Image forming apparatus and control method thereof

Abstract

To allow accurate correction of a developing bias in an image forming apparatus even if the developing bias has a high exposure area ratio.SOLUTION: A control unit (100) included in an image forming apparatus (1) executes setting processing of, when executing developing bias correction processing, setting a plurality of developing biases for a test all of which are smaller than a former developing bias being a developing bias for printing before executing the developing bias correction processing, forming processing of forming a plurality of developer images on the belt, reading processing of reading the densities of the plurality of developer images, and calculation processing of calculating the developing bias for printing according to the densities of the plurality of developer images.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus such as a printer and a control method thereof. [Background technology]

[0002] In order to maintain a target print density, an image forming apparatus is known that corrects a developing bias by creating and reading toner patches corresponding to each of a plurality of developing biases. In the technology disclosed in Patent Document 1, the most recent developing bias that is assumed to be close to the target print density is used as a reference, and toner patches corresponding to developing biases with potentials above and below the reference are generated to correct the developing bias. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2011-154146 A Summary of the Invention [Problem to be solved by the invention]

[0004] In the technology disclosed in Patent Document 1, when a toner patch for correcting the development bias with a high exposure area ratio is used, the change in the amount of reflected light is small relative to the change in the development bias, making it difficult to accurately measure the density of the toner patch. The present disclosure aims to enable accurate correction of the development bias in an image forming apparatus even with a high exposure area ratio. [Means for solving the problem]

[0005] In order to solve the above-mentioned problem, an image forming apparatus according to one embodiment of the present invention comprises a photosensitive body, a developing roller which supplies developer to the photosensitive body, a belt onto which a developer image is transferred from the photosensitive body, a sensor which reads the density of the developer image on the belt, and a control unit, and when the control unit executes a developing bias correction process which corrects a developing bias for printing to be applied to the developing roller, the control unit executes a setting process which sets multiple test developing biases which are all smaller than an old developing bias which is the developing bias for printing before the developing bias correction process is executed, a forming process which forms multiple developer images on the belt using the multiple test developing biases set in the setting process, a reading process which reads the densities of the multiple developer images formed in the forming process by the sensor, and a calculation process which calculates the developing bias for printing according to the densities of the multiple developer images read in the reading process.

[0006] According to the above-mentioned configuration, the developer image having a density smaller than the density corresponding to the target developing bias is read by the sensor, so that the reading accuracy is improved, and therefore, even when the exposure area ratio is high, the developing bias can be accurately corrected in the image forming apparatus.

[0007] In the setting process, the control unit may set a three-value test developing bias as the plurality of test developing biases.

[0008] According to the above-described configuration, by performing curve interpolation on coordinate values ​​defined by the developing bias and the corresponding print density, it is possible to more accurately correct the developing bias.

[0009] In the setting process, when the three-value test developing biases are set to a first value, a second value and a third value, in order from the smallest, the control unit may set the first value, the second value and the third value so that a first difference value between the first value and the second value is equal to a second difference value between the second value and the third value.

[0010] According to the above-mentioned configuration, the test developing bias can be easily set.

[0011] In the setting process, the control unit may set the first value, the second value, and the third value so that a third difference value between the old developing bias and the third value is greater than the first difference value and the second difference value.

[0012] According to the above configuration, a plurality of test developing biases having a large difference from the developing bias for printing set in the previous developing bias correction process are used, so that more accurate correction of the developing bias in the image forming apparatus is possible.

[0013] The image forming apparatus may further include a temperature sensor that acquires an ambient temperature of the image forming apparatus, and the control unit may change a magnitude of the third difference value in the setting process in accordance with the ambient temperature acquired by the temperature sensor.

[0014] According to the above-described configuration, for example, by setting the third difference value to be larger as the atmospheric temperature is lower, it is possible to form a developer image using a development bias suitable for the atmospheric temperature.

[0015] In the calculation process, the control unit may set the printing development bias when developing an electrostatic latent image on the photosensitive member corresponding to a solid image to a value that moves only a portion of the developer on the developing roller to the electrostatic latent image.

[0016] According to the above configuration, it is possible to control the amount of developer used for printing independently of the circumferential speed of the developing roller, thereby suppressing changes in print density due to fluctuations in the circumferential speed of the developing roller.

[0017] The control unit may, in the forming process, form developer images on the belt corresponding to exposure area ratios of 60% or more and less than 100%, respectively, using the plurality of test developing biases.

[0018] According to the above-described configuration, by using a high-density developer image as a reference, it is possible to suitably obtain gradation in a high-density region.

[0019] The image forming apparatus may further include a charger that charges the surface of the photosensitive member, and the control unit may execute a charging bias setting process that sets the magnitude of a charging bias to be applied to the charger in accordance with the magnitude of the test developing bias.

[0020] According to the above-mentioned configuration, a developer image can be formed on the belt using an appropriate charging bias.

[0021] After executing the development bias correction process, the control unit may perform a gamma correction process to form developer images corresponding to multiple different exposure area ratios on the belt using the development bias for printing set in the development bias correction process, and to set an exposure area ratio corresponding to the gradation of an input image.

[0022] According to the above-described configuration, it is possible to perform correction for the print density corresponding to an exposure area ratio different from that used as the reference in the development bias correction process.

[0023] The control unit may execute the developing bias correction process when a predetermined number of print processes have been executed after executing a previous developing bias correction process.

[0024] According to the above-described configuration, when the print density changes due to a change in the charge amount or other characteristics of the developer caused by continued printing, it is possible to execute the development bias correction process.

[0025] The image forming apparatus may be provided with a temperature sensor that acquires an ambient temperature, and the control unit may execute the development bias correction process when the ambient temperature acquired by the temperature sensor has changed by a predetermined amount or more after executing the previous development bias correction process.

[0026] According to the above configuration, when the print density changes due to a change in the characteristics of the developer, such as the charge amount, caused by a change in the atmospheric temperature, the development bias correction process can be executed.

[0027] The developing device may further include a container that contains the developer, and when the container is replaced, the control unit may execute the setting process by using a developing bias of a predetermined value as the old developing bias.

[0028] According to the above-described configuration, when the print density changes as a result of replacement with a container that contains a developer with different characteristics, the development bias correction process can be executed.

[0029] In order to solve the above-mentioned problems, a control method for an image forming apparatus according to one embodiment of the present invention is a control method for an image forming apparatus including a photosensitive member, a developing roller that supplies developer to the photosensitive member, and a belt onto which a developer image is transferred from the photosensitive member, and when performing a developing bias correction process to correct a developing bias for printing applied to the developing roller, the control method includes a forming step of forming multiple developer images on the belt by setting multiple test developing biases that are all smaller than an old developing bias, which is the developing bias for printing before performing the developing bias correction process, and a calculation step of calculating the developing bias for printing according to the density of the multiple developer images on the belt.

[0030] According to the above configuration, the same effects as those of the image forming apparatus according to one aspect of the present invention are achieved. Effect of the Invention

[0031] According to one aspect of the present invention, it is possible to accurately correct the development bias in an image forming apparatus even at a high exposure area ratio. [Brief description of the drawings]

[0032] [Figure 1] 1 is a diagram showing a schematic configuration of a color printer. [Diagram 2]FIG. 2 is a block diagram showing the functional configuration of a color printer. [Diagram 3] 4 is a flowchart showing a process flow in a control method for a color printer when a development bias correction process and a gamma correction process are executed. [Figure 4] 11A and 11B are diagrams showing the relationship between the development bias and the density of a developer image corresponding to each exposure area ratio, and diagrams showing the range in which the density of a developer image changes corresponding to each exposure area ratio; [Diagram 5] FIG. 4 is a diagram showing an example of a toner patch formed on a belt. [Figure 6] FIG. 11 is a diagram showing the relationship between the density of each toner patch read in the reading process and the developing bias corresponding to each density. [Figure 7] FIG. 4 is a diagram showing the relationship between the developing bias and the density of a developer image. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] Hereinafter, an embodiment of the present invention will be described in detail. For the sake of convenience, the same components are denoted by the same reference numerals, and their names and functions are also the same. Therefore, detailed description thereof will not be repeated.

[0034] (Color Printer 1 Configuration) Fig. 1 is a diagram showing a schematic configuration of a color printer 1 according to one embodiment of the present invention. As shown in Fig. 1, color printer 1, which is an example of an image forming device, includes a housing 10, a sheet supply unit 20, and an image forming unit 30. In Fig. 1, the left side of color printer 1 is designated as the "front", the right side as the "rear", the upper side as the "top", and the lower side as the "bottom".

[0035] The housing 10 includes an opening 10A and a cover 11 capable of opening and closing the opening 10A. The cover 11 is disposed on the opposite side of the side on which the photoconductor 51 of the exposure head 40 is disposed in the up-down direction, specifically, on the upper side of the housing 10. The cover 11 is supported rotatably about a rotation shaft 11A provided at the rear of the housing 10, and opens and closes the opening 10A formed in the housing 10. The cover 11 is rotatable between a closed position (position in FIG. 1) where the opening 10A is closed and an open position where the opening 10A is opened. The cover sensor 12 is a sensor that detects whether the cover 11 is opened and closed. When the cover 11 is opened, the cover sensor 12 outputs an ON signal until the cover 11 is closed.

[0036] Additionally, the housing 10 is provided with a temperature sensor 92 for acquiring the ambient temperature of the color printer 1, near the inside of an air intake port (not shown).

[0037] The sheet supply unit 20 is provided at the bottom inside the housing 10. The sheet supply unit 20 includes a sheet tray 21 and a pickup roller 22. The sheet tray 21 stores sheets P. The pickup roller 22 supplies the sheets P from the sheet tray 21 to the image forming unit 30. The sheets P in the sheet tray 21 are separated one by one by the pickup roller 22 and supplied to the image forming unit 30.

[0038] The image forming unit 30 is disposed above the sheet tray 21 within the housing 10. The image forming unit 30 includes a plurality of exposure heads 40, a plurality of process cartridges PC as an example of a plurality of photosensitive cartridges, a transfer unit 70, and a fixing unit 80. In FIG. 1, the number of the exposure heads 40 and the process cartridges PC are each four.

[0039] The exposure head 40 is an exposure device that exposes the photoconductor 51, and is supported by the cover 11 so as to be suspended from the cover 11. The exposure head 40 is disposed above and facing the photoconductor 51 when the cover 11 is closed. In detail, the exposure head 40 is capable of moving between an exposure position (position in FIG. 1) where the photoconductor 51 is exposed and a retracted position that is farther away from the photoconductor 51 than the exposure position, by opening and closing the cover 11.

[0040] The multiple process cartridges PC are arranged in parallel in the front-rear direction between the cover 11 and the sheet tray 21. The multiple process cartridges PC are detachable from the housing 10 through an opening 10A when the cover 11 is open. The process cartridge PC includes a drum unit 50 and a developing cartridge 60 as an example of a storage portion.

[0041] The drum unit 50 includes a photoconductor 51, a charger 52, a pressure spring 53, a cleaning roller 54, a drum frame 55, a memory 56, and a cartridge-side terminal 57. The photoconductor 51 is a cylindrical drum that carries toner, which is an example of a developer. The charger 52 charges the photoconductor 51. The pressure spring 53 biases the developing cartridge 60 toward the photoconductor 51 in the process cartridge PC. The cleaning roller 54 removes foreign matter, such as toner, remaining on the photoconductor 51. The cleaning roller 54 is in contact with the photoconductor 51 and is rotatable. The drum frame 55 supports the photoconductor 51 and the like in the process cartridge PC.

[0042] The memory 56 stores information about the process cartridge PC. The cartridge side terminal 57 is electrically connected to the memory 56. Each of the plurality of housing side terminals 13 is provided in the housing 10 for each process cartridge PC. When the corresponding process cartridge PC is provided in the housing 10, the cartridge side terminal 57 is connected to the cartridge side terminal 57 corresponding to the process cartridge PC.

[0043] The developing cartridge 60 is a cartridge that is detachably attached to the drum unit 50. The developing cartridge 60 includes a toner storage section 61 and a developing roller 62. The toner storage section 61 stores therein a toner of a specific color as an example of a developer. In detail, each toner storage section 61 stores a toner of any one of cyan (C), magenta (M), yellow (Y), and black (K). The developing roller 62 supplies the toner in the corresponding toner storage section 61 to the corresponding photoconductor 51. The toner is a non-magnetic, single-component, positively charged toner.

[0044] The process cartridge PC is attached to and detached from the housing 10 with a detachable developing cartridge 60 attached to the process cartridge PC. The memory 56 may be provided in the developing cartridge 60 and connected to the housing side terminal 13 via a cartridge side terminal 57 of the process cartridge PC. Not limited to this, the developing cartridge 60 may be detachably attached to the housing 10 separately from the process cartridge PC. Alternatively, the process cartridge PC may include a toner storage section 61 and a developing roller 62 as an example of a storage section that is integrated with the process cartridge PC. In this example, the toner storage section 61 and the developing roller 62 are not detachable from the process cartridge PC.

[0045] The transfer unit 70 is provided between the sheet tray 21 and the process cartridge PC. The transfer unit 70 includes a drive roller 71, a driven roller 72, a belt 73, a plurality of transfer rollers 74, and a sensor 75. The belt 73 is an endless belt stretched between the drive roller 71 and the driven roller 72. The outer surface of the belt 73 is in contact with each photoconductor 51. A toner image made of toner is transferred from each photoconductor 51 to the outer surface of the belt 73. Here, the toner image is an example of a developer image in the present disclosure. Each transfer roller 74 is disposed inside the belt 73 so as to sandwich the belt 73 between each photoconductor 51. The sensor 75 is a sensor for performing a reading process, etc., which will be described later, and is disposed outside the belt 73 and near the drive roller 71. The sensor 75 is an optical sensor that irradiates the belt 73 with light and receives reflected light from the belt 73 to read the density of the toner image formed on the belt 73.

[0046] The fixing unit 80 is provided behind the process cartridge PC and the transfer unit 70. The fixing unit 80 includes a heating roller 81 and a pressure roller 82. The pressure roller 82 is disposed opposite to the heating roller 81 and presses the heating roller 81.

[0047] The color printer 1 forms an image on the sheet P according to the following procedure. First, the image forming unit 30 uniformly charges the surface of each photoconductor 51 with the charger 52. Next, the photoconductor 51 is exposed to light by the exposure head 40, forming an electrostatic latent image on the photoconductor 51 based on the image data. Then, a developing bias is applied to the developing roller 62, which supplies toner to the photoconductor 51, thereby making the electrostatic latent image visible. The developing bias has the same polarity as the toner, and is the bias amount of the voltage applied to the developing roller 62. A toner image is formed on the photoconductor 51 by applying the developing bias. The image forming unit 30 transfers the toner images formed on each photoconductor 51 in order, superimposed on the sheet P, which is transported on the belt 73 by the driving roller 71 and the driven roller 72. The transfer unit 70 transports the sheet P, to which the toner image has been transferred, between the heating roller 81 and the pressure roller 82. The fixing unit 80 heats the sheet P with the heating roller 81, thereby thermally fixing the toner image to the sheet P. Thereafter, the color printer 1 ejects the sheet P from inside the housing 10 to the outside with the conveying roller 91, and places the sheet P on the ejection sheet tray 11B provided on the upper surface of the cover 11.

[0048] The image forming unit 30 may form an image on the sheet P by an intermediate transfer method. According to the intermediate transfer method, the image forming unit 30 first forms a toner image on the outer surface of the belt 73, and then transfers the toner image on the belt 73 to the sheet P.

[0049] (Functional configuration of color printer 1) Fig. 2 is a block diagram showing the functional configuration of a color printer 1 according to an embodiment of the present invention. However, in Fig. 2, some of the members included in the color printer 1 shown in Fig. 1 are omitted. As shown in this figure, the color printer 1 includes a control unit 100, an image forming unit 30 having a sensor 75 and the like, a display unit 110, an operation unit 120, a communication unit 130, and a temperature sensor 92. The control unit 100 is electrically connected to the image forming unit 30, the display unit 110, the operation unit 120, and the communication unit 130. The control unit 100 includes a CPU (Central Processing Unit) 101, a ROM (Read Only Memory) 102, a RAM (Random Access Memory) 103, and an NVM (Non-Volatile Memory) 104 as an example of a storage unit.

[0050] CPU 101 performs processing according to programs read from ROM 102, and controls each part of color printer 1 while storing the results of that processing in RAM 103 or NVM 104. ROM 102 stores various programs, including, for example, programs for controlling each part of color printer 1. RAM 103 is used as a working area when CPU 101 executes the various programs and as a temporary storage area for data. NVM 104 is a rewritable nonvolatile memory.

[0051] Display unit 110 has a liquid crystal display and displays various setting screens and the operating status of color printer 1. Operation unit 120 has multiple buttons and accepts various input instructions from a user. Communication unit 130 communicates with external device 200 such as a PC (Personal Computer) by wireless communication or wired communication and receives data. Communication unit 130 is an interface such as a LAN (Local Area Network). Note that communication unit 130 may communicate with multiple external devices 200.

[0052] (Control method for color printer 1) 3 is a flowchart showing a method for controlling color printer 1 according to one embodiment of the present invention, illustrating the flow of processing when executing a development bias correction process for correcting the development bias for printing applied to development roller 62, and a gamma correction process. The processing shown in the flowchart of FIG. 3 can be executed, for example, when color printer 1 is started up, or when cover sensor 12 detects that cover 11 is closed. Note that the order in which each step of the processing shown in the flowchart of FIG. 3 is executed is not limited to that described below.

[0053] In S101, the control unit 100 determines whether or not the conditions for starting the development bias correction process are satisfied. Here, examples of the cases where the conditions are satisfied include the following cases. - When a certain number of prints have been printed after the previous development bias adjustment process After the previous development bias adjustment process was performed, the ambient temperature acquired by the temperature sensor 92 changed by a predetermined temperature or more. When the developing cartridge 60 that contains the toner is replaced When the operation unit 120 or the communication unit 130 receives a command to execute the development bias correction Although not particularly limited, the predetermined number may be, for example, 1000 sheets, and the predetermined temperature may be, for example, 6 degrees Celsius.

[0054] In S101, when the control unit 100 determines that the condition is satisfied, it subsequently determines in S102 whether or not the condition is satisfied by replacement of the developing cartridge 60. In addition, when the control unit 100 determines that the condition is not satisfied, the process based on the flowchart of FIG. 3 ends.

[0055] In S102, when the control unit 100 determines that the above condition is satisfied due to a factor other than replacement of the developing cartridge 60, in S103, the control unit 100 sets the developing bias for printing set in the previous developing bias correction process as the old developing bias that is the correction reference for the current developing bias correction process. The old developing bias is the developing bias for printing before the current developing bias correction process was executed.

[0056] In addition, the control unit 100 may perform a process of changing the developing bias for printing in accordance with an increase in the number of printed sheets after setting the developing bias for printing in the previous developing bias correction process. In this case, the developing bias for printing immediately before the current developing bias correction process is executed may be set as the old developing bias.

[0057] If the control unit 100 determines in S102 that the above condition is satisfied by replacing the developing cartridge 60, then in S104 the control unit 100 sets a predetermined value of the developing bias as the old developing bias. In other words, when the developing cartridge 60 is replaced, the control unit 100 uses the predetermined value of the developing bias as the old developing bias, instead of the developing bias for printing set in the previous developing bias correction process. The predetermined value of the developing bias may be a fixed value specified for a new developing cartridge 60, or may be a value according to information recorded in the memory 56 mounted in the developing cartridge 60.

[0058] Next, the control unit 100 executes a setting process for setting multiple test developing biases that are all smaller than the old developing bias. For example, in the setting process, the control unit 100 may set three-value test developing biases as multiple test developing biases by the following steps S105 to S107.

[0059] Hereinafter, the three test developing bias values ​​are described in ascending order as Vb1, Vb2, and Vb3. The first difference between Vb1 and Vb2 is ΔV1, the second difference between Vb2 and Vb3 is ΔV2, and the third difference between the old developing bias and Vb3 is ΔV3.

[0060] In addition, in the case of a configuration in which negatively charged toner is used and a negative developing bias is applied, a small developing bias means that the absolute value of the developing bias is small, that is, the amount of deviation from the reference potential is small.

[0061] In S105, the temperature sensor 92 acquires the ambient temperature of the color printer 1.

[0062] In S106, the control unit 100 sets ΔV3. In other words, the control unit 100 sets Vb3. In addition, in the setting process, the control unit 100 may change the magnitude of ΔV3 according to the ambient temperature acquired by the temperature sensor 92. For example, the lower the ambient temperature, the larger ΔV3 is set, so that a toner image can be formed using a suitable developing bias according to the ambient temperature. Usually, the lower the ambient temperature, the higher the charge amount of the toner, and the smaller the change in density of the toner image with respect to the change in the developing bias. Therefore, in an environment with a low ambient temperature, it is desirable to set ΔV3 large in order to reduce the density of the toner image.

[0063] In S107, the control unit 100 sets ΔV1 and ΔV2. In other words, the control unit 100 sets Vb1 and Vb2.

[0064] Furthermore, in the setting process, the control unit 100 may set Vb1, Vb2, and Vb3 so that ΔV1 is equal to ΔV2. Furthermore, in the setting process, the control unit 100 may set Vb1, Vb2, and Vb3 so that ΔV3 is greater than ΔV1 and ΔV2. This allows multiple test developing biases with large differences from the old developing biases to be used, making it possible to reduce the density of the toner image and correct the developing bias more accurately in the color printer 1. As an example, the control unit 100 may set ΔV1 to 40 (V), ΔV2 to 40 (V), and ΔV3 to 100 (V).

[0065] The values ​​of Vb1, Vb2, and Vb3 may be different for each color. That is, for example, Vb1 corresponding to a first color and Vb1 corresponding to a second color do not necessarily have to be the same.

[0066] In S108, the control unit 100 executes a charging bias setting process for setting the magnitude of the charging bias applied to the charger 52 in accordance with the magnitudes of the test developing biases, that is, Vb1, Vb2, and Vb3.

[0067] In S109, the control unit 100 controls the image forming unit 30 to apply the developing bias Vb1 to the developing roller 62, and forms a toner patch "Patch1" which is an example of a toner image for each color on the surface of the belt 73. Next, in S110, the control unit 100 reads the density dens1 of the toner patch formed using the developing bias Vb1 by the sensor 75. The values ​​of dens1 and dens2 and dens3 described later may differ from one another for each color.

[0068] In S111, the control unit 100 controls the image forming unit 30 to apply the developing bias Vb2 to the developing roller 62, and forms a toner patch "Patch2" of each color on the surface of the belt 73. Next, in S112, the control unit 100 reads the density dens2 of the toner patch formed using the developing bias Vb2 by the sensor 75.

[0069] In S113, the control unit 100 controls the image forming unit 30 to apply the developing bias Vb3 to the developing roller 62, and forms a toner patch "Patch3" of each color on the surface of the belt 73. Next, in S114, the control unit 100 causes the sensor 75 to read the density dens3 of the toner patch formed using the developing bias Vb3.

[0070] A supplementary explanation will be given regarding the toner patches formed on the belt 73 in the steps S109, S111, and S113. Fig. 4(A) is a diagram showing the relationship between the development bias and the density for toner patches corresponding to the respective exposure area ratios of 100%, 75%, and 70%. Fig. 4(B) is a diagram showing the range in which the density of the toner patch changes corresponding to each of the above exposure area ratios.

[0071] In each process, the toner patch on the belt 73 is formed with the same exposure area ratio for each color. The exposure area ratio means the ratio of the area of ​​the photoconductor 51 that is exposed to light within the range in which an electrostatic latent image corresponding to a binarized image is formed on the photoconductor 51.

[0072] For each color, it is desirable that the toner patch is formed with an exposure area ratio that includes a target density, which is a target density, within the range in which the density changes, and that the range in which the density changes is large so that the sensor 75 can obtain an appropriate gradation value. In the example shown in Fig. 4, since the target density is 1.38, it is desirable to form a toner patch that corresponds to an exposure area ratio of 75%.

[0073] Fig. 5 is a diagram showing an example of toner patches formed on the belt 73. The successive "YMCK" shown in Fig. 5 are toner patches of each color formed by S109, S111 and S113, respectively. Also, a dummy toner patch "d" in Fig. 5 indicates that the toner patches of each color YMCK formed with different test developing biases are spaced apart.

[0074] In other words, the above-mentioned processes of S109 to S114 are executed by the control unit 100 in the following forming process and reading process. That is, in the forming process, the control unit 100 forms a plurality of toner patches on the belt 73 using a plurality of test developing biases Vb1 to Vb3 set in the setting process of S105 to S107. In the reading process, the control unit 100 reads the densities dens1 to dens3 of the plurality of toner patches formed in the forming process by the sensor 75.

[0075] As described above, in the formation process, the control unit 100 may use a plurality of test developing biases to form toner patches corresponding to exposure area ratios of 60% or more and less than 100% on the belt 73. By using a high-density toner patch as a reference for the development bias correction process, it is possible to suitably obtain density gradation in the high-density region.

[0076] In S115, the control unit 100 performs curve interpolation, for example, by polynomial approximation, based on the coordinate values ​​defined by the test developing biases Vb1 to Vb3 and the densities of the toner patches formed using each developing bias and read in the reading process for the target color. Fig. 6 is a diagram showing the relationship between the densities and the developing biases corresponding to each density.

[0077] Regarding Figure 6, for example, when x1=dens1, x2=dens2, x3=dens3, y1=Vb1, y2=Vb2, and y3=Vb3, the curve y=ax passing through the points (dens1, Vb1), (dens2, Vb2), and (dens3, Vb3) is 2 The coefficients a, b, and c of +bx+c may be derived for each color, for example, by the following formula. a=(x2*y3-x1*y3-x3*y2+x1*y2+x3*y1-x2*y1) / ((x2-x1)*(x3-x1)*(x3-x2)) b=-(x2^2*y3-x1^2*y3-x3^2*y2+x1^2*y2+x3^2*y1-x2^2*y1) / ((x2-x1)*(x3-x1)*(x3-x2)) c=(x1*x2^2*y3-x1^2*x2*y3-x1*x3^2*y2+x1^2*x3*y2+x2*x3^2*y1-x2^2*x3*y1) / ((x2-x1)*(x3-x1)*(x3-x2)) In S116, the control unit 100 executes a calculation process to calculate a development bias V0 for printing according to the densities of the multiple toner patches read in the reading process for the target color. The development bias V0 is derived by substituting dens[YMCK], which is a value indicating the target density of the target color, into the above-mentioned curve equation as follows: V0=a*(dens[YMCK])^2+b*(dens[YMCK])+c Furthermore, in the calculation process, the control unit 100 may be configured to set the development bias for printing when developing an electrostatic latent image on the photoconductor 51 corresponding to a solid image to a value that moves only a portion of the toner on the development roller 62 to the electrostatic latent image. In other words, the control unit 100 may be configured to set the development bias for printing when developing an electrostatic latent image on the photoconductor 51 corresponding to a solid image to a value that does not move all of the toner on the development roller 62 to the electrostatic latent image. Here, the exposure area ratio of the electrostatic latent image on the photoconductor 51 corresponding to the above-mentioned solid image is 100%.

[0078] Fig. 7 is a diagram showing the relationship between the developing bias and the density of a toner image. In the configuration shown in the comparative example of Fig. 7, when an electrostatic latent image on the photoconductor 51 corresponding to a solid image is developed, all of the toner carried on the developing roller 62 and movable by an electric field moves to the electrostatic latent image. In the configuration of the comparative example, for example, when the speed of the developing roller 62 fluctuates, the amount of toner supplied to the electrostatic latent image fluctuates, and there is a risk of a change in print density occurring due to the fluctuation in the peripheral speed of the developing roller. Therefore, the occurrence of a change in print density due to the fluctuation in the peripheral speed of the developing roller cannot be suppressed.

[0079] On the other hand, in this embodiment shown in Fig. 7, that is, in the configuration described above in the present application, even when an electrostatic latent image on the photoconductor 51 corresponding to a solid image is developed, toner carried on the developing roller 62 remains. For example, if the "target value" in Fig. 7 is a density corresponding to a solid image, V1 is set as the developing bias for printing, but in this embodiment, the toner that can be supplied from the developing roller 62 does not reach the upper limit until the developing bias exceeds V2. As a result, even if the speed of the developing roller 62 fluctuates, the amount of toner supplied from the developing roller 62 is an amount that depends on the electric field, that is, an amount that depends on the developing bias V1 for printing, so that it is possible to suppress the occurrence of a change in print density.

[0080] In S117, the control unit 100 executes the development bias correction process for the target color, and then executes the following gamma correction process. That is, the control unit 100 forms toner patches corresponding to a plurality of different exposure area ratios on the belt 73 using the development bias for printing set in the development bias correction process, and sets the exposure area ratio corresponding to the gradation of the input image. This makes it possible to correct the print density corresponding to an exposure area ratio different from the exposure area ratio used when forming the toner patch on the belt 73. In addition, by executing the gamma correction process after correcting the print density corresponding to a high exposure area ratio, the print density can be corrected so that the exposure area ratio near the intermediate gradation is also stable as the output gradation.

[0081] In S118, the control unit 100 judges whether the development bias correction process and the gamma correction process are completed for all the colors to be processed. If the control unit 100 judges that the process is completed for each color, the process based on the flowchart of Fig. 3 ends. If the control unit 100 judges that the process is not completed for a certain color, the process from S115 onwards is executed for that color.

[0082] Note that the gamma correction process may be performed after the development bias correction process is completed for all colors to be processed.

[0083] As described above, the control method of color printer 1 includes at least the following forming step and calculation step when executing the development bias correction process. That is, in the forming step, as shown in S106 and S107, control unit 100 sets multiple test development biases that are all smaller than the old development bias. Then, as shown in S109, S111, and S113, control unit 100 forms multiple toner patches on belt 73. Also, in the calculation step, as shown in S116, control unit 100 calculates the development bias for printing according to the densities of the multiple toner patches on belt 73.

[0084] According to the above configuration, since the toner patch having a density smaller than the density corresponding to the target developing bias is read by the sensor 75, the change in the amount of reflected light from the belt 73 becomes relatively large in response to the change in the developing bias, improving the reading accuracy. This enables accurate correction of the developing bias in the color printer 1 even at a high exposure area ratio.

[0085] [Software implementation example] The control unit 100 may be realized by a logic circuit (hardware) formed on an integrated circuit (IC chip) or the like, or may be realized by software.

[0086] In the latter case, the control unit 100 includes a computer that executes instructions of a program, which is software that realizes each function. The computer includes, for example, one or more processors, and a computer-readable recording medium that stores the program. The object of the present invention is achieved by the processor reading the program from the recording medium and executing it in the computer. The processor may be, for example, a CPU (Central Processing Unit). The recording medium may be a "non-transient tangible medium," such as a ROM (Read Only Memory), tape, disk, card, semiconductor memory, programmable logic circuit, or the like. The computer may further include a RAM (Random Access Memory) that expands the program. The program may be supplied to the computer via any transmission medium (such as a communication network or broadcast waves) that can transmit the program. Note that one aspect of the present invention may also be realized in the form of a data signal embedded in a carrier wave, in which the program is embodied by electronic transmission.

[0087] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0088] In this embodiment, color printer 1 is configured to include exposure head 40 as an exposure device that exposes photoconductor 51, but is not limited to this, and may be configured to include a laser scanner as the exposure device.

[0089] In addition, in this embodiment, the color printer 1 is configured to use positively charged toner and apply a positive developing bias, but this is not limited to this, and the color printer 1 may be configured to use negatively charged toner and apply a negative developing bias. [Explanation of symbols]

[0090] 1. Color printer (image forming device) 51 Photoreceptor 52 Charger 62 Developing roller 73 Belt 75 Sensors 92 Temperature Sensor 100 Control section

Claims

1. A photoconductor; a developing roller for supplying a developer to the photoconductor; a belt onto which a developer image is transferred from the photoreceptor; a sensor for reading the density of the developer image on the belt; a temperature sensor for acquiring an ambient temperature of the image forming apparatus; A control unit, The control unit is When executing a developing bias correction process for correcting a developing bias for printing applied to the developing roller, a setting process for setting a plurality of test developing biases which are all smaller than an old developing bias which is a developing bias for printing before the developing bias correction process is executed; a forming process for forming a plurality of developer images on the belt using the plurality of test developing biases set in the setting process; a reading process for reading densities of the plurality of developer images formed in the forming process by the sensor; a calculation process for calculating a development bias for printing in accordance with the densities of the plurality of developer images read in the reading process; In the setting process, a three-value test developing bias is set as the multiple test developing biases, and when the three-value test developing biases are set to a first value, a second value and a third value in order from the smallest, the magnitude of a third difference value between the old developing bias and the third value is increased as the ambient temperature acquired by the temperature sensor becomes lower.

2. The control unit is 2. The image forming apparatus according to claim 1, wherein in the setting process, the first value, the second value and the third value are set so that a first difference value between the first value and the second value is equal to a second difference value between the second value and the third value.

3. The control unit is 3 . The image forming apparatus according to claim 2 , wherein in the setting process, the first value, the second value, and the third value are set so that the third difference value is greater than the first difference value and the second difference value.

4. The control unit is 4. The image forming apparatus according to claim 1, wherein in the calculation process, the developing bias for printing when developing an electrostatic latent image on the photosensitive member corresponding to a solid image is set to a value that moves only a portion of the developer on the developing roller to the electrostatic latent image.

5. The control unit is 5. The image forming apparatus according to claim 1, wherein in the forming process, developer images corresponding to exposure area ratios of 60% or more and less than 100% are formed on the belt using the multiple test developing biases.

6. a charger for charging the surface of the photoreceptor, The control unit is 6. The image forming apparatus according to claim 1, further comprising a charging bias setting process for setting a magnitude of the charging bias applied to the charger in accordance with a magnitude of the test developing bias.

7. The control unit is 7. An image forming apparatus as described in any one of claims 1 to 6, wherein after performing the development bias correction process, developer images corresponding to a plurality of different exposure area ratios are formed on the belt using the development bias for printing set in the development bias correction process, and a gamma correction process is performed to set an exposure area ratio corresponding to the gradation of an input image.

8. The control unit is 7. The image forming apparatus according to claim 1, wherein the developing bias correction process is executed when a predetermined number of print processes have been executed after the previous developing bias correction process has been executed.

9. a temperature sensor for acquiring an ambient temperature of the image forming apparatus; The control unit is 9. The image forming apparatus according to claim 1, wherein the developing bias correction process is executed when the ambient temperature acquired by the temperature sensor has changed by a predetermined amount or more after the previous developing bias correction process is executed.

10. a container for containing the developer, The control unit is 10. The image forming apparatus according to claim 1, wherein when the container is replaced, the setting process is executed using a developing bias of a predetermined value as the old developing bias.

11. A method for controlling an image forming apparatus including a photoconductor, a developing roller for supplying a developer to the photoconductor, a belt onto which a developer image is transferred from the photoconductor, and a temperature sensor for acquiring an ambient temperature of the image forming apparatus, comprising the steps of: When executing a developing bias correction process for correcting a developing bias for printing applied to the developing roller, a forming step of forming a plurality of developer images on the belt by setting a plurality of test developing biases which are all smaller than an old developing bias which is a developing bias for printing before the developing bias correction process is executed; a calculation step of calculating a developing bias for printing in accordance with the densities of the plurality of developer images on the belt, In the forming process, a three-value test developing bias is set as the multiple test developing biases, and when the three-value test developing biases are set to a first value, a second value and a third value in order from the smallest, the magnitude of a third difference value between the old developing bias and the third value is increased as the ambient temperature acquired by the temperature sensor becomes lower.

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