixing discharge sensor 23 and monitors for jams in the fixing device 28 or the recording paper 40 wrapping around the heating roller 29. When it detects that the rear end of the recording paper 40 has reached the paper stacker section 48, it stops driving the heat motor 84 and temperature control of the heater 31, and completes the printing operation.

Before describing the density correction process performed by the image forming device 11 according to this embodiment, a comparative example of density correction process will be described with reference to the flowchart in FIG. 11. FIG. 11 is a flowchart showing the flow of a density correction process that is a comparative example of the density correction process of the image forming device 11.

<Step S201> The concentration sensor light emission amount adjustment unit 64d of the mechanism control unit 64 adjusts the light emission current of the infrared LED 36a to absorb changes in the light emission characteristics of the infrared LED 36a due to the temperature of the concentration sensor 36 itself and variations in the light emission and light reception sensitivity of the concentration sensor 36 itself that may occur during manufacturing. As described above, in concentration sensor calibration, the light emission current of the infrared LED 36a is adjusted so that the output voltage of the phototransistor 36b for receiving specular reflected light from the reference reflector for calibration and the phototransistor 36c for receiving diffuse reflected light is a preset value.

Here, the light emission current adjustment range of the infrared LED 36a is set to 15 mV to 25 mV, and the output voltage range of the phototransistors 36b and 36c is set to 0 V to 3 V. In addition, a sensor cover 37 (Figure 1) disposed between the density sensor 36 and the transfer belt 26 is used as a reference reflector for calibrating the light emission current of the infrared LED 36a when detecting the densities of yellow (Y), magenta (M), and cyan (C).

The sensor cover 37 has a predetermined standard diffuse reflection characteristic. The light emission current of the infrared LED 36a is adjusted so that the output voltage of the phototransistor 36c becomes a set value when the infrared LED 36a irradiates the sensor cover 37 with infrared light. Here, the light emission intensity of the infrared LED 36a is determined so that the output voltage of the phototransistor 36c for receiving diffuse reflected light becomes a set value of 2.00 [V] when the sensor cover 37 is irradiated with infrared light.

The transfer belt 26 is used as a reference reflector for calibrating the light emission current of the infrared LED 36a when detecting the density of black (K). The transfer belt 26 has a predetermined reference specular reflection characteristic, and the light emission current of the infrared LED 36a is set so that the output voltage of the phototransistor 36b becomes a set value when the infrared LED 36a irradiates the transfer belt 26 with infrared light. Here, the light emission intensity of the infrared LED 36a is determined so that the output voltage of the phototransistor 36b for receiving specular reflected light becomes a set value of 2.50 [V].

によって求められる。マゼンタ（Ｍ）、シアン（Ｃ）、ブラック（Ｋ）の現像電圧初期値ＭＤＢ_０、ＣＤＢ_０、ＫＤＢ_０、の算出過程も、イエロー（Ｙ）の場合と同様である。
一方、ＬＥＤ駆動時間は予め決められた初期値ＤＫ_０［ｓ］に設定する。 <Step S202> The density correction control unit 64b of the mechanism control unit 64 reads out the average value DBH [V] of the history of the development voltage correction values for the past several times as the correction value information stored in the density correction history storage unit 64e for each preset density correction process execution condition such as changes in the outside air temperature and humidity detected by the environment sensor 25, and adds the development voltage value to the predetermined initial value DBI [V]. As a result, the correction operation is performed from a density close to the target density. Therefore, the initial development voltage value _YDB0 for yellow (Y) is

The calculation process of the initial development voltage values MDB ₀ , CDB ₀ , and KDB ₀ for magenta (M), cyan (C), and black (K) is similar to that for yellow (Y).
On the other hand, the LED drive time is set to a predetermined initial value DK ₀ [s].

<Step S203> The density correction control unit 64b of the mechanism control unit 64 reads the sensor detection voltage/density value conversion table 113 (Figure 6), the target print density value data table 114 (Figure 7), the development voltage value/density sensitivity table 115 (Figure 8), and the LED drive time/density sensitivity table 116 (Figure 9) stored in the memory unit 64c.

<Step S204> When the density correction control unit 64b of the mechanism control unit 64 receives a signal to perform density detection, it starts printing the density detection pattern PAT1, which is pre-stored in the memory unit 64c, onto the transfer belt 26.

FIG. 12 is a diagram showing an example of a density detection pattern PAT1 used by the image forming apparatus 11. As shown in FIG.
The density detection pattern PAT1 is arranged in the order of yellow (Y), magenta (M), cyan (C), and black (K) from the downstream side in the transport direction (arrow A direction), and the toner development area ratio is 100%. The toner development area ratio is the ratio of the developed toner to a specific area on the transfer belt 26, and is called "duty". Each pattern portion constituting the density detection pattern PAT1 has a length Lp [mm]. The pattern portions constituting the density detection pattern PAT1 are arranged without any gaps in the transport direction.

The mechanism control unit 64 causes the infrared LED 36a of the density sensor 36 to emit light according to the color of the pattern portion being read, and irradiates the density detection pattern PAT1 with infrared light. The phototransistor 36b and the phototransistor 36c are driven by a sensor circuit (not shown) and output a current signal proportional to the received light energy. This current signal is converted into a voltage signal by the sensor circuit and read by the mechanism control unit 64. When the read pattern portion is yellow (Y), magenta (M), or cyan (C), the mechanism control unit 64 reads the output voltage of the phototransistor 36c for receiving diffuse reflected light, and when the read pattern portion is black (K), the mechanism control unit 64 reads the output voltage of the phototransistor 36b for receiving specular reflected light.

Here, the first pattern portion detected is the 100% duty pattern portion PY100, so the mechanism control unit 64 reads the output voltage of the phototransistor 36c. Next, the mechanism control unit 64 moves the transfer belt 26 by the pattern portion length Lp [mm] to align the center of the 100% duty pattern portion PM100 with the detection position of the density sensor 36, and reads the output voltage of the phototransistor 36c. From then on, the mechanism control unit 64 sequentially reads the output voltage for all pattern portions of the density detection pattern PAT1.

The mechanism control unit 64 converts the read output voltage into a density value using a sensor detection voltage/density value conversion table 113 (Figure 6). The table values of the sensor detection voltage/density value conversion table 113 are experimentally determined optimum values of coefficients A and B, which are coefficients of a linear approximation equation that shows the relationship between the sensor detection voltage and the density value. Here, the calculation process for the density value of yellow (Y) is explained. The calculation process for the density values of magenta (M), cyan (C), and black (K) is the same as that for yellow (Y).

尚、濃度値ＹＯＤ_１００のように、文字右下に表す数値は、そのときの濃度検出パターンのデューティを示す。 If the sensor detection voltage of the yellow (Y) pattern portion PY100 with a duty of 100% of the read density detection pattern PAT1 is _YV100 , the coefficient A is Y(A), and the coefficient B is Y(B), then the density value _YOD100 is expressed by the following formula (2).

Incidentally, the numerical value shown at the bottom right of the character, such as density value YOD ₁₀₀ , indicates the duty of the density detection pattern at that time.

<Step S205> The mechanism control unit 64 compares the density value calculated in step S204 with the target density value in the target print density value data table 114 (FIG. 7), and calculates how much the development voltage value of each color should be increased or decreased from the difference between them. This calculation uses the development voltage value/density sensitivity table 115 (FIG. 8) stored in the memory unit 64c. The table value _ΔDB50 in the development voltage value/density sensitivity table 115 is the amount of change in density value when the development voltage value changes by 1 V. Here, the development voltage value/density sensitivity table 115 stored in the device is set so as to enable better correction near the target density.

By changing the development voltage value, the thickness of the developed toner layer can be changed, and this can be used to increase or decrease the density from the low duty portion to the high duty portion. Here, the development voltage value control amount YDB(A) as the correction value for yellow (Y) when the duty is 100% is calculated by the following formula (3) using the density value YOD ₁₀₀ and the target density value YOD _T100 for yellow (Y). The development voltage value control amounts MDB(A), CDB(A), and KDB(A) for magenta (M), cyan (C), and black (K) are calculated in the same way as for yellow (Y).

マゼンタ（Ｍ）、シアン（Ｃ）、ブラック（Ｋ）の補正後の現像電圧値ＭＤＢ_１［Ｖ］、ＣＤＢ_１［Ｖ］、ＫＤＢ_１［Ｖ］も、イエロー（Ｙ）の場合と同様に算出される。
尚、今回、濃度補正履歴記憶部６４ｅに記憶される現像電圧補正値は、数式（４）で算出した現像電圧値ＹＤＢ_１［Ｖ］から初期値ＤＢＩを引いた電圧値となる。 The mechanism control unit 64 issues an instruction to the high voltage control unit 71 to increase or decrease the developing voltage based on the developing voltage value control amounts YDB(A), MDB(A), CDB(A), and KDB(A) for each color calculated by the above formula (3). The developing voltage generating unit 73 calculates a developing voltage value YDB1 [V] as a corrected developing bias after correction by adding the developing voltage value control amount YDB(A) to the developing voltage initial value _YDB0 during printing operation in the case of yellow (Y) by the following formula (4), and supplies _it to the developing roller 16 of the ID unit 12Y.

The corrected development voltage values MDB ₁ [V], CDB ₁ [V], and KDB ₁ [V] for magenta (M), cyan (C), and black (K) are calculated in the same manner as for yellow (Y).
Incidentally, the developing voltage correction value stored in the density correction history storage unit 64e this time is a voltage value obtained by subtracting the initial value DBI from the developing voltage value YDB ₁ [V] calculated by the formula (4).

<Step S206> FIG. 13 is a diagram showing an example of the density detection pattern PAT2 used by the image forming apparatus 11. As shown in FIG.
When the mechanism control unit 64 receives a signal instructing execution of density detection, it prints the density detection pattern PAT2, which is stored in advance in the storage unit 64c, on the transfer belt 26. The density detection pattern PAT2 has pattern portions arranged in the order of yellow (Y), magenta (M), cyan (C), and black (K) from the downstream side in the transport direction (arrow A direction), and has a duty of 50%. The development voltage values at this time are the development voltage values YDB ₁ [V], MDB ₁ [V], CDB ₁ [V], and KDB ₁ [V] after development voltage correction.

Like density detection pattern PAT1, density detection pattern PAT2 is printed with a pattern portion length Lp [mm], with no gaps between the end of each pattern portion and the next pattern portion. The dot arrangement of the dither pattern is, for example, a 45° halftone dither (as shown in Figure 15 (c) described below).

マゼンタ（Ｍ）、シアン（Ｃ）、ブラック（Ｋ）の濃度値ＭＯＤ′_５０、ＣＯＤ′_５０、ＫＯＤ′_５０も、イエロー（Ｙ）の場合と同様に求められる。 The mechanism control unit 64 reads the output voltage of the pattern portion of each color by the density sensor 36, and converts it into a density value by the sensor detection voltage/density value conversion table 113 (FIG. 6). If the sensor detection voltage of the read density detection pattern PAT2 with a duty of 50% is _YV'50 [V], the density value _YOD'50 of yellow (Y) can be calculated by the following formula (5).

The density values _MOD'50 , _COD'50 and _KOD'50 of magenta (M), cyan (C) and black (K) are obtained in the same manner as in the case of yellow (Y).

<Step S207> The mechanism control unit 64 compares the density value calculated by the above formula (5) with the target print density value data table 114 (Figure 7), and calculates how much to increase or decrease the individual LED drive time of the LED head 15 of each color based on the difference between them. This calculation uses the LED drive time/density sensitivity table 116 (Figure 9) stored in the memory unit 64c. The table values of the LED drive time/density sensitivity table 116 are the amount of change in density value when the LED drive time changes by 1%. By changing the LED drive time, it is possible to increase or decrease the density mainly from the low duty section to the medium duty section.

マゼンタ（Ｍ）、シアン（Ｃ）、ブラック（Ｋ）のＬＥＤ駆動時間制御量ＭＤＫ（Ａ）、ＣＤＫ（Ａ）、ＫＤＫ（Ａ）は、イエロー（Ｙ）の場合と同様に求められる。 In this embodiment, the yellow (Y) LED drive time control amount YDK(A) is calculated by the following formula (6) using the density value _YOD'50 and the target density value YOD-- _T50 .

The LED drive time control amounts MDK(A), CDK(A), and KDK(A) for magenta (M), cyan (C), and black (K) are determined in the same manner as for yellow (Y).

The mechanism control unit 64 issues an instruction to the LED head interface unit 63 to increase or decrease the drive time of each LED head 15 based on the LED drive time control amounts YDK(A), MDK(A), CDK(A), and KDK(A) for each color calculated using formula (6) above. During printing operations, the mechanism control unit 64 exposes each LED head 15 to an LED drive time that is the initial value of the LED drive time plus a correction value based on the LED drive time control amounts YDK(A), MDK(A), CDK(A), and KDK(A).

マゼンタ（Ｍ）、シアン（Ｃ）、ブラック（Ｋ）の補正後のＬＥＤ駆動時間ＭＤＫ_１［ｓ］、ＣＤＫ_１［ｓ］、ＫＤＫ_１［ｓ］は、イエロー（Ｙ）の場合と同様に求められる。 For example, in the case of yellow (Y), the corrected LED drive time YDK ₁ [s] is calculated by the following formula (7) using the initial value YDK ₀ [s] of the LED drive time and the LED drive time control amount YDK(A).

The corrected LED drive times MDK ₁ [s], CDK ₁ [s], and KDK ₁ [s] for magenta (M), cyan (C), and black (K) are determined in the same manner as for yellow (Y).

<Step S208> FIG. 14 is a diagram showing an example of a density detection pattern PAT3 used by the image forming apparatus 11. As shown in FIG.
When the mechanism control unit 64 receives the instruction signal to perform density detection, it starts printing the density detection pattern PAT3, which is stored in advance in the memory unit 64c, on the transfer belt 26. The development voltage values and LED drive times at this time are development voltage corrected values YDB ₁ [V], MDB ₁ [V], CDB ₁ [V], and KDB ₁ [V], and LED drive times corrected values YDK ₁ [s], MDK ₁ [s], CDK ₁ [s], and KDK ₁ [s].

The density detection pattern PAT3 has six sets of multiple pattern parts arranged in the order of yellow (Y), magenta (M), cyan (C), and black (K) from the downstream side in the transport direction (arrow A direction), and the duties of each set are 15%, 30%, 50%, 70%, 85%, and 100%. Like the density detection pattern PAT1, the density detection pattern PAT3 is printed with a pattern part length Lp [mm] and no space between the end of each pattern part and the next pattern part. However, in FIG. 14, some of the patterns with duties of 30%, 50%, 70%, and 85% are omitted.

Figures 15(a) to (f) are diagrams showing examples of dither patterns P15, P30, P50, P70, P85, and P100 with a duty of 15%, 30%, 50%, 70%, 85%, and 100%. The dot arrangement of the dither pattern of the density detection pattern PAT3 in Figure 14 is as shown in Figures 15(a) to (f), and is a 45° halftone dither. Note that the density detection patterns PAT1 to PAT3 used for density detection are not limited to the patterns shown in Figures 15(a) to (f), and the type and configuration of the dither pattern, the combination of duties, the order of colors, the length of each pattern portion, the interval, etc. may be changed as needed.

The mechanism control unit 64 receives the output voltage of the pattern portion of each color from the density sensor 36 and converts the output voltage into a density value based on the sensor detection voltage/density value conversion table 113 (Figure 6).

マゼンタ（Ｍ）、シアン（Ｃ）、ブラック（Ｋ）の濃度値も、イエロー（Ｙ）の場合と同様に求められる。 If the sensor detection voltages of the 15%, 30%, 50%, 70%, 85%, and ₁₀₀ % duty pattern portions of the read density detection pattern PAT3 are _YV''15 , _YV''30 , _YV''50 , _YV''70 , YV''85, and _YV''100 , respectively, the yellow (Y) density values _YOD''15 , _YOD''30 , _YOD''50 , _YOD''70 , _YOD''85 , and _YOD''100 are expressed by the following mathematical expressions (8) to (13).

The density values of magenta (M), cyan (C) and black (K) are obtained in the same manner as for yellow (Y).

<Step S209> The density gradation correction control unit 62a of the command/image processing unit 62 receives density values YOD''15, _YOD''30 , YOD''50, YOD''70, YOD''85, and _YOD''100 of the pattern portions of the density detection pattern PAT3 with duties of 15%, 30%, 50%, 70%, 85%, and ₁₀₀ % read by the mechanism control unit 64 in the density detection pattern _printing and detection process (step _S208 ). The density gradation correction control unit 62a of the command/ _image processing unit 62 calculates density values for 256 gradation levels from the density values of the pattern portions of each duty received.

If each duty of 15%, 30%, 50%, 70%, 85%, and 100% is expressed in 256 gradation levels from 0 to 255, it is as follows. A duty of 0% is 0 gradation level, a duty of 15% is 38 gradation level, and a duty of 30% is 77 gradation level. A duty of 50% is 128 gradation level, and a duty of 70% is 179 gradation level. A duty of 85% is 217 gradation level, and a duty of 100% is 255 gradation level.

マゼンタ（Ｍ）、シアン（Ｃ）、ブラック（Ｋ）の各階調レベルの濃度値も、イエロー（Ｙ）の場合と同様に表される。 For yellow (Y), if the density value of the 0 gradation level is YODG ₀ , the density value of the 38 gradation level is YODG 38, the density value of the 77 gradation level is YODG ₇₇ , the density value of the 128 gradation level is YODG ₁₂₈ , the density value of the 179 gradation level is YODG ₁₇₉ , the density value of the 217 gradation level is YODG ₂₁₇ , and the density value of the 255 gradation level is YODG ₂₅₅ , the density values of each gradation level are expressed by the following formulas (14) to (20).

The density values of each tone level of magenta (M), cyan (C) and black (K) are expressed in the same manner as in the case of yellow (Y).

マゼンタ（Ｍ）、シアン（Ｃ）、ブラック（Ｋ）の階調レベルの濃度値も、イエロー（Ｙ）の場合と同様に表される。 Furthermore, when the density value of the l gradation level (l=1 to 37) between the 1st gradation level and the 37th gradation level is YODG _l , the density value of the m gradation level (m=39 to 76) between the 39th gradation level and the 76th gradation level is YODG _m , the density value of the n gradation level (n=78 to 127) between the 78th gradation level and the 127th gradation level is YODG _n , the density value of the o gradation level (o=129 to 178) between the 129th gradation level and the 178th gradation level is YODG _o , the density value of the p gradation level (p=180 to 216) between the 180th gradation level and the 216th gradation level is YODG _p , and the density value of the q gradation level (q=218 to 254) between the 218th gradation level and the 254th gradation level is YODG _q , each density value is expressed by the following mathematical formulas (21) to (26).

The density values of the gradation levels of magenta (M), cyan (C) and black (K) are expressed in the same manner as for yellow (Y).

Next, the density tone correction control unit 62a of the command/image processing unit 62 compares the density values of each of the calculated 256 gradation levels with the gradation levels in the target gradation density value table 111 (Figure 4). The density tone correction control unit 62a finds gradation levels from each of the calculated 256 gradation levels that match the density values of each of the 256 gradation levels in the target gradation density value table 111, and updates the gradation correction value table 112 by setting the gradation levels in the target gradation density value table 111 as input gradation values and setting each of the calculated 256 gradation levels that match the density values in the target gradation density value table 111 as output gradation values.

Here, the density tone correction control will be further explained. For example, if the calculated 256 gradation levels that match the density value of the 100 gradation level at the target gradation density value are 90 gradation levels, the calculated density value of the 100 gradation level at each of the 256 gradation levels will be darker than the density value of the 100 gradation level at the target gradation density value.

In this case, if an input signal of 100 gradation level is printed as an output signal of 100 gradation level, the density value when actually printed will be printed darker than the target density value. Therefore, by replacing the input signal of 100 gradation level with the calculated 90 gradation level, which matches the density value of 100 gradation level at the target gradation density value, as the output signal and performing image processing, the density value when actually printed can be corrected to the target density value.

Figure 16 is a diagram (graph) for explaining the problem that the present invention is intended to solve. The horizontal axis shows the density (density before density correction) obtained by converting the voltage detected by the density sensor 36 of the density detection pattern using formula (2) in step S204 of the density correction operation flow in Figure 11, and the vertical axis shows the density (density after density correction) obtained by converting the voltage detected by the density sensor of the density detection pattern using formula (2) in step S206 after the development voltage correction in step S205.

When the pre-correction density is close to the target density, the post-correction density is likely to be corrected to be close to the target density, but as the pre-correction density becomes thinner, the post-correction density gradually shifts toward the darker side than the target density. Furthermore, when the pre-correction density is darker than the target density and exceeds a certain density, there is a tendency for the post-correction density to become suddenly darker.

Figure 17 is a graph (grab) used to explain the mechanism of the phenomenon described in Figure 16. The horizontal axis shows the development voltage, and the vertical axis shows the density obtained by converting the voltage detected by the density sensor 36 of the density detection pattern using formula (2).

As shown in the graph, the density saturates as the absolute value of the development voltage increases. When the result of printing/detection in <Step S204> is A, the mechanism control unit 64 compares the read density value with the target density value in the target print density value data table 114, calculates the amount by which the development voltage value for each color should be increased or decreased from the difference between them, and corrects the development voltage by ΔDB to move toward the target density B. However, in reality, in accordance with the relationship between the development voltage and density of the device, the density moves toward point B' and becomes high. This phenomenon occurs significantly when the target density is close to the saturation density.

Since the development voltage value is set by adding the development voltage control amount DB(A), which is the result of the previous density correction, to a predetermined initial value _DB0 [V], the correction operation is performed from a density close to the target density, but since the density after correction has a certain degree of variation, it may fluctuate toward the darker side. If the density correction is started in a dark state once, the above phenomenon occurs, and the density often settles at a darker density in subsequent density corrections.

Therefore, the density detection pattern detected before performing the development voltage correction in step S204 is intentionally made thinner, and the development voltage correction is performed from a point where the density is thinner than the target density, thereby solving the above problem.

Hereinafter, the density correction process of the first embodiment of the present invention for solving the above problem will be described with reference to the flowchart in FIG. 18. FIG. 18 is a flowchart showing the flow of the density correction process of the first embodiment of the present invention.

Here, steps S301 to S302 of this flowchart are substantially the same as steps S201 to S202 of the flowchart shown in FIG. 11, and steps S304 to S310 are substantially the same as steps S203 to S209 of the flowchart shown in FIG. 11, so a description of the processing content of each of these steps will be omitted. The differences will be described later.

<Step S303> Here, the development voltage value set in step S302 is adjusted to make the density lighter. However, if the development voltage value is adjusted too much and the pre-correction density becomes excessively light, as explained in Figure 16, it is known that the post-correction density will shift to the darker side, so it is necessary to set an appropriate adjustment amount.

マゼンタ（Ｍ）、シアン（Ｃ）、ブラック（Ｋ）のＭＤＢ（Ｓ）、ＣＤＢ（Ｓ）、ＫＤＢ（Ｓ）も、イエロー（Ｙ）の場合と同様に求められる。尚、ΔＹＤＢ_１００は、現像電圧値が１Ｖ変化するときの濃度値の変化量である。 For this reason, in this embodiment, the development voltage value adjustment amount DB(S) as an adjustment value is a value obtained by converting the experimentally obtained density variation ODσ after density correction into a development voltage value based on the development voltage value/density sensitivity table 115 shown in Fig. 8. Here, YDB(S) for yellow (Y) is obtained by the following formula (27).

The MDB(S), CDB(S), and KDB(S) of magenta (M), cyan (C), and black (K) can be calculated in the same manner as for yellow (Y). Note that ΔYDB ₁₀₀ is the amount of change in density value when the development voltage value changes by 1 V.

For example, if the experimentally obtained density variation YODσ is 0.1 and ΔYDB ₁₀₀ is 6.67×10 ⁻³ , then
YDB(S)=15[V]
This can be calculated as follows.

この計算により、現像電圧値の絶対値は、元の数式（１）より適度に小さくなり、目標濃度より薄い濃度からステップＳ３０６の現像電圧補正を実施できる。これにより、補正後濃度を目標濃度により近づけることができる。尚、ここではステップＳ３０６（ステップＳ２０５に相当）における数式（４）において、ＹＤＢ_０がＹＤＢ´_０となる。 Based on the calculated development voltage adjustment amount, in step S303, the development voltage initial value _YDB'0 as the initial development bias when printing the density detection pattern is calculated according to the following formula (28).

By this calculation, the absolute value of the developing voltage value becomes appropriately smaller than the original formula (1), and the developing voltage correction in step S306 can be performed from a density lower than the target density. This makes it possible to bring the corrected density closer to the target density. Note that here, in formula (4) in step S306 (corresponding to step S205), YDB ₀ becomes YDB' ₀ .

In addition, since the post-correction density variation σ and the development voltage value adjustment table value _ΔDB100 vary depending on the environment, a development voltage value adjustment amount table may be prepared for each environment. Table 1 shows an example of a development voltage value adjustment amount table. In this table, NN stands for medium temperature and medium humidity, HH stands for high temperature and high humidity, and LL stands for low temperature and low humidity.

Furthermore, if the density correction conditions for the previous density correction, such as the surface temperature of the transfer belt 26 measured by the belt thermistor 38 stored in the memory unit 64c and the temperature and humidity outside the apparatus measured by the environmental sensor 25, are significantly different from the density correction conditions when the density correction is next performed, the adjustment amount of the developing voltage value adjustment amount DB(S) calculated by formula (27) may be insufficient, and correction may not be possible from a point where the density is lower than the target value. In such a case where the density correction conditions are significantly different from the previous one, an operation may be performed to further adjust the developing voltage value in the absolute direction smaller than the developing voltage initial value _YDB'0 calculated by formula (28).

As described above, according to the image forming apparatus of this embodiment, the initial developing bias (initial value of developing voltage) in the density correction process is changed from YDB ₀ shown in formula (1), which is close to the corrected developing bias (developing voltage value YDB ₁ ) obtained from past performance, to YDB' ₀ calculated by formula (28), which is smaller than YDB ₀ by the adjustment value (developing voltage value adjustment amount DB(S)) obtained by formula (27). As a result,
Absolute value of initial development bias (initial value of development voltage)
<The absolute value of the corrected developing bias (developing voltage value)>

As described above, according to the image forming device 11 of this embodiment, the absolute value of the initial developing voltage when starting density correction using the developing bias voltage can be set to be smaller than the absolute value of the corrected developing voltage value, making it possible to correct the density with high accuracy, and also reducing toner consumption since the density is no longer maintained in a high state.

Embodiment 2.
The density correction process according to the second embodiment of the present invention will be described with reference to the flowchart of Fig. 19. Fig. 19 is a flowchart showing the flow of the density correction process according to this embodiment.

The image forming apparatus of this embodiment differs mainly from the image forming apparatus of the first embodiment described above in that the flow of the density correction process shown in the flowchart of FIG. 19 is partially different from the flow of the density correction process of the first embodiment shown in the flowchart of FIG. 18 described above. Therefore, parts of the image forming apparatus of this embodiment that are common to the image forming apparatus 11 of the first embodiment described above are given the same reference numerals or the drawings are omitted to omit the explanation, and the differences are mainly described. Note that the essential configuration of the image forming apparatus of this embodiment is common to the essential configuration of the image forming apparatus of the first embodiment other than as described above, so reference will be made to FIGS. 1 to 3 as necessary.

Here, step S401 in the flowchart of FIG. 19 is the same as step S301 in the flowchart shown in FIG. 18, and steps S404 to S410 are the same as steps S304 to S310 in the flowchart shown in FIG. 18, so a description of the processing content of each of these steps will be omitted.

<Step S402> In this embodiment, the density correction control section 64b of the mechanism control section 64 shown in Fig. 3 repeats the operation of printing the density detection pattern PAT1 stored in advance in the storage section 64c on the transfer belt 26 multiple times while decreasing the absolute value of the development voltage value by a predetermined amount of change. The mechanism control section 64 converts the multiple output voltages that it has read into density values using the sensor detection voltage/density value conversion table 113 (Fig. 6). The coefficients A and B in the table differ depending on the duty of the pattern to be detected, and six types are prepared for each color, such as _Y15 (A)... _Y100 (A) for coefficient A.

<Step S403> In this step, the multiple density value data are compared with the target density values in the target print density value data table ₁₁₄ (Figure 7), and the developing bias voltage value when printing the density detection pattern PAT1 that is lower than the target density value and closest to the target density value is supplied to the developing roller 16 of the ID unit 12Y as the developing voltage initial value YDB'0 (see formula (28)).

As described above, in the image forming apparatus of this embodiment, the absolute value of the initial developing voltage when starting density correction using the developing bias voltage can be set to be smaller than the absolute value of the corrected developing voltage value, making it possible to correct the density with high precision, and since the density is no longer maintained in a high state, it is also possible to reduce toner consumption. Also, instead of printing multiple density detection patterns, there is no need to store the history of past adjustment voltage values.

Embodiment 3.
The density correction process according to the third embodiment of the present invention will be described with reference to the flowchart of Fig. 20. Fig. 20 is a flowchart showing the flow of the density correction process according to this embodiment.

The image forming apparatus of this embodiment differs mainly from the image forming apparatus of the first embodiment described above in that the flow of the density correction process shown in the flowchart of FIG. 19 is partially different from the flow of the density correction process of the first embodiment shown in the flowchart of FIG. 18 described above. Therefore, parts of the image forming apparatus of this embodiment that are common to the image forming apparatus 11 of the first embodiment described above are given the same reference numerals or the drawings are omitted to omit the explanation, and the differences are mainly described. Note that the essential configuration of the image forming apparatus of this embodiment is common to the essential configuration of the image forming apparatus of the first embodiment other than as described above, so reference will be made to FIGS. 1 to 3 as necessary.

Here, steps S501 to S503 in the flowchart of FIG. 20 are the same as steps S201 to S203 in the flowchart shown in FIG. 11, and steps S506 to S509 are the same as steps S206 to S209 in the flowchart shown in FIG. 11, so a description of the processing content of each of these steps will be omitted.

<Step S504> In this embodiment, the density correction control unit 64b of the mechanism control unit 64 shown in FIG. 3 receives a signal to perform density detection, and then repeats the operation of printing the density detection pattern PAT1 stored in advance in the memory unit 64c on the transfer belt 26 multiple times while decreasing the absolute value of the development voltage value by a predetermined amount of change. The mechanism control unit 64 converts the multiple output voltages that it has read into density values using the sensor detection voltage/density value conversion table 113 (FIG. 6).

<Step S505> In this step, the multiple density value data are compared with the target density values in the target print density value data table ₁₁₄ (Figure 7), and the developing bias voltage value when printing the density detection pattern PAT1 that is lower than the target density value and closest to the target density value is supplied to the developing roller 16 of the ID unit 12Y as the developing voltage value YDB1 (see formula (4)).

As described above, in this embodiment, multiple density detection patterns with varying development voltages are printed and detected, a density detection pattern that is lighter than the target density but close to the target density is detected, and the corresponding development bias voltage value is used, making it possible to accurately correct the density. In addition, by reducing the number of times that density detection patterns are printed during correction, toner consumption and correction time can be reduced.

In the above embodiment, an example was described in which the image forming device 11 is a color printer, but the image forming device 11 may be a multifunction printer, a facsimile, or a copier. Also, an example was described in which the image forming device 11 is a direct transfer type printer, but it may be an intermediate transfer type printer.

REFERENCE SIGNS LIST 11 image forming apparatus, 12 ID unit, 13 photoconductor drum, 14 charging roller, 15 LED head, 16 developing roller, 17 transfer roller, 18 toner supply roller, 19 developing blade, 20 toner cartridge, 21 transfer unit, 21a transfer belt drive roller, 21b transfer belt driven roller, 22 transfer discharge sensor, 23 fixing discharge sensor, 24 paper feed tray, 25 environment sensor, 26 transfer belt, 27 cleaning blade, 28 fixing device, 29 heating roller, 30 pressure roller, 31 heater, 32 fixing thermistor, 34 belt cleaning blade, 35 belt cleaner container, 36 density sensor, 36a infrared LED, 36b phototransistor for receiving specular reflected light, 36c phototransistor for receiving diffuse reflected light, 37 sensor cover, Reference Signs List 38 Belt thermistor, 40 Recording paper, 41 Paper guide section, 42 Paper guide section, 43 Hopping roller, 44 Paper detection sensor, 45 Registration roller pair, 48 Paper stacker section, 49 Paper detection sensor, 61 Communication section, 62 Command/image processing section, 62a Density tone correction control section, 62b Memory section, 63 LED head interface section, 64 Mechanism control section, 64a Density correction process execution determination section, 64b Density correction control section, 64c Memory section, 64d Density sensor light emission amount adjustment section, 64e Density correction history memory section, 71 High voltage control section, 71a Memory section, 72 Charging voltage generation section, 73 Development voltage generation section, 74 Supply voltage generation section, 75 Transfer voltage generation section, 81 Hopping motor, 82 Registration motor, 83 Belt motor, 84 Heat motor, 85 drum motor, 111 target gradation density value table, 112 gradation correction value table, 113 sensor detection voltage/density value conversion table, 114 target print density value data table, 115 development voltage value/density sensitivity table, 116 LED drive time/density sensitivity table.

Claims (10)

a developing means for developing the electrostatic latent image carried by the image carrier with a developer;
a transfer belt that faces the image carrier and transports a medium;
a transfer section for transferring the developer image formed on the image carrier;
a density detection means for detecting the developer image transferred onto the transfer belt;
a density correction means for printing a density correction pattern of a predetermined duty on the transfer belt with an initial developing bias and determining a correction value for correcting the initial developing bias from a detected density of the printed density correction pattern and a stored target density,
The image forming apparatus according to claim 1, wherein the density correcting means makes the absolute value of the initial developing bias smaller than the absolute value of a corrected developing bias corrected by the correction value. The image forming apparatus according to claim 1, characterized in that the density correction means includes a history storage unit that stores a history of correction value information including the correction value, predicts the corrected development bias to be obtained next based on the correction value information, and reduces the absolute value of the predicted corrected development bias by a predetermined adjustment value to set it as the initial development bias. The image forming apparatus according to claim 2, characterized in that the density correction means predicts the corrected development bias based on an average of the history of the correction value information. An environment detection means for measuring the temperature and humidity of the outside air is provided.
3. The image forming apparatus according to claim 2, wherein the history of the correction value information stored in the history storage section is stored for each environment when each piece of correction value information was obtained. The image forming apparatus according to claim 2, characterized in that the adjustment value is a value obtained by converting the variation in the detected density of the density correction pattern printed with the corrected development bias, which is obtained through an experiment, into a development voltage value. An environment detection means for measuring the temperature and humidity of the outside air is provided.
3. The image forming apparatus according to claim 2, wherein the adjustment value is a value that is stored in advance for each environment. The image forming apparatus according to claim 1 or 2, characterized in that the specified duty is 100%. The image forming apparatus according to claim 1, characterized in that the density correction means prints the density correction pattern multiple times with different densities, compares the detected density of each detected density correction pattern with the target density, and sets the development bias voltage value when a density detection pattern that is lower than the target density and closest to the target density is printed as the initial development bias. The image forming apparatus according to claim 8, characterized in that the specified duty is 100%. a developing means for developing the electrostatic latent image carried by the image carrier with a developer;
a transfer belt that faces the image carrier and transports a medium;
a transfer section for transferring the developer image formed on the image carrier;
a density detection means for detecting the developer image transferred onto the transfer belt;
a density correction means for printing a density correction pattern of a predetermined duty on the transfer belt and determining a development bias from a detected density of the printed density correction pattern and a stored target density,
an image forming apparatus characterized in that the density correction means prints the density correction pattern multiple times with different densities, compares the detected density of each detected density correction pattern with the target density, and sets the developing bias voltage value when a density detection pattern that is lower than the target density and closest to the target density is printed as the developing bias.

Images