Image forming device

Abstract

To provide an image formation device that can suppress image density highly accurately.SOLUTION: An image formation device comprises: an image formation unit that forms images on the basis of an image formation condition; an image density sensor 50 that reads test images formed by the image formation unit; and a CPU 401 that detects image density of a first test image on the basis of a reading result of the first test image by the image density sensor 50, and adjusts the image formation condition on the basis of the image density of the detected first image. The CPU 401 is configured to predict a charge amount of toner a developer accommodates; forms a plurality of first test images by the image formation unit in a plurality of image formation conditions based on the predicted charge amount thereof, and determine an image formation condition on the basis of a detection result by the image density sensor 50 of the first test image.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a printer, a copier, or a multifunction peripheral. [Background technology]

[0002] Image forming apparatuses are required to have stable image density for images printed on sheets. For example, electrophotographic image forming apparatuses adjust image forming conditions so that the maximum density of the image becomes a target density (hereinafter referred to as "target maximum density"). To achieve this, the image forming apparatus adjusts the image forming conditions so that the maximum density of the image becomes the target maximum density based on the results of reading test images formed under multiple different image forming conditions with an optical sensor. This type of image density adjustment control is called "DMAX control."

[0003] Patent Document 1 discloses an image forming apparatus that determines image formation conditions (maximum density condition values) that result in a target maximum density based on the image density of test patterns formed on a sheet under multiple image formation conditions. This image forming apparatus performs correction control to correct the maximum density condition value based on the image density of test patterns formed on a photosensitive member each time a predetermined number of sheets are passed. When the image forming apparatus next performs processing to determine the maximum density condition value, it forms test patterns on a sheet under multiple image formation conditions that are determined based on environmental conditions (temperature, humidity) so that the maximum density condition value corrected by the most recent correction control is included. This processing enables the maximum density condition value to be controlled with high precision. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-84053 Summary of the Invention [Problem to be solved by the invention]

[0005] Due to changes in the user's operating environment and diversification of print modes, the target maximum density may not be included within the density range of the test pattern formed under the predetermined range of image forming conditions. In this case, the image forming conditions cannot be determined with high accuracy. If the range of image forming conditions is widened too much so that the target maximum density is included within the image density range of the test pattern, the accuracy of image density control may decrease.

[0006] In Patent Document 1, the maximum density condition value is corrected by correction control, but for test patterns with high image density, the sensitivity of the detected image density to the development contrast is low, resulting in low correction accuracy. As a result, the accuracy of the maximum density condition value decreases as correction control is repeatedly performed. Furthermore, because the state of the developer in the developing device is unknown, the maximum density condition value cannot be adjusted with high precision.

[0007] SUMMARY OF THE INVENTION In view of the above problems, it is a primary object of the present invention to provide an image forming apparatus capable of controlling image density with high precision. [Means for solving the problem]

[0008] The image forming apparatus of the present invention includes an image forming unit that forms an image based on image forming conditions, a reading unit that reads a test image formed by the image forming unit, and a reading unit that detects an image density of the first test image based on a result of reading the first test image by the reading unit, and a reading unit that reads the image density of the first test image based on the detected image density of the first test image. The aforementioned and a control means for adjusting image formation conditions, wherein the image forming means comprises a photosensitive body having a photosensitive layer, a charging means for uniformly charging the photosensitive layer of the photosensitive body, an exposure means for irradiating the charged photosensitive layer with light to form an electrostatic latent image on the photosensitive layer, and a developing means for containing toner and causing the toner to adhere to the electrostatic latent image to develop the electrostatic latent image, the image forming conditions include a development contrast, The control means predicts the charge amount of the toner contained in the developing means, and The toner Charge amount determining a plurality of development contrasts based on the Multiple Development contrast forming a plurality of first test images by the image forming means; The control meansResults of reading the plurality of first test images by the reading means Development contrast that can achieve the target maximum density from The method is characterized in that: [Effects of the Invention]

[0009] According to the present invention, the image density can be controlled with high precision by adjusting the image forming conditions. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram illustrating the configuration of an image forming apparatus. [Figure 2] FIG. [Figure 3] FIG. 1 is a diagram illustrating the configuration of an image reading device. [Figure 4] FIG. [Figure 5] FIG. 2 is a diagram illustrating the configuration of an image density sensor. [Figure 6] FIG. [Figure 7] An explanatory diagram of ATR control. [Figure 8] 10 is a flowchart showing an initialization control process for a developing unit. [Figure 9] FIG. 4 is an explanatory diagram of a toner supply amount control unit. [Figure 10] FIG. 10 is an example of a test image. [Figure 11] 10 is a flowchart showing DMAX control. [Figure 12] 10 is a graph showing the relationship between detected density and the charge state of toner. [Figure 13] FIG. 10 is an explanatory diagram of a method for determining the range of development contrast. [Figure 14] 6A to 6C are explanatory diagrams of image formation conditions for each test pattern. [Figure 15] FIG. 10 is a diagram illustrating a Vc conversion table. [Figure 16] 6A and 6B are diagrams illustrating the relationship between the image density value of a test pattern and the development contrast. [Figure 17] Illustrative diagram of the effect on linear interpolation error (ΔD). [Figure 18] FIG. 10 is an explanatory diagram of a method for determining the range of development contrast. [Figure 19] 6A and 6B are diagrams illustrating the relationship between the image density value of a test pattern and the development contrast. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

[0012] (First embodiment) FIG. 1 is a configuration diagram of an image forming apparatus according to this embodiment. The image forming apparatus 100 is an electrophotographic color multifunction printer that employs a contact charging method and a two-component contact development method. The image forming apparatus 100 has an image reading device 110 and an operation unit 120 on top. The operation unit 120 is a user interface having an input interface and an output interface. The image forming apparatus 100 prints an image on a sheet S in accordance with instructions and settings input from the operation unit 120. The image forming apparatus 100 displays a setting screen and messages via the operation unit 120. The image reading device 110 reads an image from a document and generates image data representing the read image.

[0013] The image forming apparatus 100 includes an exposure unit 3, multiple (four in this embodiment) image forming units Pa, b, Pc, and Pd, an intermediate transfer belt 11, a secondary transfer roller 12, and a fixing unit 9. The exposure unit 3 has two light sources 31 and one rotary polygon mirror 32. The exposure unit 3 deflects laser beams output from the two light sources 31 using the rotary polygon mirror 32 and supplies laser beams La, Lb, Lc, and Ld to the image forming units Pa, Pb, Pc, and Pd. The image forming unit Pa forms a yellow (Y) toner image based on the laser beam La. The image forming unit Pb forms a magenta (M) toner image based on the laser beam Lb. The image forming unit Pc forms a cyan (C) toner image based on the laser beam Lc. The image forming unit Pd forms a black (K) toner image based on the laser beam Ld. Toner containers 8a, 8b, 8c, and 8d are connected to the image forming units Pa, Pb, Pc, and Pd to replenish the toner consumed in forming toner images. When it is not necessary to distinguish between toner colors, the suffixes a, b, c, and d are omitted from the reference numerals.

[0014] The toner images of each color formed at the image forming stations Pa, Pb, Pc, and Pd are transferred in a superimposed manner onto an intermediate transfer belt 11. The intermediate transfer belt 11 is an image carrier that carries the toner images of each color transferred from the image forming stations Pa, Pb, Pc, and Pd. The intermediate transfer belt 11 rotates in the direction of the arrow and transports the toner images of each color that it carries to a secondary transfer roller 12.

[0015] The image forming apparatus 100 has a sheet cassette 14 that stores sheets S, a feed roller 15 for feeding the sheets S, and multiple conveying roller pairs 16. The feed roller 15 supplies the sheets S from the sheet cassette 14 to a conveying path. The multiple conveying roller pairs 16 convey the sheets S supplied to the conveying path to a secondary transfer roller 12. The conveying roller pair 16 closest to the secondary transfer roller 12 conveys the sheet S to the secondary transfer roller 12 in accordance with the timing at which the toner image carried on the intermediate transfer belt 11 is conveyed to the secondary transfer roller 12.

[0016] The secondary transfer roller 12 forms a secondary transfer nip by contacting with the intermediate transfer belt 11. When the sheet S passes through the secondary transfer nip, the toner image is transferred from the intermediate transfer belt 11 to the sheet S. The secondary transfer roller 12 transports the sheet S with the transferred toner image to the fixing device 9. The fixing device 9 fixes the toner image transferred to the sheet S. In this way, the image is printed on the sheet S. The sheet S with the printed image is discharged to the outside of the image forming apparatus 100 by, for example, a discharge section 17 having a pair of discharge rollers.

[0017] An image density sensor 50 is disposed near the intermediate transfer belt 11, between the image forming units Pa to Pd and the secondary transfer roller 12. The image density sensor 50 is used to detect the image density (optical density) of the toner image carried by the intermediate transfer belt 11. For example, the image forming apparatus 100 uses the image density sensor 50 to detect the image density of a test image for detecting the image density. The image forming apparatus 100 performs control (DMAX control) of the target maximum image density (target maximum density) based on the detected image density. Through DMAX control, the image formation conditions are adjusted so that the maximum density of the image formed on the sheet S (image density when the density signal is 100%) becomes the target maximum density. The image formation conditions adjusted for image density control include the exposure amount, charging bias voltage, developing bias voltage, and development contrast. The image forming apparatus 100 also includes an environmental sensor 60 for detecting environmental conditions such as environmental temperature and environmental humidity. The environmental conditions detected by the environmental sensor 60 can also be used for image density control.

[0018] FIG. 2 is a diagram illustrating the configuration of image forming units Pa, Pb, Pc, and Pd. The image forming units Pa, Pb, Pc, and Pd have the same configuration and form toner images using the same operation. Here, the color of the toner image they form will not be distinguished, and they will be described as image forming unit P. The image forming unit P includes a photosensitive drum 1, a charging roller 2, a developing unit 4, and a primary transfer roller 7. The photosensitive drum 1 is a drum-shaped photosensitive body having a photosensitive layer on its surface, and is rotatable around the drum axis in the direction of arrow Y. Around the photosensitive drum 1, the charging roller 2, the developing unit 4, and the primary transfer roller 7 are arranged, in this order from the upstream side in the direction of arrow Y. The photosensitive drum 1 is an image carrier that carries the toner image formed by the charging roller 2, the exposure unit 3, and the developing unit 4.

[0019] The charging roller 2 is a charger that receives a charging bias voltage Vd from a high-voltage power supply 101 and uniformly charges the surface of the photosensitive drum 1. The charging roller 2 has a core metal whose ends are rotatably held by bearing members (not shown). Both ends of the core metal of the charging roller 2 are biased toward the photosensitive drum 1 by pressure springs 21. In other words, the charging roller 2 is pressed against the surface of the photosensitive drum 1 with a predetermined pressure. This causes the charging roller 2 to rotate in response to the rotation of the photosensitive drum 1. The charging bias voltage Vd is, for example, an oscillating voltage obtained by superimposing a DC voltage and an AC voltage. The waveform of the AC voltage is, for example, a sine wave. The frequency of the AC voltage is 1.3 kHz. The peak-to-peak voltage Vpp of the AC voltage is 1.5 kV. The DC voltage is, for example, -600 V. In this case, the surface potential of the photosensitive drum 1 becomes −600 [V] (dark potential), which is the same as the DC voltage applied to the charging roller 2.

[0020] The uniformly charged surface of the photosensitive drum 1 is irradiated with laser light L from an exposure device 3. The exposure device 3 irradiates the surface of the photosensitive drum 1 with laser light L modulated according to image data, and forms an electrostatic latent image corresponding to the image data on the surface of the photosensitive drum 1. The image data is input from an external device such as an image reading device 110 or a personal computer.

[0021] The developing device 4 reversely develops the toner image by adhering toner to the electrostatic latent image. The developing device 4 has a container 40 that contains toner, an agitating screw 42 that agitates the toner and carrier, and a developing sleeve 41 that is disposed opposite the photosensitive drum 1. The container 40 contains, for example, a two-component developer in which non-magnetic toner and magnetic carrier are mixed at a predetermined ratio. A developing bias voltage Vdc is applied to the core of the developing sleeve 41 from a high-voltage power supply 102. The developing bias voltage Vdc is a voltage for developing the electrostatic latent image.

[0022] The non-magnetic toner used in the developing device 4 is, for example, negative polarity. The developing sleeve 41 is a non-magnetic developer carrier and rotates in the direction of arrow X. A portion of the outer circumferential surface of the developing sleeve 41 is exposed to the outside of the developing device 4. The closest distance (SD gap) between the developing sleeve 41 and the photosensitive drum 1 is, for example, 260 μm. The electrostatic latent image formed on the photosensitive drum 1 is developed at a position facing the developing sleeve 41.

[0023] The developing bias voltage Vdc is an oscillating voltage obtained by superimposing a DC voltage and an AC voltage. The DC voltage is, for example, -450 [V]. The AC voltage has a frequency of 8.0 [kH]. The AC voltage has a peak-to-peak voltage Vpp of 1.8 [kV]. The AC voltage has a rectangular waveform. The toner is transported from the developing sleeve 41 to the surface of the photosensitive drum 1 by an electric field generated between the developing bias Vdc and the potential on the surface of the photosensitive drum 1, and the electrostatic latent image is reverse-developed.

[0024] The toner image developed by the developing device 4 is transported to a primary transfer nip formed by the contact between the primary transfer roller 7 and the photosensitive drum 1 as the photosensitive drum 1 rotates. A high-voltage power supply 103 applies a primary transfer bias voltage Vpr to the primary transfer roller 7. Application of the primary transfer bias voltage Vpr promotes the transfer of the toner image from the photosensitive drum 1 to the intermediate transfer belt 11. The primary transfer bias voltage Vpr is a voltage of positive polarity, which is opposite to the charge polarity (negative polarity) of the toner. For example, the primary transfer bias voltage Vpr is +1 kV.

[0025] The image forming unit P may be provided with an image density sensor that detects the image density of the toner image formed on the photosensitive drum 1. In this case, the image density sensor is provided facing the surface of the photosensitive drum 1 between the position where the toner image is developed by the developing unit 4 and the position where the toner image is transferred onto the intermediate transfer belt 11 by the primary transfer roller 7.

[0026] 3 is a configuration diagram of image reading device 110. Image reading device 110 includes a first mirror unit 301, a second mirror unit 302, an image sensor 303, a lens 304, a motor 305, a size sensor 306, and a home position sensor 307 within its housing. A platen glass 310 is provided on the top of the housing of image reading device 110. First mirror unit 301 includes an illumination light source 311 and a first mirror 312. Second mirror unit 302 includes a second mirror 313 and a third mirror 314. First mirror unit 301 and second mirror unit 302 are driven by motor 305 and can move back and forth in the direction of arrow Z.

[0027] Original document 390 is placed on original document glass 310 with the side on which an image is printed facing original document glass 310. When an instruction to read original document 390 is given by operation unit 120 after original document 390 has been placed on original document glass 310, motor 305 is activated to move first mirror unit 301 and second mirror unit 302 to their home positions. Home position sensor 307 detects that first mirror unit 301 and second mirror unit 302 have moved to their home positions.

[0028] When the first mirror unit 301 and the second mirror unit 302 move to their home positions, the first mirror unit 301 turns on the illumination light source 311 to irradiate light onto the reading surface (the surface on which an image is printed) of the original 390. As the first mirror unit 301 and the second mirror unit 302 move in the direction of the arrow Z, the first mirror 312, the second mirror 313, and the third mirror 314 guide the reflected light from the original 390 to the lens 304. The lens 304 forms an image of the reflected light on the light receiving surface of the image sensor 303. The image sensor 303 receives the reflected light and converts it into an electrical signal. Predetermined processing is performed on this electrical signal to generate image data representing the image of the original 390.

[0029] (Control unit) 4 is a configuration diagram of a control unit for controlling the operation of image forming apparatus 100 configured as above. Control unit 400 includes a CPU (Central Processing Unit) 401. CPU 401 executes a control program to control the operation of each unit of image forming apparatus 100 and print an image on sheet S. CPU 401 also includes a built-in memory 490 for storing various data.

[0030] A density signal generation unit 402 is connected to the CPU 401 to control the exposure unit 3. The density signal generation unit 402 converts image data supplied from a print image processing unit 414 (described later) into a density signal (laser control signal) and supplies it to the exposure unit 3. The density signal generation unit 402 also includes a generation unit that generates a density signal of a test image used to determine image formation conditions that can achieve a target maximum density. The exposure unit 3 drives the light source 31 based on the density signal acquired from the density signal generation unit 402 to output laser beams La to Ld.

[0031] A read control unit 406 and a read image processing unit 405 are connected to the CPU 401 in order to control the image reading device 110. The read control unit 406 controls the lighting of the illumination light source 311 of the image reading device 110 and the driving of the motor 305 in response to commands from the CPU 401. The read image processing unit 405 acquires an electrical signal from the image sensor 303 of the image reading device 110, generates an image signal, and transmits it to the CPU 401. The read image processing unit 405 converts the electrical signal into an image signal using, for example, a lookup table LUTid.

[0032] A motor control unit 407 for controlling various motors is connected to the CPU 401. The motor control unit 407 controls the drive of various motors that serve as drive sources for various parts of the image forming apparatus 100 in response to commands from the CPU 401. The motors controlled by the motor control unit 407 include a motor that drives the feed roller 15, a motor that drives the pair of conveying rollers 16, a motor that drives the image forming unit P, a motor that drives the intermediate transfer belt 11, and a motor that drives the fixing unit 9.

[0033] A high-voltage control unit 408 is connected to the CPU 401 to control the high-voltage power supplies 101, 102, and 103. The high-voltage control unit 408 controls the high-voltage power supplies 101, 102, and 103 in response to commands from the CPU 401. The high-voltage power supplies 101, 102, and 103 output a charging bias voltage Vd, a developing bias voltage Vdc, and a primary transfer bias voltage Vpr under the control of the high-voltage control unit 408.

[0034] The CPU 401 is connected to an I / F unit 409 that controls the interface with the operation unit 120, and a timer 410. The operation unit 120 includes a display unit 411 that is an output interface and an input unit 412 that is an input interface. The display unit 411 is a liquid crystal display, an organic EL display, or the like. The input unit 412 is a keyboard, a touch sensor, or the like. The operation unit 120 may be an external device such as a personal computer connected to the image forming apparatus 100. The timer 410 can be used to measure time during various processes.

[0035] A controller 413 and a print image processing unit 414 are connected to the CPU 401. The CPU 401 acquires an image signal 415 via the controller 413. The image signal 415 can be acquired from the image reading device 110 as described above, or from an external device 450 such as a host computer. The CPU 401 processes the image signal 415 using the print image processing unit 414 to generate image data. The image data generated by the print image processing unit 414 is sent to the density signal generation unit 402. This makes it possible to form an image on the sheet S in accordance with the image signal 415.

[0036] The print image processing unit 414 performs, for example, the development of the image signal 415 into a bitmap image and the conversion of the image signal 415 into a color space (e.g., RGB to YMCK). The density signal generation unit 402 corrects the gradation characteristics of the image data using a gamma correction table and binarizes the image data using a predetermined dither matrix, and generates a laser control signal. The laser control signal is generated for each color of the image to be formed. The exposure unit 3 controls the emission of laser beams La, Lb, Lc, and Ld by the laser control signal.

[0037] The CPU 401 is connected to a video signal counting unit 207 and a toner supply amount control unit 150. The CPU 401 generates a video count value by accumulating the pixel values ​​of each pixel of the image represented by the image signal 415 over the entire image using the video signal counting unit 207. For example, if the image signal 415 represents an image with a resolution of 600 dpi, the accumulated pixel values ​​range from 0 to 255. The video count value reaches a maximum value of 529 when, for example, the pixel values ​​of all pixels on one side of an A4 size image are 255. The video count value and a detection value by a magnetic permeability sensor 45 (described later) are sent to the toner supply amount control unit 150. The toner supply amount control unit 150 controls the supply of toner from the toner container 8 to the developing device 4 of the corresponding image forming unit P based on the video count value and the detection value by the magnetic permeability sensor.

[0038] The CPU 401 is connected to the environment sensor 60, the image density sensor 50, and the magnetic permeability sensor 45. The CPU 401 acquires the detection results from the environment sensor 60, the image density sensor 50, and the magnetic permeability sensor 45, and performs various processes. The magnetic permeability sensor 45 is used to detect the amount of toner in the developing device 4 and to predict the charge amount of the toner.

[0039] (Image density sensor) FIG. 5 is a configuration diagram of the image density sensor 50. The image density sensor 50 of this embodiment is a specular reflection type optical sensor that detects specularly reflected light from a test pattern PT, which is a test image on the intermediate transfer belt 11. The image density sensor 50 includes a light-emitting element 601, a light-receiving element 602, and a sensor control unit 603. The light-emitting element 601 is a light source that outputs light toward a measurement position. The light-emitting element 601 is, for example, a light-emitting diode. The light-receiving element 602 is an element that receives specularly reflected light from the test pattern PT or the intermediate transfer belt 11 that passes through the measurement position. The light-receiving element 602 is, for example, a photodiode. The sensor control unit 603 controls the amount of light emitted (amount of irradiation light) from the light-emitting element 601 as one of the irradiation conditions. The sensor control unit 603 controls the light-emitting element 601 in accordance with instructions from the CPU 401.

[0040] The light-emitting element 601 is disposed so that its optical axis forms a 45-degree angle with respect to the normal to the intermediate transfer belt 11. The light-receiving element 602 is disposed so that it is symmetrical to the light-emitting element 601 with respect to the normal to the intermediate transfer belt 11. The light-receiving element 602 outputs a current according to the light-receiving result (reflected light level). The sensor control unit 603 converts the current output from the light-receiving element 602 into a voltage, and further converts the voltage into a digital value and transmits it to the CPU 401. The digital value is the detection result (output value) of the image density sensor 50.

[0041] FIG. 5 illustrates an example in which a test pattern PT included in a test image passes the measurement position. The print image processing unit 414 has a conversion table that converts the output values ​​of the image density sensor 50 into image density values ​​of the test pattern PT. The output values ​​include luminance values. The CPU 401 converts the output values ​​of the image density sensor 50 into image density values ​​using the print image processing unit 414. The CPU 401 may store the conversion table in memory 490 and convert the output values ​​into image density values. The conversion table is created in advance in accordance with the output characteristics of the image density sensor 50 and is stored in advance in the memory within the print image processing unit 414 or in memory 490.

[0042] The image density sensor 50 may be a diffuse reflection type optical sensor that receives diffusely reflected light. A specular reflection type optical sensor can detect yellow, magenta, cyan, and black toner images. However, the detection accuracy of a specular reflection type optical sensor decreases when the density signal of the test pattern PT approaches 100%. A diffuse reflection type optical sensor can detect yellow, magenta, and cyan toner images even when the density signal is 100%. However, a diffuse reflection type optical sensor cannot detect black toner images. Therefore, ideally, the image density sensor 50 is configured with both a specular reflection type optical sensor and a diffuse reflection type optical sensor. Such an image density sensor 50 can accurately detect yellow, magenta, cyan, and black test patterns PT even when the density signal is 100%.

[0043] (developer) Figure 6 is a configuration diagram of the developing unit 4. Figure 6 is a cross-sectional view of the developing unit 4 in Figure 2 as seen from above. As described above, the developing unit 4 contains a two-component developer including non-magnetic toner and a magnetic carrier. To this end, the developing unit 4 has a developing container 40 that contains the two-component developer.

[0044] The interior of the developing container 40 is divided into a developing chamber 40a and an agitating chamber 40b by a partition wall 40c extending in the vertical direction. A screw member 42a is disposed in the developing chamber 40a. A screw member 42b is disposed in the agitating chamber 40b. A toner supply port 43 through which toner is replenished from the toner container 8 is provided in the agitating chamber 40b. The toner supply port 43 communicates with the toner container 8. The partition wall 40c forms, at both longitudinal ends thereof, delivery portions 46a, 46b that allow the developer to pass between the developing chamber 40a and the agitating chamber 40b.

[0045] Each of the screw members 42a and 42b is configured with a spiral blade around a magnetic shaft (rotation shaft) as a developer transport section. In addition to the spiral blade, the screw member 42b is provided with an agitating rib 42c that protrudes radially from the shaft and has a predetermined width in the developer transport direction. The agitating rib 42c agitates the developer as the shaft rotates.

[0046] The screw member 42a agitates and transports the developer in the developing chamber 40a. The screw member 40b performs automatic toner replenishment control (hereinafter referred to as "ATR (Automatic Toner Replenisher) control") to equalize the toner concentration. That is, the screw member 40b agitates and transports the toner supplied from the toner replenishing port 43 and the developer consisting of toner and magnetic carrier already in the agitating chamber 40b, thereby equalizing the toner concentration of the developer.

[0047] The screw members 42a and 42b are disposed substantially parallel to each other along the direction of the rotational axis of the developing sleeve 41. The screw members 42a and 42b transport the developer in opposite directions along the direction of the rotational axis of the developing sleeve 41. This causes the developer to circulate within the developing container 40 via the delivery portions 46a and 46b. In other words, the developer in the developing chamber 40a, whose toner concentration has decreased as a result of toner consumption in the developing process, is transported into the stirring chamber 40b via one of the delivery portions (for example, delivery portion 46b).

[0048] A magnetic permeability sensor 45 is provided in the stirring chamber 40b as a sensor for detecting the toner concentration of the developer. The CPU 401 determines whether to perform ATR control based on the toner concentration of the developer detected by the magnetic permeability sensor 45 and the image density of the toner image detected by the image density sensor 50. For example, the CPU 401 performs ATR control when the toner concentration of the developer is lower than a predetermined toner concentration or when the image density of the toner image is lower than a predetermined image density. When performing ATR control, the CPU 401 controls the operation of the toner container 8 to replenish toner from the toner container 8 to the toner replenishing port 43. The replenished toner is mixed with the developer in the stirring chamber 40b while being stirred by the screw member 42b and is then transported into the developing chamber 40a via the other delivery section (e.g., delivery section 46a). The developer in the developing chamber 40a is supplied to the developing sleeve 41 by the screw member 42b.

[0049] 7 is an explanatory diagram of ATR control. The toner container 8 has an outlet 82 for discharging toner. A replenishment conveying path 83 is connected to the outlet 82, which communicates with the toner supply port 43 of the developing unit 4. The toner discharged from the outlet 82 is supplied to the toner supply port 43 via the replenishment conveying path 83.

[0050] The toner storage container 8 includes a container portion 8a having a spiral groove formed on the inner wall of the cylindrical container, and a pump portion 8b having a pump 81 and an outlet 82. As the container portion 8a rotates, the toner is transported to the outlet 82 of the pump portion 8b. The toner transported to the outlet 82 is discharged through the outlet 82 into a replenishment conveying path 83 due to changes in air pressure generated by the pump 81, which changes the volume inside the toner storage container 8.

[0051] The magnetic permeability sensor 45 will now be described. The developer in the developing device 4 is a two-component developer containing non-magnetic toner and magnetic carrier. As the toner concentration changes, the magnetic permeability also changes depending on the mixture ratio of the non-magnetic toner and magnetic carrier. Therefore, the toner concentration of the developer can be detected by detecting the magnetic permeability. The magnetic permeability sensor 45 of this embodiment can output an output voltage corresponding to the change in magnetic permeability of the developer, that is, the toner concentration, as the detection result, using, for example, the inductance of a coil.

[0052] The magnetic permeability sensor 45 is wired with four wires: one for applying a predetermined power supply voltage (e.g., 5.0 V), one for applying voltage, one for grounding, and one for outputting the detection result. The magnetic permeability sensor 45 detects the magnetic permeability of the developer near the sensor surface. As shown in FIG. 6, the magnetic permeability sensor 45 is provided on the delivery section 46a side of the stirring chamber 40b. Therefore, the magnetic permeability sensor 45 can detect the magnetic permeability of the developer in a state where the developer circulated from the developing chamber 40a to the stirring chamber 40b and the toner newly supplied from the toner supply port 43 are mixed and sufficiently stirred. With this configuration, the magnetic permeability sensor 45 can prevent erroneous detection of the toner concentration due to insufficient stirring.

[0053] When the toner concentration of the developer in the developing unit 4 decreases, the proportion of magnetic carrier contained in the developer per unit volume increases relatively near the sensor surface of the magnetic permeability sensor 45, and the apparent magnetic permeability of the developer increases. In this case, the output voltage (detection result) output from the magnetic permeability sensor 45 increases. When the toner concentration of the developer in the developing unit 4 increases, the proportion of magnetic carrier contained in the developer per unit volume decreases relatively near the sensor surface of the magnetic permeability sensor 45, and the apparent magnetic permeability of the developer decreases. In this case, the output voltage (detection result) output from the magnetic permeability sensor 45 decreases.

[0054] The output voltage output by the magnetic permeability sensor 45 varies depending on the bulk density of the developer, even when developer with the same toner concentration is present near the sensor surface of the magnetic permeability sensor 45. For example, in a high-temperature, high-humidity environment, the amount of toner charge in the developer decreases, which reduces the Coulomb repulsive force between toner particles or carrier particles, thereby increasing the bulk density of the developer. On the other hand, in a low-temperature, low-humidity environment, the amount of toner charge in the developer increases, which increases the Coulomb repulsive force between toner particles or carrier particles, thereby decreasing the bulk density of the developer. In other words, the detection results of the magnetic permeability sensor 45 may vary depending on environmental conditions (temperature, humidity). To prevent the detection results of the magnetic permeability sensor 45 from varying depending on environmental conditions, the developing unit 4 undergoes initialization control processing.

[0055] 8 is a flowchart showing the initialization control process for the developing device 4. By performing the initialization control process, the magnetic permeability sensor 45 outputs the same output voltage regardless of the environmental conditions, as long as the toner concentration of the developer is the same. In this initialization control process, the image formation conditions when performing ATR control are determined according to the detection result of the image concentration of the toner image.

[0056] When a new developing device 4 is attached, the CPU 401 performs a developer stirring operation for a predetermined time (for example, one minute) to stabilize the charge amount of the developer contained in the developing device 4 (S141). At this time, in order to prevent the toner in the developer from flying to the photosensitive drum 1 and changing the toner concentration, a predetermined voltage is applied to the developing sleeve 41 and the photosensitive drum 1 to create a predetermined potential difference.

[0057] The CPU 401 varies the control voltage applied to the magnetic permeability sensor 45 within a predetermined range, for example, by 0.1 [V], and acquires the output voltage of the magnetic permeability sensor 45 output in response to the change. The developing device 4 contains a developer that has been adjusted to a target density before shipping. The CPU 401 sets the control voltage obtained when a predetermined output voltage (2.0 [V] in this case) corresponding to the target toner density is acquired as the control voltage (Vcnt1) to be applied to the magnetic permeability sensor 45 for proper toner density detection (S142). When the control voltage (Vcnt1) is applied to the magnetic permeability sensor 45, the magnetic permeability sensor 45 can output the same output voltage regardless of environmental conditions, as long as the toner density of the developer is the same.

[0058] The CPU 401 forms a plurality of fixed test images with an area ratio of 60% on the intermediate transfer belt 11 while changing the image forming conditions (S143). Here, the image forming conditions are fixed settings of the charging bias voltage Vd at -600 [V] and the developing bias voltage Vdc at -450 [V], and the exposure amount of the laser light L from the exposure device 3 is changed to five levels. The test images are images for detecting the image density of the toner image.

[0059] The CPU 401 detects the image density of the test image from the detection result of the test image by the image density sensor 50. The CPU 401 determines the exposure amount when the detection result of the image density (image density value) becomes a predetermined value (here, 0.8) as the exposure amount LPW-ATR of the image forming conditions for forming the test image when toner is replenished according to the detection result of the image density sensor 50. Other image forming conditions are a charging bias voltage Vd of −600 [V] and a developing bias voltage Vdc of −450 [V]. The CPU 401 determines the image forming conditions (charge bias voltage -600V, development bias voltage -450V, exposure amount LPW-ATR) to be used for forming a test image when toner is replenished according to the detection result of the image density sensor 50 (S144). Note that the image forming conditions that can be changed in the process of S143 are not limited to the exposure amount, and may be any parameter that affects toner consumption, such as the charge bias voltage Vd or the development bias voltage. Furthermore, in the process of S143, these conditions may be changed in combination.

[0060] (ATR control) In ATR control, the toner supply amount control unit 150 derives the necessary amount of toner to be supplied based on the results of detecting and estimating the amount and state of toner in the developing unit 4. The toner supply amount control unit 150 automatically supplies toner to the developing unit 4 at an appropriate timing. An example of the appropriate timing is when the toner concentration decreases due to toner consumption during image formation.

[0061] In the ATR control of this embodiment, the amount of toner replenishment is calculated by adding together the amount of replenishment based on the estimated value of toner consumption using the video count value and the amount of replenishment based on the toner concentration detection result by the magnetic permeability sensor 45. Toner replenishment amount control based on the estimated value of toner consumption using the video count value is called "video count replenishment control." Toner replenishment amount control based on the replenishment amount based on the toner concentration detection result by the magnetic permeability sensor 45 is called "inductance replenishment control."

[0062] At this time, the target value of the toner concentration (toner concentration target) detected by the magnetic permeability sensor 45 is not constant but is constantly corrected. The toner concentration target is corrected by forming a fixed test image with an area ratio of 60% on the intermediate transfer belt 11 under the image forming conditions determined in the initialization control process, and appropriately controlling the detection of the image density by the image density sensor 50. This type of control of the toner concentration target is called "ATR patch control."

[0063] In ATR patch control, a fixed test image is formed under the same image forming conditions, so the image density detected by the image density sensor 50 reflects the toner charge state (Q / M). The toner charge state (Q / M) is the amount of toner charge per unit mass. If the toner charge state (Q / M) is low, the detected image density will be high, and if the toner charge state (Q / M) is high, the detected image density will be low.

[0064] In this embodiment, a test image for ATR patch control (hereinafter referred to as "ATR patch control pattern") is formed at predetermined intervals. For example, an ATR patch control pattern is formed every time a predetermined number of sheets S are passed through (for example, every 200 A4-sized sheets). The ATR patch control pattern is formed under image forming conditions determined by the initialization control process for the developer 4. In this embodiment, the image forming conditions are a charging bias voltage of -600 [V], a developing bias voltage of -450 [V], and an exposure amount LPW-ATR. In this embodiment, the target image density of the ATR patch control pattern is set to approximately 0.90. This is because this image density range is suitable for detecting the toner charge state (Q / M).

[0065] FIG. 9 is an explanatory diagram of the toner supply amount control unit 150. As described above, the toner supply amount is the sum of the supply amounts calculated by the video count supply control and the inductance supply control. The toner supply amount is calculated in units of the mass [mg] of the toner to be supplied. The toner supply amount control unit 150 includes a video count supply control unit 160, an inductance supply control unit 170, and a toner patch control unit 180. The toner supply amount control unit 150 is connected to a video signal count unit 207, a magnetic permeability sensor 45, an image density sensor 50, and a toner container drive control unit 190.

[0066] In the video count replenishment control, the video count replenishment control unit 160 acquires the video count value generated by the video signal count unit 207. The video count replenishment control unit 160 converts the acquired video count value into the mass of the toner to be replenished using a conversion table stored in advance, thereby deriving the amount of toner to be replenished.

[0067] In inductance replenishment control, the inductance replenishment control unit 170 acquires an output voltage value corresponding to the toner concentration from the magnetic permeability sensor 45. The inductance replenishment control unit 170 performs AD conversion on the acquired output voltage value to generate an 8-bit digital signal value. Hereinafter, this digital signal value will be referred to as the "toner concentration output value." The replenishment amount is calculated from the difference between the toner concentration output value and its target value. The target value is generated by the toner patch control unit 180. The toner patch control unit 180 determines the target value according to the output value of the image density sensor 50.

[0068] Toner supply amount control unit 150 adds up the supply amounts derived by video count supply control unit 160 and inductance supply control unit 170, and derives the amount of toner to be supplied to developing unit 4. Toner supply amount control unit 150 transmits the derived amount of toner to be supplied to toner container drive control unit 190. Toner container drive control unit 190 operates toner container 8 to supply the developing unit 4 with the amount of toner to be supplied.

[0069] Video count replenishment control is a feedforward control, and the replenishment amount is always a positive value. Inductance replenishment control is a feedback control performed by detecting the toner concentration in the developer 4 using the magnetic permeability sensor 45, and the replenishment amount can be either a positive or negative value. Any excess or deficiency in the replenishment amount calculated by video count replenishment control is adjusted by the replenishment amount calculated by inductance replenishment control, so fluctuations in the toner concentration in the developer 4 can be suppressed.

[0070] (DMAX control) Image density correction is broadly divided into "DMAX control" and "gradation correction control." DMAX control is performed by correcting the development contrast Vc by adjusting image formation conditions related to image density, such as the exposure amount LPW, charging bias voltage Vd, and developing bias voltage Vdc. Gradation correction control is performed by correcting image data using a gamma correction table. The development contrast Vc is the potential difference between the potential of the electrostatic latent image (electrostatic latent image potential) generated by irradiating the surface of the photosensitive drum 1, which is charged with the charging bias voltage Vd, with laser light of exposure amount LPW, and the developing bias voltage Vdc. The amount of toner used to develop the electrostatic latent image changes depending on the development contrast Vc. Note that if the charging bias and developing bias contain AC components, their DC components become the charging bias voltage Vd and the developing bias voltage Vd.

[0071] In this embodiment, when performing DMAX control, which adjusts image formation conditions based on the detection results of image density of test images formed under multiple different image formation conditions, the charge state of toner in the developing unit 4 is predicted with high accuracy. The range of variation of image formation conditions during DMAX control is determined based on the predicted result of the toner charge amount. This allows for highly accurate adjustment of image formation conditions without increasing control time. DMAX control is performed every time several thousand sheets S are passed through, or in response to a user instruction from the operation unit 120.

[0072] In this embodiment, a case will be described in which a test image is formed on the intermediate transfer belt 11 and DMAX control is performed based on the results of detecting this test image with the image density sensor 50. However, DMAX control can also be performed based on the results of forming a test image on the sheet S and reading it with the image reading device 110, or the results of forming a test image on the photosensitive drum 1 and detecting it with the image density sensor 50. Furthermore, DMAX control is performed for each of the colors yellow, magenta, cyan, and black.

[0073] In this embodiment, the toner charge state (Q / M) is predicted based on, for example, a prediction model of the toner charge amount based on the toner supply amount, toner consumption amount, etc., or on the detected density by ATR patch control. In this embodiment, the accuracy of DMAX control can be improved by changing the variation range of the image formation conditions during DMAX control based on the predicted value of the toner charge state (Q / M).

[0074] FIG. 10 is an example of a test image for adjusting the development contrast to achieve a target maximum density. The test image 700 includes a plurality of (here, N) test patterns PT_Yi, PT_Mi, PT_Ci, and PT_Ki formed with yellow, magenta, cyan, and black toners. "i" is an integer between 1 and N. In the example of FIG. 10, N=5. The minimum value of N is 2. The larger N is, the more improved the control accuracy is, but the longer the control time is. Therefore, N is determined depending on the relationship between the control accuracy and the control time targeted in the design.

[0075] The test patterns PT are formed in a row in the rotation direction F1 of the intermediate transfer belt 11. The image density sensor 50 is disposed so that its detection position is the position of each test pattern PT on the intermediate transfer belt 11. The image density sensor 50 detects the test patterns PT that pass the detection position as the intermediate transfer belt 11 rotates.

[0076] The test patterns PT are rectangular, each measuring, for example, 25 mm x 25 mm. N test patterns PT are formed for each color, but the image formation conditions used to form each test pattern PT are different. For example, when forming N test patterns PT, N combinations of exposure amount LPW, charging bias voltage Vd, and development bias voltage Vdc are prepared in advance. The N combinations correspond one-to-one to the N development contrasts Vc. In this example, five test patterns PT are formed based on five development contrasts Vc1, Vc2, Vc3, Vc4, and Vc5. Therefore, the test image 700 has a total of 20 test patterns PT for all colors.

[0077] The density signals (density gradients) of the 20 test patterns PT are all the same. In other words, if the image formation conditions are the same, the test patterns PT for each color will all have the same image density. If the density signals of the test patterns PT are too high, it will be difficult for the specular reflection type image density sensor 50 to detect the test patterns PT accurately. For this reason, in this embodiment, the density signals of the test patterns PT are set to 100%.

[0078] 11 is a flowchart showing the DMAX control. The DMAX control is started when an instruction to execute the DMAX control is input from the operation unit 120 or when the number of image forming operations reaches a predetermined number. The DMAX control is performed independently for each of the colors yellow, magenta, cyan, and black.

[0079] The CPU 401 starts DMAX control by executing a control program stored in the memory 490 (S901). The CPU 401 acquires a predicted value of the toner charge state (Q / M) in the developing unit 4 (S902). As described above, the predicted value of the toner charge state (Q / M) can be acquired based on a prediction model of the toner charge amount or on the detected density by ATR patch control. Here, we will explain the case where the predicted value of the toner charge state (Q / M) is obtained based on the image density obtained by detecting the latest ATR patch control pattern. Figure 12 is a graph showing the relationship between the image density (detected density) detected from the ATR patch control pattern and the toner charge state (Q / M).

[0080] This graph shows an approximation curve (solid line) obtained from the detected density and charge state (Q / M) values ​​detected in advance through experiments. A function representing this approximation curve or a conversion table representing the approximation curve is stored in memory 490. CPU 401 references the function or conversion table stored in memory 490 and obtains the toner charge state (Q / M) corresponding to the image density obtained by detecting the latest ATR patch control pattern.

[0081] As shown by the dashed line, the detected density varies greatly at high densities, resulting in lower accuracy. This is due to the characteristics of the image density sensor 50. In this embodiment, the target density of the ATR patch control pattern is set to approximately 0.90, enabling highly accurate detection of the toner charge state (Q / M).

[0082] In this embodiment, the image density of the ATR patch control pattern is used as the toner charge state (Q / M) because the ATR patch control pattern is a fixed image pattern formed under the same image forming conditions, and the detected density reflects the toner charge state (Q / M). Another reason is that a density range with high detection accuracy (approximately 0.90) is used.

[0083] The CPU 401 determines the range of the development contrast Vc of the multiple test patterns PT formed by DMAX control based on the predicted value of the toner charge state (Q / M) (S903). Fig. 13 is an explanatory diagram of a method for determining the range of the development contrast Vc of the multiple test patterns PT. Fig. 13 explains the method of this embodiment in comparison with a comparative example.

[0084] In the comparative example, the predicted value of the toner charge state (Q / M) is unknown. Therefore, the toner charge amount is predicted based on the environmental conditions (temperature, humidity, etc.) detected by the environmental sensor 60. Based on this predicted value of the toner charge amount, a range (Vc1 to Vc5) of the development contrast Vc corresponding to a range including the upper limit (60) and lower limit (20) of the toner charge state (Q / M) is set. In this embodiment, the development contrast Vc is set by selecting from the range (Vc1 to Vc5) of the development contrast Vc in accordance with the predicted range of the toner charge state (Q / M) based on the detected density (ATRDens) of the ATR patch control pattern. The method of setting the range of the development contrast Vc will be described below in (1) to (3).

[0085] (1) The range of the toner charge state (Q / M) is determined based on the range of the detected density (ATRDens) of the ATR patch control pattern. (2) The predicted range of the toner charge state (Q / M) during DMAX control is wider than the predicted range of (1). This is because the toner is replaced by toner replenishment control as the number of sheets on which images are formed increases after the ATR patch control pattern is detected. The amount of change in the toner charge state (Q / M) varies depending on the frequency of the ATR patch control pattern, the amount of toner in the developer 4, etc. (3) Set a range (Vc1 to Vc5) of development contrast Vc corresponding to the predicted range of the toner charge state (Q / M) in (2). In this embodiment, the range of development contrast Vc is selected corresponding to the predicted range of the toner charge state (Q / M) of three types, but the predicted range of the toner charge state (Q / M) does not need to be limited to three types.

[0086] As described above, in this embodiment, it is possible to set a narrower range (Vc1 to Vc5) of development contrast Vc than in the comparative example. Note that, because the detected density of the ATR patch control pattern reflects the toner charge state (Q / M), the range (Vc1 to Vc5) of development contrast Vc may be set according to the detected density of the ATR patch control pattern. Furthermore, although the detected density of the ATR patch control pattern has been described in this embodiment, it is not necessarily the detected density; the toner charge state (Q / M) may be estimated based on the detection result of the ATR patch control pattern.

[0087] After determining the range of development contrast Vc (Vc1 to Vc5), CPU 401 transmits a command to form test pattern PT to density signal generator 402, thereby forming test image 700 on intermediate transfer belt 11 (S904). In accordance with the command, density signal generator 402 transmits a density signal for forming test image 700 to exposure device 3, and sets image formation conditions for each test pattern PT in exposure device 3 and high-voltage power supplies 101 and 102. FIG. 14 is an explanatory diagram of image formation conditions for each test pattern PT.

[0088] Fig. 14(a) shows the combination of the development contrast Vc of each test pattern PT, the charging bias voltage Vd, the development bias voltage Vdc, and the exposure amount LPW. These combinations are stored in advance in memory 490. Fig. 14(b) shows a comparative example, and Fig. 14(c) shows an example of the image forming conditions of this embodiment.

[0089] 15 is a diagram illustrating a Vc conversion table showing the development contrast Vc corresponding to the charging bias voltage Vd and the exposure amount LPW. The Vc conversion table 1100 is created in advance in accordance with the characteristics of the photosensitive drum 1, and is stored in the print image processing unit 414 or the memory 490. For example, the development contrast Vc1 in this embodiment is LPW1=0.16 [μJ / cm 2 ] and Vd1=-500[V]. Therefore, Vc1=90[V]. In this embodiment, the combination of charging bias voltage Vd and exposure amount LPW shown in the shaded area in FIG. 15 is not used, and the image forming conditions are set so as not to change the exposure amount LPW as much as possible. This is because the thickness of thin lines changes depending on the exposure amount LPW, so the exposure amount LPW should not be changed as long as the same image density can be maintained.

[0090] After forming the test image, the CPU 401 detects the image density of the test pattern PT using the image density sensor 50 (S905). The CPU 401 converts the output value of the image density sensor 50 into an image density value using a conversion table that converts the output value of the image density sensor 50 into the image density value of the test pattern PT.

[0091] The CPU 401 determines the development contrast Vc that can achieve the target maximum density (S906). For example, if the target maximum density is 1.50 when the density signal is 100%, the CPU 401 determines the development contrast Vc that will result in a target maximum density of 1.50 when the density signal is 80%. Figure 16 is an explanatory diagram of the relationship between the image density values ​​of the five test patterns PT detected in the process of S905 and the development contrast Vc. Figure 16 shows an example of a comparative example and this embodiment.

[0092] The graph in Fig. 16 plots the image density values ​​D1 to D5 detected from each of the five test patterns PT1 to PT5 for the development contrasts Vc1 to Vc5 set to generate the five test patterns PT1 to PT5. The dotted line indicates the ideal characteristic of the image density value Di relative to the development contrast Vci, and is an upward convex curve.

[0093] In the comparative example, the relationship between the development contrast and image density value when test patterns PT1 to PT5 are formed with development contrasts Vc1r to Vc5r according to the environmental conditions is shown by white circles, and the coordinates of the development contrast Vc that achieves the target maximum density through linear interpolation are shown by black circles. In this embodiment, when the predicted center value of the toner charge state (Q / M) is 25, DMAX control is performed within the range of development contrasts Vc1 to Vc5, which corresponds to a prediction range of 20≦Q / M≦50. The relationship between the development contrasts Vc1 to Vc5 obtained in this case and the image density value is shown by crosses, and the coordinates of the development contrast Vc that achieves the target density through linear interpolation are shown by black triangles.

[0094] The CPU 401 determines the two image density values ​​Di and Di+1 that are closest to the target maximum density from among the image density values ​​D1 to D5 detected from the test patterns PT1 to PT5. In this example, the two image density values ​​closest to the target maximum density are D1r and D2r in the comparative example and D1 and D2 in the present embodiment. Therefore, the CPU 401 interpolates the coordinates (Vc1r, D1r) and (Vc2r, D2r) in the comparative example, and the coordinates (Vc1, D1) and (Vc2, D2) in the present embodiment to calculate the development contrast Vc_A corresponding to the target maximum density. Note that (Vc1r, D1r) in the comparative example and (Vc1, D1) in the present embodiment are the same coordinates. (Vc2r, D2r) in the comparative example and (Vc3, D3) in the present embodiment are the same coordinates.

[0095] As a result, in the comparative example, the development contrast Vc_A is 113 [V]. In the present embodiment, the development contrast Vc_A is 107 [V]. As can be seen from the enlarged view, the difference between the coordinates of the development contrast Vc of the present embodiment, indicated by the black triangles, and the original characteristics, indicated by the dotted line, is smaller than the difference between the coordinates of the development contrast Vc of the comparative example, indicated by the black circles, and the original characteristics, indicated by the dotted line. The difference between the original characteristics, indicated by the dotted line, becomes the linear interpolation errors (ΔD) and (ΔDr) due to linear interpolation.

[0096] After determining the development contrast, the CPU 401 determines the image forming conditions A based on the determined development contrast Vc_A (S907). The CPU 401 references the Vc conversion table 1100 in FIG. 15 and determines the charging bias voltage Vd and exposure amount LPW corresponding to the determined development contrast Vc_A as the image forming conditions A. In this embodiment, the charging bias voltage Vd_A is −500 [V], the development bias voltage Vdc_A is −350 [V], and the exposure amount LPW_A=0.18 [μJ / cm 2 The CPU 401 stores the determined image forming conditions A in the memory 490. The image forming conditions A include a development contrast Vc_A, a charging bias voltage Vd_A, a development bias voltage Vdc_A, and an exposure amount LPW_A.

[0097] In the present embodiment described above, the toner charge state (Q / M) is predicted based on the detected density of the ATR patch control pattern. This allows the range of variation of the development contrast Vc during DMAX control to be set to a narrower range than when the toner charge state (Q / M) is not predicted. As a result, the development contrast Vc can be determined with higher accuracy without increasing the control time. This allows the image formation conditions to be determined with high accuracy.

[0098] FIG. 17 is an explanatory diagram of the effect of DMAX control of this embodiment on linear interpolation error (ΔD). In the comparative example, the development contrast Vc_A is 113 [V]. In this case, the image density is 1.289 according to the dotted line characteristics in FIG. 16, while the target maximum density of an image with an 80% density signal is 1.20. As a result, the linear interpolation error (ΔD) is 0.089. In this embodiment, the image density is 1.231 according to the dotted line characteristics, while the target maximum density is 1.20. As a result, the linear interpolation error (ΔD) is 0.031. As can be seen, this embodiment reduces the linear interpolation error (ΔD).

[0099] (Second embodiment) The configuration of the image forming apparatus 100 and the configuration of the control unit 400 of the second embodiment are the same as those of the first embodiment, so a description of the configuration of the image forming apparatus 100 and the configuration of the control unit 400 of the second embodiment will be omitted.

[0100] When using a toner charge prediction model, the toner charge state (Q / M) can be predicted as needed. However, when predicting the toner charge state (Q / M) based on the image density detected from the ATR patch control pattern, the toner charge state (Q / M) is predicted at predetermined intervals. For example, the toner charge state (Q / M) is predicted every 200 A4-size sheets. Therefore, changes in the toner charge state (Q / M) during this time are not predicted. To accommodate these changes, the fluctuation range (Vc1 to Vc5) of the development contrast Vc must be set wide. In particular, when the interval between predictions of the toner charge state (Q / M) is long or when the amount of toner consumed during printing is large, the deviation of the predicted value of the toner charge state (Q / M) from the actual value becomes large.

[0101] Therefore, the image forming apparatus 100 may use a predetermined calculation model to predict the toner charge state (Q / M) for a predetermined interval by the CPU 401. The CPU 401 constantly derives a predicted value of the toner charge state (Q / M). This allows the fluctuation range of the development contrast Vc to be set narrower than when the toner charge state (Q / M) is predicted based on the image density detected from the ATR patch control pattern.

[0102] (Prediction of toner charge state (Q / M) using a computational model) The CPU 401 predicts the toner charge state (Q / M) using a calculation model as described below, which is just one example. The CPU 401 predicts the charge amount Q of the toner particles in the developing unit 4 at regular time intervals T using the following formula and stores the predicted value in the memory 490.

[0103] - When developing sleeve rotates 41 times (during development) (Q / M)=(Qp / M)(1-T / α-C / Mass)+β·T / α+(Qp / M) …(1) When developing sleeve 41 is stopped (non-developing process) (Q / M)=(Qp / M)·(1-γ) …(2) Mass is the amount of toner in the developing device 4, (Qp / M) is the charge amount of the toner particles in the developing device 4 predicted in the previous calculation, and C is the amount of toner consumed since the previous calculation. The initial value of the charge amount of the toner particles is set to, for example, 80% of the saturated charge amount.

[0104] Furthermore, α is the saturated charge amount of toner particles under the operating environment, and is a value based on actual measurements at the lower limit of the toner-carrier ratio and the lowest saturated humidity environment during continuous operation. β is an index of the speed at which triboelectric charging, or static elimination, occurs, and γ is an index of the speed at which charge leakage from toner particles occurs. α, β, and γ are determined according to the charging characteristics of the toner and are stored in advance in memory 490. The toner amount Mass in developer 4 is calculated, for example, from the amount of toner consumed (developed amount) and the amount of toner replenished. The calculation formula is not limited to the above formulas (1) and (2), and other formulas that can predict the charge amount of toner particles in developer 4 may be used.

[0105] In this embodiment, when DMAX control is executed, CPU 401 acquires the latest predicted value of the charge amount of toner particles from memory 490. CPU 401 executes the flowchart showing DMAX control in Fig. 11, but the processes in S902 and S903 are different from those in the first embodiment. The different processes will be described.

[0106] The CPU 401 obtains a predicted value of the toner charge state (Q / M) (S902). The CPU 401 obtains the toner charge state (Q / M) using the above-described calculation model. By using the calculation model, there is no time lag between the prediction of the toner charge state (Q / M) and the execution of DMAX control, and the accuracy of the predicted value of the toner charge state (Q / M) can be maintained at a high level.

[0107] Based on the acquired predicted value of the toner charge state (Q / M), the CPU 401 determines the range (Vc1 to Vc5) of the development contrast Vc for forming each toner pattern PT used in DMAX control (S903). FIG. 18 is an explanatory diagram of a method for determining the range of the development contrast Vc for multiple test patterns PT. In the second embodiment, the toner charge state (Q / M) based on the calculation model is predicted within a narrower range than in the first embodiment (see FIG. 13), and the range (Vc1 to Vc5) of the development contrast Vc can be set more precisely. This is because the time elapsed from the prediction of the toner charge state (Q / M) to the execution of DMAX control is sufficiently short, and there is no toner replacement due to toner replenishment.

[0108] 18, the development contrast Vc is selected and set from a range of seven types of development contrast Vc. This allows DMAX control to be performed by setting a narrower range of development contrast Vc. Furthermore, CPU 401 may directly determine development contrasts Vc1, Vc2, Vc3, Vc4, and Vc5 in DMAX control using an arithmetic expression in accordance with a predicted value of the toner charge state (Q / M).

[0109] FIG. 19 is an explanatory diagram of the relationship between the image density value of the test pattern PT and the development contrast Vc. FIG. 19 shows examples of a comparative example, the first, and the second embodiments. In the second embodiment, when the predicted center value of the toner charge state (Q / M) is 25, the predicted range of the toner charge state (Q / M) is 20≦Q / M≦30, which is narrower than in the first embodiment. The relationship between the image density value and the development contrast Vc obtained in this case is represented by white squares, and the coordinates of the development contrast Vc that achieve the target maximum density through linear interpolation are represented by black squares. It can be seen that the black squares, which represent the coordinates of the development contrast Vc, almost overlap with the original characteristics shown by the dotted line. In the second embodiment, the development contrast Vc_A calculated by linear interpolation is 104 [V], and the linear interpolation error (ΔD) is 0.005. As can be seen, the second embodiment can reduce the linear interpolation error (ΔD) more than the first embodiment.

[0110] In the above description, in the processes of S904 and S905, the test image 700 is formed on the intermediate transfer belt 11 and detected by the image density sensor 50. However, the test image 700 may be formed on the photosensitive drum 1. In this case, the test image 700 is formed on the photosensitive drum 1 corresponding to each color. For example, the yellow test image 700 (test pattern PT_Yi) is formed on the photosensitive drum 1a of the image forming unit Pa and detected by the image density sensor provided in the image forming unit Pa. The image density is detected based on the detection result of the test image 700 by the image density sensor provided in each of the image forming units Pa to Pd.

[0111] The test image 700 may also be printed on a sheet S. In this case, the sheet S on which the test image 700 is printed is read by the image reading device 110. Based on the reading result of the test image 700 by the image reading device 110, the image density is detected.

[0112] The toner charge amount may be predicted using the magnetic permeability sensor 45, or may be predicted based on the video count value from the video signal count unit 207, the detection results (environmental conditions) from the environmental sensor 60, the driving time and the standing time measured by the timer 410, etc. Furthermore, the toner charge amount may be predicted based on at least one of the detection results from the magnetic permeability sensor 45, the video count value, the environmental conditions, the driving time and the standing time, etc., or a combination of two or more thereof.

Claims

1. an image forming means for forming an image based on image forming conditions; reading means for reading the test image formed by the image forming means; a control unit that detects an image density of the first test image based on a result of reading the first test image by the reading unit, and adjusts the image forming conditions based on the detected image density of the first test image, The image forming means a photoreceptor having a photosensitive layer; a charging means for uniformly charging the photosensitive layer of the photoreceptor; an exposure unit that irradiates the charged photosensitive layer with light to form an electrostatic latent image on the photosensitive layer; a developing means for storing toner and for developing the electrostatic latent image by causing the toner to adhere to the electrostatic latent image, the image forming conditions include a development contrast, the control means predicts the charge amount of the toner contained in the developing means, determines a plurality of development contrasts based on the predicted charge amount of the toner, and forms a plurality of first test images by the image forming means at the plurality of development contrasts; the control means determines a development contrast that can achieve a target maximum density from the results of reading the plurality of first test images by the reading means. Image forming device.

2. the image forming means further includes a detecting means for detecting the charge amount of the toner contained in the developing means based on the magnetic permeability of the toner, The control means predicts the charge amount of the toner contained in the developing means based on the detection result by the detection means.

2. The image forming apparatus according to claim 1.

3. the image forming means forms a plurality of second test images under the same image forming conditions; The reading means reads the second test image, the control means detects the image density of the second test image based on the reading result of the second test image by the reading means, and predicts the charge amount of the toner based on the detected image density of the second test image.

2. The image forming apparatus according to claim 1.

4. the control means includes a storage means for storing information indicating a correspondence relationship between the image density of the second test image and the charge amount of the toner, and the control means predicts the charge amount of the toner using the information.

4. The image forming apparatus according to claim 3.

5. Further comprising a toner container for replenishing toner to said developing means, the image forming means forms a second test image used to control toner supply from the toner container to the developing means; the control unit detects the second test image and predicts the charge amount of the toner based on the detection result of the second test image.

2. The image forming apparatus according to claim 1.

6. the control means includes a storage means for storing information indicating a relationship between the image density of the second test image and the charge amount of the toner, and the control means predicts the charge amount of the toner based on the information.

6. The image forming apparatus according to claim 5.

7. the control means performs a process of predicting the charge amount of the toner contained in the developing means based on the image density of the second test image at predetermined intervals, and predicts the charge amount of the toner during the predetermined intervals using a predetermined calculation model.

6. The image forming apparatus according to claim 5.

Images