JP2005017542A - Image forming apparatus and its control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To overcome the problem that detecting accuracy as a whole is deteriorated by being affected by the worse detecting accuracy on the staining of a sensor and the staining of ground because of merely using the mean value of output from two sensors as the result of the detection of a test pattern for controlling density in the conventional density control. <P>SOLUTION: The image forming apparatus is equipped with an image carrier and a plurality of sensors for detecting the density at positions different from each other on the image carrier. While an image is not formed on the image carrier, the output from the each sensor is examined (step S12). When the output from the sensor is low, it is considered as the influence by the staining. Then, at least one sensor whose output is high is selected, and the density control is performed by using at least one selected sensor (steps S13 and S14). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus and a control method thereof, and more particularly to a density control technique in an image forming apparatus.
[0002]
[Prior art]
The need for outputting documents and images in color has expanded with the progress of computerization, and various types of printers have been put on the market. As a color image forming method, a sublimation type, a thermal transfer type, an ink jet method, or the like is used, and an electrophotographic method is said to be most excellent for forming an image at high speed.
[0003]
In the electrophotographic image forming apparatus, there is a problem that the image density greatly fluctuates depending on the temperature and humidity used, variations in characteristics of the photosensitive member and the developer, and durability of the developing device. In particular, the color image forming apparatus has a problem that the color changes.
[0004]
In view of these problems, a test pattern for density control is formed in advance on one of the photoconductor, intermediate transfer body, and transfer body, and the density is detected by using a density detection sensor, thereby charging bias, developing bias, In general, density control is performed to stabilize image density by controlling image forming process conditions such as exposure amount.
[0005]
Further, in the color image forming apparatus, there is a problem of color misregistration due to a change in the writing start position (registration) of each color due to a design tolerance or the like. Since color misregistration is one of the important factors that determine image quality, registration detection patterns are formed and detected in advance on the photosensitive member, intermediate transfer member, and transfer member, and each color writing position in the sub-scanning direction, In general, registration control for correcting the writing start position of each color in the main scanning direction and the main scanning width of each color is performed.
[0006]
In the registration control, it is necessary to provide two sensors in the main scanning direction in order to correct each color writing position in the main scanning direction and the main scanning width of each color. Here, the sensor used for density control and the sensor used for registration control generally use an optical sensor composed of a light emitting part and a light receiving part. In recent years, a sensor for density detection and registration detection from the viewpoint of cost. Tend to be common (see, for example, Patent Document 1).
In consideration of developability and transferability unevenness in the longitudinal direction, density control is performed using two optical sensors.
[0007]
[Patent Document 1]
Japanese Patent No. 2573855
[0008]
[Problems to be solved by the invention]
However, the image forming apparatus that performs density control using a plurality of sensors has the following problems.
[0009]
When the sensor is contaminated with toner or the like, there is a problem that detection accuracy deteriorates. In addition, the detection accuracy also deteriorates when the photoconductor, intermediate transfer member, and transfer member serving as a base on which the density control test pattern is formed are dirty. Although both the sensor contamination and the background contamination become worse with the years of use of the image forming apparatus, the toner scattered in the image forming apparatus flows into the air flow and the contamination of only one of the two sensors Tend to get worse. However, in the conventional density control, the average value of the two sensor outputs is simply used as the detection result of the test pattern for density control. There is a problem that gets worse. Furthermore, in the unlikely event that one of the sensors fails and cannot be detected, the density control cannot be performed, resulting in a problem that the color changes.
[0010]
The present invention has been made to solve the above-described problems, and aims to improve the accuracy of density detection by a density detection sensor and thereby improve the stability of image density by density control. .
[0011]
[Means for Solving the Problems]
According to an aspect of the present invention, an image carrier, a plurality of sensors for detecting densities at different positions on the image carrier, and an image formed on the image carrier are not formed. There is provided an image forming apparatus comprising: selection means for selecting at least one sensor based on the output of each sensor; and density control means for performing density control using the selected at least one sensor. The
[0012]
According to another aspect of the present invention, there is provided a control method for an image forming apparatus including an image carrier and a plurality of sensors for detecting densities at different positions on the image carrier. Selecting at least one sensor based on the output of each sensor when no image is formed on the image carrier, and performing density control using the selected at least one sensor. An image forming apparatus control method is provided.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.
[0014]
(First embodiment)
FIG. 1 is a schematic cross-sectional view of an image forming apparatus according to the present embodiment.
[0015]
The image forming apparatus includes a rotating drum type photosensitive drum 1 as an image carrier. Around the photosensitive drum 1, a charging roller 2, a developing device 4, a rotary intermediate transfer drum 6, and a photosensitive drum cleaning device 7 are disposed, and the upper portion between the charging roller 2 and the developing device 4 is exposed. A device 3 is arranged.
[0016]
The intermediate transfer drum 6 as an image carrier is in contact with the surface of the photosensitive drum 1 at the primary transfer nip portion, and is further in contact with the surface of the secondary transfer belt 8 at the secondary transfer nip portion. An intermediate transfer drum cleaning roller 10 is disposed on the outer periphery of the intermediate transfer drum 6. A fixing device 9 is disposed on the downstream side in the transport direction of the secondary transfer belt 8. A density detection sensor 11 is disposed at a position facing the intermediate transfer drum 6. The arrangement of the density detection sensor 11 will be described in more detail later.
[0017]
In the present embodiment, the photosensitive drum 1 is, for example, an OPC photosensitive drum having a diameter of 62 mm. An undercoating layer, a charge injection preventing layer, a charge generation layer, and a charge transport layer are provided on an aluminum drum. It is rotationally driven in the direction of arrow a at a peripheral speed (for example, 100 mm / sec), and receives a uniform negative charge by the charging roller 2 that contacts in the rotation process. A charging roller 2 as a charging unit is rotatably contacted with the surface of the photosensitive drum 1 and charges the photosensitive drum 1 to a predetermined polarity and potential by a charging bias applied from a charging bias power source (not shown).
[0018]
The exposure apparatus 3 includes a laser driver (not shown), a laser diode, a polygon mirror, and the like, and a laser beam modulated in response to a time-series electric digital image signal of image information input to the laser driver is a laser diode. The laser beam is scanned with a polygon mirror that is rotated at a high speed, and the surface of the photosensitive drum 1 is subjected to image exposure L via a reflection mirror (not shown), thereby forming an electrostatic latent image corresponding to the image information. .
[0019]
The developing device 4 includes a yellow (Y) developing device 4a, a magenta (M) developing device 4b, a cyan (C) developing device 4c, and a black (Bk) developing device 4d as non-magnetic one-component developing devices mounted on a rotating body 5. The yellow (Y) developing unit 4a, the magenta (M) developing unit 4b, the cyan (C) developing unit 4c, and the black (Bk) developing unit 4d are rotated by the rotation of the rotating body 5 by a rotation driving device (not shown). Is disposed at a position facing the photosensitive drum 1 during the development process. The toner is attached to the electrostatic latent image formed on the photosensitive drum 1 by these developing devices 4a, 4b, 4c, and 4d, and developed as a toner image.
[0020]
Each color toner used in the yellow (Y) developing unit 4a, the magenta (M) developing unit 4b, and the cyan (C) developing unit 4c is a capsule-type spherical non-mag toner manufactured by a polymerization method and encapsulating wax. The black toner used in the black (Bk) developing device 4d is obtained by subjecting a pulverized toner having a particle diameter of 6 um to spheroidizing treatment, and adding 100 parts of magnetite to the polyester binder, in addition to a charge control agent, a lubricant, and the like. Internally added.
[0021]
In the present embodiment, a bias in which a rectangular wave having a frequency of 2000 Hz and a peak-to-peak voltage of 2000 Vpp is applied to each of the developing devices 4a, 4b, 4c, and 4d with a direct current of −350V, and an exposed portion on the surface of the photosensitive drum 1 Is developed with a negatively chargeable negative toner to reveal an electrostatic latent image.
[0022]
The photosensitive drum cleaning device 7 removes and collects primary transfer residual toner remaining on the photosensitive drum 1 without being primarily transferred to the intermediate transfer drum 6.
[0023]
In the present embodiment, the intermediate transfer drum 6 has a circumference that can write an image corresponding to paper (A3 size in the present embodiment) as a transfer material having a diameter of 186 mm and a maximum sheet passing size. And rotate in the direction of arrow b. A primary transfer bias power source (not shown) is connected to the intermediate transfer drum 6, and a predetermined primary transfer bias (for example, +200 V in the present embodiment) is applied to a core metal (not shown) of the intermediate transfer drum 6. As a result, the toner image on the photosensitive drum 1 is primarily transferred onto the intermediate transfer drum 6 by the potential difference between the photosensitive drum 1 and the intermediate transfer drum 6 at the primary transfer nip.
[0024]
In the present embodiment, the intermediate transfer drum 6 is formed, for example, by forming an elastic resistance layer made of a medium-resistance rubber material having a thickness of 5 mm on the outer peripheral surface of an aluminum drum and further ensuring releasability on the surface. Fluorine resin is coated. The rubber material is made of NBR and ethylene oxide, and the resistance is reduced to 1E7 Ωcm by ethylene oxide. The fluorine-based resin coated on the surface has a volume resistance value of 1E14 Ωcm. The volume resistance value of the intermediate transfer drum 6 is obtained by converting from a current measured by bringing a metal drum having a diameter of 62 mm into contact with the entire length of the intermediate transfer drum 6 with a nip width of 7 mm and applying a voltage of 1000 V therebetween. It was.
The secondary transfer belt 8 is stretched and suspended by the transfer roller 12 and the drive roller 13, and the upper surface of the belt rotates in the direction of arrow c by the rotation of the drive roller 13. The secondary transfer belt 8 is disposed so as to be in contact with and away from the intermediate transfer drum 6. Secondary transfer onto the transfer material P and then the secondary transfer belt 8 contacts the intermediate transfer drum 6.
[0025]
In order to attract the transfer material P, the secondary transfer belt 8 is coated with 30 μm PVDF on a conductive urethane belt to increase the electrostatic capacity, and 10 cm on the belt surface. ² The resistance value measured by applying a voltage of 1000 V between this region and the belt substrate was 1E10Ω. The secondary transfer roller 12 and the driving roller 13 are low resistance rubber rollers, and the impedance of the secondary transfer belt 8 substantially depends only on the resistance of the surface layer of the secondary transfer belt 8. A secondary transfer bias power source (not shown) is connected to the secondary transfer roller 12, and a transfer current is passed from the secondary transfer bias power source (not shown) to the secondary transfer belt 8 to transfer a toner image to a transfer material. Transfer on top.
[0026]
A bias power supply (not shown) is connected to the intermediate transfer drum cleaning roller 10 to remove secondary transfer residual toner remaining on the surface of the intermediate transfer drum 6 without being transferred to the transfer material P. The intermediate transfer drum cleaning roller 10 charges the secondary transfer residual toner on the intermediate transfer drum 6 with a reverse polarity, and the secondary transfer residual toner charged with the reverse polarity is stored on the photosensitive drum 1. Simultaneously with the primary transfer of the toner image onto the intermediate transfer drum 6, the toner image is transferred from the intermediate transfer drum 6 to the photosensitive drum 1 and removed and collected by the cleaning device 7.
[0027]
The fixing device 9 includes a fixing roller 9a and a pressure roller 9b, and nipping and conveying a transfer material P on which an unfixed toner image is transferred to a fixing nip portion between the fixing roller 9a and the pressure roller 9b. Then, the transfer material P is heated and pressurized to fix the toner image.
[0028]
The CPU 17 comprehensively controls an image forming operation including density control, which will be described later, and includes a ROM 17a and a RAM 17b. The CPU 17 is connected to the density detection sensor 11 as shown in the figure, and is also connected to the exposure apparatus 3.
[0029]
Next, an image forming operation of the above-described image forming apparatus will be described.
[0030]
Image exposure with a laser beam is given to the charged photosensitive drum 1 by the exposure device 3, and an electrostatic latent image corresponding to a first color component image (for example, a yellow component image) of a target color image is formed. . At this time, the electrostatic latent image formed on the photosensitive drum 1 in the image-exposed portion is developed with yellow toner as the first color by the yellow (Y) developing device 4a.
[0031]
The yellow toner image of the first color formed and supported on the photosensitive drum 1 passes through the primary transfer nip portion between the photosensitive drum 1 and the intermediate transfer drum 6 and is primary on the outer peripheral surface of the intermediate transfer drum 6. It will be transcribed.
[0032]
The primary transfer residual toner remaining on the photosensitive drum 1 after the primary transfer of the yellow toner image is removed by the photosensitive drum cleaning device 7 and used for forming the next yellow toner image.
[0033]
Similarly, the magenta (M) developing unit 4b, the cyan (C) developing unit 4c, and the black (Bk) developing unit 4d are respectively formed and supported on the photosensitive drum 1 by the second color magenta toner image and the third color. The cyan toner image and the fourth color black toner image are successively superimposed on the intermediate transfer drum 6 to form a composite color toner image corresponding to the target color image.
[0034]
The secondary transfer belt 8 and the intermediate transfer drum cleaning roller 10 are separated from the intermediate transfer drum 6 in the sequential superimposing transfer process of the first to fourth color toner images from the photosensitive drum 1 to the intermediate transfer drum 6. .
[0035]
Then, a transfer material P such as paper is conveyed at a predetermined timing in accordance with the leading end of the composite color toner image on the intermediate transfer drum 6. Then, at the timing when the transfer material P passes through the paper feed path to the secondary transfer nip portion, the secondary transfer belt 8 swings so as to contact the intermediate transfer drum 6 and a secondary transfer bias power source (not shown). Thus, a predetermined secondary transfer bias is applied to the transfer roller 12, and the composite color image is secondarily transferred onto the transfer material P at once.
[0036]
The transfer material P onto which the composite color toner image has been transferred is subjected to curvature separation on the downstream side in the conveyance direction of the secondary transfer belt 8 and is nipped and conveyed between the fixing roller 9a and the pressure roller 9b in the fixing device 9 to be heated and heated. The composite color toner image is heat-fixed on the surface and output.
[0037]
Further, the secondary transfer residual toner remaining on the intermediate transfer drum 6 without being secondary transferred is converted to a reverse polarity by the intermediate transfer cleaning roller 10 to which a bias is applied, and is electrostatically applied to the photosensitive drum 1. The intermediate transfer drum 6 is cleaned. The secondary transfer residual toner adsorbed on the photosensitive drum 1 is then collected by the photosensitive drum cleaning device 7.
[0038]
FIG. 2 is a schematic diagram showing the arrangement relationship of the density detection sensor 11 with respect to the photosensitive drum 1 and the intermediate transfer drum 6 of the image forming apparatus. The density detection sensor 11 in the present embodiment includes, for example, two density detection sensors 11a and 11b, and 11a and 11b are arranged at positions separated by a certain distance in the rotation axis direction of the intermediate transfer drum.
[0039]
Next, the density detection sensors 11a and 11b will be described with reference to FIG. The density detection sensors 11a and 11b have the same configuration, and include a light emitting unit 20 and a light receiving unit 21 as shown in FIG. 3, and a light emitting unit is provided on the density control density patch 22 formed on the surface of the intermediate transfer drum 6. The spot light is irradiated from 20 and the reflected light is received by the light receiving unit 21, and the density is detected by the received light quantity.
[0040]
FIG. 4 is a diagram showing the relationship between the density of the Bk density patch and the output of the density detection sensor. The density is the density on the transfer material P when the toner image is transferred onto the transfer material under the same conditions. When the amount of applied toner on the intermediate transfer drum 6 is 0, the output value of the density detection sensor is 4V. When the amount of applied toner increases, the light emitted from the light emitting unit 20 is absorbed and diffused by the toner. The amount of reflected light incident on the unit 21 decreases, and as a result, the output value of the density detection sensor decreases. The relationship shown in the figure is experimentally obtained, and this relationship is stored as a density conversion table for each color component in, for example, the ROM 17a, and the output value of the density detection sensor is referred to by referring to this density conversion table. To concentration.
[0041]
Next, the density control process in this embodiment will be described. FIG. 5 is a flowchart illustrating an example of the density control process in the present embodiment. This density control is started by the CPU 17 at an appropriate timing such as when the main body is turned on, when a predetermined time has elapsed since the power was turned on, or when the number of printed sheets reaches a predetermined number.
[0042]
First, in step S11, the outputs of the density detection sensors 11a and 11b are detected in a state where no toner is on the intermediate transfer drum 6, and it is determined whether or not both density detection sensors are out of order. If the concentration detection sensor is out of order, spot light from the light emitting unit 20 is not irradiated, so there is no light incident on the light receiving unit 21, and the resulting sensor output value is close to 0V. Therefore, the threshold value for failure determination is set to Th (for example, 1V), and if the output of the concentration detection sensor is lower than Th, it is determined that the failure has occurred, and the outputs of the two concentration detection sensors 11a and 11b are both lower than Th. In the case, the density control is not executed and the process ends.
[0043]
If it is determined in step S11 that the output of at least one of the concentration detection sensors is equal to or greater than Th and no failure is recognized, the process proceeds to step S12. In step S12, it is determined which of the density detection sensors 11a and 11b is in better condition. As described above, when the density detection sensor becomes dirty, the sensor output decreases. In addition, even when the intermediate transfer drum 6 is dirty, the amount of reflected light decreases, so the sensor output decreases. Therefore, the sensor output is detected in a state where the toner is not on the intermediate transfer drum 6, the sensor outputs of the density detection sensors 11a and 11b are compared, and the larger sensor output is determined as a good density sensor. Here, when the output of the density detection sensor 11a exceeds the output of the density detection sensor 11b, the process proceeds to step S13, where density control is executed using the density detection sensor 11a, and the output of the density detection sensor 11a becomes the density detection sensor 11b. If the output is equal to or lower than the output, the process proceeds to step S14 to execute density control using the density detection sensor 11b.
[0044]
In the density control in step S13 or step S14, a density patch, which will be described later, is formed on the intermediate transfer drum 6, the output of the density detection sensor selected in step S12 is examined, and based on this, a lookup for halftone density correction is performed. It includes processing for creating a table (hereinafter referred to as “LUT”). This will be specifically described below.
[0045]
FIG. 6 shows a test image for density control formed on the intermediate transfer drum 6. Hereinafter, the test image for density control is referred to as “density patch”. As shown in the figure, this density patch is formed by forming images of eight levels of density for each of K, C, M, and Y for two columns, a and b. Here, the density patches K1a to K8a, C1a to C8a, M1a to M8a, and Y1a to Y8a in the a row are formed at positions that can be detected by the density detection sensor 11a, and the density patches K1b to K8b, C1b in the b row are formed. ˜C8b, M1b to M8b, and Y1b to Y8b are formed at positions that can be detected by the density detection sensor 11b. Further, for example, K1a and K1b are the same pattern, and similarly, the left and right density patches are the same pattern after K2a and K2b. Further, for example, in K1a to K8a, K1a has the lowest concentration, and the concentration gradually increases up to K8a. The same applies to density patches of other color components. Such density patches are stored in advance in the ROM 17a, for example.
[0046]
First, image data corresponding to the density patches K1 to K8 of the Bk component is read from the ROM 17a, and the data is sent to a laser driver (not shown) in the exposure apparatus 3, and the density of Bk formed on the intermediate transfer drum 6 is read. The patches K1 to K8 are measured at an appropriate timing by the density detection sensor selected in step S2. Next, the outputs sK1 to sK8 of the density detection sensor are stored in the RAM 17b. Then, by referring to the density conversion table stored in advance in the ROM 17a, the densities DK1 to DK8 corresponding to these outputs sK1 to sK8 are obtained. As a result, the relationship between the image signal and the density when actually output (so-called γ) can be obtained. Ideally, γ should not always change, but γ changes depending on the durability of the photosensitive drum and the developing device and the atmosphere environment in which the main body is used. As a result, the image density and color change, and the image quality is greatly impaired. Therefore, a density patch is formed and the current γ is obtained by measuring the density. Next, for example, so-called γ correction is performed to create an LUT for adjusting the correspondence between the input image signal and the laser output signal so that a linear relationship between the image signal and the actual density can be obtained. Therefore, an LUT for correcting the obtained density patch densities DK1 to DK8 to an appropriate value is created. Similarly, the LUT is created for the Y, M, and C components. Each created LUT is stored in the RAM 17b, for example. Note that the order of the color components for generating the LUT may be arbitrary.
[0047]
Thereafter, the halftone density can be corrected using the LUT created according to the degree of contamination of the density detection sensor as described above.
[0048]
In order to confirm the effect of this embodiment, the following evaluation was performed. One of the two density detection sensors was soiled with toner, and the output of the density detection sensor was set to 2 V with no toner on the intermediate transfer drum 6. Next, the patch density on the intermediate transfer drum 6 is changed in four stages, the sensor state of this embodiment is judged, density detection is performed based on the sensor output with the better state, and two sensor outputs as a conventional example. The detection error was compared with the case of density detection based on the average value. The result is shown in FIG.
As can be seen from the figure, it was confirmed that the detection error was smaller in this embodiment than in the conventional example.
[0049]
As described above, according to the present embodiment, a plurality of density detection sensors are provided, one density detection sensor having a good state is selected, and density detection is performed using the density detection sensor. Improvement can be achieved.
[0050]
(Second Embodiment)
In the first embodiment described above, one density detection sensor having a high output is selected and density control is performed using the density detection sensor. However, no abnormality is recognized in either density detection sensor. If the state is good, the density control may be performed using both density detection sensors.
[0051]
FIG. 7 is a flowchart showing an example of density control processing in the present embodiment. Steps similar to those in the flowchart according to the first embodiment shown in FIG. 5 are given the same reference numerals.
[0052]
The flowchart of FIG. 7 differs from the flowchart of FIG. 5 in that the processes of steps S21 and S23 indicated by a broken line are added between steps S11 and S12. In the following, description will be given in order.
[0053]
First, in step S11, the outputs of the density detection sensors 11a and 11b are detected in a state where no toner is on the intermediate transfer drum 6, and it is determined whether or not both density detection sensors are out of order. Here, for example, the determination is made based on whether or not the outputs of the two concentration detection sensors 11a and 11b are both lower than a first threshold Th1 (for example, 1 V) for failure determination. If the outputs of the two density detection sensors 11a and 11b are both lower than Th1, the density control is not executed and the process is terminated.
[0054]
If it is determined in step S11 that the output of at least one of the concentration detection sensors is equal to or greater than Th1, and no failure is recognized, the process proceeds to step S21. In step S21, it is determined whether or not the outputs of the two density detection sensors 11a and 11b exceed a second threshold Th2 (for example, 3V) higher than Th1 in a state where there is no toner on the intermediate transfer drum 6. Here, when the outputs of the density detection sensors 11a and 11b both exceed Th2, it is judged that both the density detection sensors 11a and 11b are in good condition with little dirt on the sensor itself and dirt on the intermediate transfer drum 6. In step S23, density control is performed using both the density detection sensors 11a and 11b. Specifically, the average value of the outputs of the density detection sensors 11a and 11b for each density patch is calculated for each color component using the density patch as shown in FIG. 6, and stored in the ROM 17a in advance. The density corresponding to these average output values is obtained by referring to the density conversion table, and an LUT is created based on the density.
[0055]
On the other hand, if at least one of the density detection sensors 11a and 11b is equal to or less than Th2 in step S21, the density detection sensor becomes dirty or its output is equal to or less than Th2, or the intermediate transfer drum 6 at the detection position thereof. It is determined that the state is bad because it is dirty, and the process proceeds to step S12, where processing as described in the first embodiment is performed.
[0056]
In order to confirm the effect of this embodiment, the following evaluation was performed. One of the two density detection sensors was soiled with toner, and the output of the density detection sensor was set to 2 V with no toner on the intermediate transfer drum 6. Next, the density patch density on the intermediate transfer drum 6 is changed in four stages and density detection is performed according to the present embodiment, and the density detection is always performed based on the average value of two sensor outputs as a conventional example. The detection error was compared. The result is shown in FIG. As can be seen from the figure, it was confirmed that the detection error was smaller in this embodiment than in the conventional example.
[0057]
As described above, in this embodiment, when the density detection sensor is faulty or the sensor is dirty, or the intermediate transfer drum is greatly affected by the dirt, density control is performed using one density detection sensor that does not have a problem, and any density detection is performed. When neither sensor nor failure was found and the condition was good, density control was performed using both density detection sensors, so that the density stability by density control could be further improved.
[0058]
(Third embodiment)
In the second embodiment described above, when the process proceeds to step S23 and density control is performed using both of the density detection sensors 11a and 11b, the output of either sensor can be used reliably. Thus, it is possible to shorten the time required to detect the density patch by changing the density patch pattern.
[0059]
The density patch shown in FIG. 6 has eight levels of density patches for each of K, C, M, and Y. The density patch is divided into two columns, a row that can be detected by the density detection sensor 11a and b row that can be detected by the density detection sensor 11b. Although it was formed, this was based on the premise that one of the two density detection sensors is not used. On the other hand, since both density detection sensors can be used reliably in step S23, as shown in FIG. 9, four density patches of the eight density patches of each color component can be detected by the density detection sensor 11a. If the other four-stage density patches are formed in a row and formed in a row that can be detected by the density detection sensor 11b, the conveyance length of the intermediate transfer drum 6 for detecting all the patches is almost halved. Thus, the time required to detect all density patches can be reduced by almost half. In the illustrated example, density patches K1 to K4, C1 to C4, M1 to M4, and Y1 to Y4 are formed in a row that can be detected by the density detection sensor 11a, and the density patches K5 to K4 are formed in a row that can be detected by the density detection sensor 11b. K8, C5-C8, M5-M8, Y5-Y8 are formed.
[0060]
In short, in the present embodiment, a plurality of density patches having different densities are distributed and formed at each position on the intermediate transfer drum 6 that can be detected by each sensor used.
[0061]
FIG. 8 is a flowchart showing an example of the density control process in the present embodiment. The density control process in the present embodiment is substantially the same as the density control process in the second embodiment shown in FIG. 7 except that the process of step S22 is added before step S23. In step S22, the density patch pattern as shown in FIG. 6 is changed to the density patch pattern as shown in FIG.
[0062]
Thus, according to the present embodiment, the time for detecting all density patches can be reduced to about half, so that the time required for density control can be shortened.
[0063]
(Other embodiments)
In the above-described embodiment, the density detection sensor is arranged to detect the density patch formed on the intermediate transfer drum 6 as the image carrier, but the density formed on the photosensitive drum which is also the image carrier. You may arrange | position so that a patch may be detected.
[0064]
Moreover, although the above-mentioned embodiment demonstrated what has two density | concentration detection sensors, this invention does not limit the number of sensors to two, You may provide three or more. In that case, for example, in step S12, at least one sensor having the larger output may be selected, and in steps S13 and S14, concentration control may be performed using the selected at least one sensor. In that case, in step S21 (see FIG. 8), it is determined whether or not all the sensor outputs exceed the threshold value Th2. If all the sensor outputs exceed the threshold value Th2, all the sensor outputs are determined in step S23. The density control is performed by using this. Furthermore, it goes without saying that the density patch formed on the intermediate transfer drum 6 is formed according to the position of the sensor to be used.
[0065]
【The invention's effect】
According to the present invention, it is possible to improve the density detection accuracy by the density detection sensor, and to increase the stability of the image density by the density control.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a configuration of an image forming apparatus according to an embodiment.
FIG. 2 is a diagram showing an arrangement mode of density detection sensors in the embodiment.
FIG. 3 is a diagram illustrating a configuration example of a density detection sensor in the embodiment.
FIG. 4 is a diagram illustrating an example of a relationship between a density patch density and an output of a density detection sensor in the embodiment.
FIG. 5 is a flowchart showing an example of density control processing in the first embodiment.
FIG. 6 is a diagram showing a density patch in the first embodiment.
FIG. 7 is a flowchart illustrating an example of density control processing according to the second embodiment.
FIG. 8 is a flowchart illustrating an example of density control processing according to the third embodiment.
FIG. 9 is a diagram illustrating an alternative density patch according to a third embodiment.
FIG. 10 is a diagram for explaining the effect of the first embodiment.
FIG. 11 is a diagram for explaining the effect of the second embodiment.

Claims (7)

An image carrier;
A plurality of sensors for detecting densities at different positions on the image carrier, and at least one sensor is selected based on the output of each sensor when no image is formed on the image carrier. A selection means;
An image forming apparatus comprising: density control means for performing density control using the selected at least one sensor. The image forming apparatus according to claim 1, wherein the selection unit selects at least one sensor having a larger output. The image forming apparatus according to claim 2, wherein the selection unit selects all the sensors when the output of each sensor exceeds the first threshold value. The density control by the density control means is not performed when the output of each sensor is less than a second threshold value that is lower than the first threshold value. The image forming apparatus described. The concentration control means includes
Image forming means for forming a plurality of test images having different densities at each position on the image carrier that can be detected by each sensor selected by the selection means;
Lookup table creation means for creating a lookup table for density correction based on the output of each sensor selected by the selection means for each test image formed on the image carrier. The image forming apparatus according to claim 1. The image forming unit distributes and forms the plurality of test images at positions on the image carrier that can be detected by the plurality of sensors when a plurality of sensors are selected by the selection unit. The image forming apparatus according to claim 5. A control method for an image forming apparatus comprising an image carrier and a plurality of sensors for detecting densities at different positions on the image carrier,
Selecting at least one sensor based on the output of each sensor when no image is formed on the image carrier;
And a step of performing density control using the selected at least one sensor.

Images