JP4221966B2 - Image forming apparatus and image forming method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
According to the present invention, an electrostatic latent image is formed on an image carrier, and the toner is transferred from a toner carrier carrying toner to the surface of the image carrier to visualize the electrostatic latent image, thereby forming a toner image. The present invention relates to an image forming apparatus and an image forming method.
[0002]
[Prior art]
Image forming apparatuses such as copying machines, printers, and facsimile machines that apply electrophotographic technology are of the contact development type in which an image carrier and a toner carrier that are rotated and moved in a predetermined direction are held in contact with each other. And a non-contact developing type in which they are held apart. Among these, in a contact development type image forming apparatus, a developing bias in which an AC voltage is superimposed on a DC voltage or a DC voltage is applied to the toner carrier, and the toner carried on the surface of the image bearing device is static on the image carrier. When contacting the electrostatic latent image, a part of the surface moves to the image carrier in accordance with the surface potential to form a toner image.
[0003]
Further, in the non-contact developing type image forming apparatus, an alternating electric field as a developing bias is applied to the toner carrier, whereby an alternating electric field is formed in the gap between the image carrier and the toner by the action of this alternating electric field. The toner image is formed by flying.
[0004]
In such an image forming apparatus, the image density of the toner image may differ due to individual differences of the apparatus, changes with time, and changes in the surrounding environment of the apparatus such as temperature and humidity. In view of this, various techniques for stabilizing the image density have been proposed. As such a technique, for example, a test small image (patch image) is formed on an image carrier, and a density control factor that affects the image density is optimized based on the density of the patch image. There is. This technology forms a predetermined toner image on the image carrier while variously changing the density control factor and transfers the toner image on the image carrier or the toner image to an intermediate such as an intermediate transfer medium. The toner image is detected as a patch image, the image density is detected, and a density control factor is adjusted so that the patch image density matches a preset target density, thereby obtaining a desired image density. Is.
[0005]
[Problems to be solved by the invention]
In such an image forming apparatus, the image density of the formed toner image may periodically fluctuate due to a variation factor in the apparatus configuration. Examples of factors that cause such density fluctuation include eccentricity, deformation, and surface scratches of the toner carrier or the image carrier. Also, in an image forming apparatus that forms an electrostatic latent image by forming the surface of an image carrier with a photoconductor and exposing the surface with a light beam, the sensitivity of the photoconductor varies within the plane of the image carrier. In addition, the image density may periodically change due to the temperature change.
[0006]
Therefore, the density of the toner image formed as a patch image also changes not only with the set value of the density control factor but also with the above-described density fluctuation. If the value detected as the patch image density includes the influence of such density fluctuation, it is impossible to correctly grasp the correspondence between the density control factor and the image density. Therefore, even if the density control factor is optimized based on the patch image density, it is difficult to set the density control factor to an appropriate value.
[0007]
In the conventional image forming apparatus, the influence of density fluctuation due to such an apparatus configuration on the patch image density is not sufficiently considered, and density control is performed based on the density of the patch image including the influence of density fluctuation. Since the factors are set, image formation may be performed under image formation conditions that deviate from the original optimum conditions. As a result, a toner image with sufficient image quality may not necessarily be formed. there were.
[0008]
SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and can suppress the influence of patch image density fluctuation caused by fluctuation factors in the apparatus configuration and can stably form a toner image with good image quality. An object is to provide an apparatus and an image forming method.
[0009]
[Means for Solving the Problems]
The present invention comprises an image carrier that is formed in an endless shape and moves in a predetermined direction to convey an electrostatic latent image carried on its surface to a predetermined development position, while carrying toner on its surface. A toner carrier that transports the toner to the development position by rotating in a predetermined direction, and moving the toner carried on the toner carrier to the image carrier to convert the electrostatic latent image into toner In the image forming apparatus that visualizes the toner image and forms a toner image, in order to achieve the above object, the image forming condition is changed in multiple stages by changing and setting the density control factor that affects the image density in multiple stages. A toner image as a patch image is formed under each image forming condition, the toner density is detected, the density control factor is optimized based on the detection result, and the multi-stage image is detected. In at least one selected image forming condition among the forming conditions, the patch image covers all of a plurality of detection regions arranged on the outer circumferential surface of the image carrier so as to be different from each other in the circumferential direction. Each of the plurality of detection areas formed has a length corresponding to the circumferential length of the toner carrier in the patch length direction corresponding to the moving direction of the image carrier. An average value of the toner density is obtained for each, and a value obtained by further averaging the average value for each detection area is set as the toner density of the patch image. It is characterized by that.
[0010]
In the image forming apparatus configured as described above, the density of the toner image developed at the development position varies somewhat depending on the image carrier and the structure or characteristic variations of the toner carrier. In addition, since each of these is moving around, the density of the toner image formed as a patch image is complicated due to variations in the structure or characteristics of the image carrier and the toner carrier, and also corresponding to these movement cycles. Will fluctuate.
[0011]
According to the present invention, out of density fluctuations appearing in the patch image, those that are caused by the structure and characteristics of the image carrier and those that are caused by the structure and characteristics of the toner carrier are separately extracted. Is possible. That is, the toner density at each point on the patch image appears to be superimposed with the density fluctuation in the movement period of the toner carrier and the density fluctuation in the movement period of the image carrier. In the length corresponding to the circumferential length, density fluctuations appear in the moving period of the toner carrier. Therefore, by obtaining the toner concentration in the detection region having a length corresponding to the circumferential length of the toner carrier, it is possible to know the state of density fluctuation in the movement cycle of the toner carrier. On the other hand, the toner density detected in each detection area is superimposed with fluctuations in the moving period of the image carrier, so the difference in density between a plurality of detection areas with different positions can be seen. It is possible to know the state of concentration fluctuation in the moving period of the carrier.
[0012]
Therefore, in this image forming apparatus, it is possible to individually cope with density fluctuations caused by variations in the structure and characteristics of the image carrier and the toner carrier, and the influence of these density fluctuations on the patch image is affected. Can be eliminated by taking appropriate action it can. Specifically, an average value of toner density is obtained for each detection area, and a value obtained by further averaging the average value obtained for each detection area is set as the toner density of the patch image. as a result, In this invention, By setting the density control factor to an optimum state, it is possible to stably form a toner image with good image quality.
[0013]
The “length corresponding to the circumferential length of the toner carrying member” in the present invention means a moving distance in which one point on the outer circumferential surface of the image carrying member advances in the circumferential direction while the toner carrying member makes one round. This is the length of the surface area that passes through the development position during one round of the toner carrier on the outer peripheral surface of the image carrier. Therefore, when the moving speeds (peripheral speeds) of the surfaces of the toner carrier and the image carrier are equal, this length is equal to the circumferential length of the toner carrier, but in other cases, the length depends on the circumferential speed ratio. Longer or shorter than this.
[0014]
In the image forming apparatus configured as described above, it is sufficient that the toner image as the patch image is formed so as to cover at least each of the detection regions, and whether or not the toner image is formed in other regions. Is optional. Therefore, for example, the patch image formed under the selected image forming condition may be composed of a plurality of patch pieces corresponding to each of the detection regions, or may extend continuously in the patch length direction. It is also possible to form a strip shape that collectively covers the plurality of detection areas.
[0015]
Further, regarding the patch image formed as described above, it is possible to determine in detail the state of density change by detecting the toner density at a plurality of detection locations. The average value of the toner density at each detection position obtained based on the detection results at a plurality of different detection positions in one detection area among the plurality of detection areas is used as the toner density in the detection area of the patch image. be able to. In this way, by averaging the toner density within each detection region having a length corresponding to the peripheral length of the toner carrier, the influence of the density variation that appears in the movement cycle of the toner carrier can be canceled. .
[0016]
Further, if the average value of the toner density obtained in each detection area is set as the toner density of the patch image, the influence of density fluctuation that appears in the moving period of the image carrier can be canceled. It is possible to correctly obtain the patch image density under the image forming conditions by eliminating the influence of density fluctuations that occur in the moving periods of the toner carrier and the image carrier.
[0017]
Further, in the image forming apparatus configured as described above, in order to accurately grasp the density fluctuation that appears in the moving period of the image carrier and effectively eliminate the influence thereof, each of the detection regions includes the image It is desirable that they are arranged at regular intervals within a range corresponding to the circumference of the carrier.
[0018]
Further, it is desirable that the circumference of the image carrier is an integral multiple of the length corresponding to the circumference of the toner carrier. In such a case, each point on the surface of the image carrier is opposed to the same position on the toner carrier at the development position in every round, and the fluctuation cycle of the image density is the movement cycle of the image carrier. It will be the same. Therefore, if the state of density fluctuation is obtained for one period of the image carrier, it is possible to grasp in advance how the density fluctuation appears in the subsequent toner image formation. If a correction process is appropriately performed in response to the density fluctuation, a toner image in which the density fluctuation is further suppressed can be formed.
[0019]
Such density fluctuation tends to appear in an image formed under an image forming condition in which the image density is relatively low. Therefore, it is preferable that the low-density side image forming condition at which the image density is lowest among the multi-stage image forming conditions is at least the selected image forming condition.
[0020]
Further, the surface of the image carrier is formed by a photoconductor, and in the image forming apparatus for forming the electrostatic latent image by exposing the surface of the photoconductor with a light beam, due to the nature of the material, the surface of the photoconductor Since the characteristics may have a relatively large variation in the plane, it is particularly effective to apply the invention to an image forming apparatus having such an image carrier.
[0021]
In addition, in the image forming apparatus further including a bias applying unit that applies a predetermined developing bias to the toner carrier, the developing bias can be used as the density control factor.
[0022]
Furthermore, in an image forming apparatus further comprising an intermediate body configured to be able to temporarily carry a toner image visualized on the surface of the image carrier, the toner image as a patch image carried on the surface of the intermediate body It may be configured to detect the toner concentration. That is, for the toner image visualized on the image carrier as a patch image, the toner density may be directly detected on the image carrier, or after the toner image is transferred to the intermediate You may make it detect above.
[0025]
The image forming method according to the present invention forms an electrostatic latent image on the surface of an image carrier that is endlessly formed and moves around in a predetermined direction, and carries a toner on the surface in a predetermined direction. In the image forming method of forming the toner image by developing the electrostatic latent image with the toner by moving the toner from the rotating toner carrier to the image carrier, the image density is adjusted to achieve the above object. By changing and setting the density control factor that has an effect in multiple stages, while changing the image formation conditions in multiple stages, a toner image is formed as a patch image under each image formation condition, and the toner density is detected, and the detection result And the density control factor is optimized based on the moving direction of the image carrier in at least one of the multi-stage image forming conditions. All of the plurality of detection areas having a length corresponding to the circumferential length of the toner carrier in the patch length direction and arranged at different positions on the outer circumferential surface of the image carrier in the circumferential direction. The patch image is formed so as to cover the detection areas. An average value of the toner density is obtained for each, and a value obtained by further averaging the average value for each detection area is set as the toner density of the patch image. It is characterized by that.
[0026]
In the image forming method configured as described above, as in the above-described apparatus, the density fluctuation that appears corresponding to the moving period of the toner carrier and the density fluctuation that appears corresponding to the moving period of the image carrier are patch images. By effectively canceling the influence on the density and optimizing the density control factor while eliminating these influences, the image quality can be improved by performing the image formation under the optimized image forming conditions. The stable toner image can be stably formed.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a diagram showing an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus of FIG. This image forming apparatus forms a full color image by superposing four color toners of yellow (Y), cyan (C), magenta (M), and black (K), or uses only black (K) toner. This is a non-contact developing type image forming apparatus for forming a monochrome image. In this image forming apparatus, when an image signal is given to the main controller 11 from an external device such as a host computer in response to an image formation request from a user, the engine controller 10 causes the engine unit EG to respond to a command from the main controller 11. Are controlled to form an image corresponding to the image signal on the sheet S.
[0028]
In the engine unit EG, the photosensitive member 2 is provided so as to be rotatable in an arrow direction D1 in FIG. Further, a charging unit 3, a rotary developing unit 4 and a cleaning unit 5 are arranged around the photosensitive member 2 along the rotation direction D1. The charging unit 3 is applied with a charging bias from the charging controller 103 and uniformly charges the outer peripheral surface of the photoreceptor 2 to a predetermined surface potential.
[0029]
Then, the light beam L is irradiated from the exposure unit 6 toward the outer peripheral surface of the photosensitive member 2 charged by the charging unit 3. The exposure unit 6 exposes the light beam L onto the photoconductor 2 in accordance with a control command given from the exposure control unit 102 to form an electrostatic latent image corresponding to the image signal on the photoconductor 2. For example, when an image signal is given from an external device such as a host computer to the CPU 111 of the main controller 11 via the interface 112, the CPU 101 of the engine controller 10 sends a control signal corresponding to the image signal to the exposure control unit 102 at a predetermined timing. In response to this, a light beam L is irradiated onto the photosensitive member 2 from the exposure unit 6, and an electrostatic latent image corresponding to the image signal is formed on the photosensitive member 2. When forming a patch image, which will be described later, as necessary, a control signal corresponding to a predetermined pattern of patch image signals is given from the CPU 101 to the exposure control unit 102, and the electrostatic image corresponding to the pattern is output. A latent image is formed on the photoreceptor 2. Thus, in this embodiment, the photoreceptor 2 functions as the “image carrier” of the present invention.
[0030]
The electrostatic latent image thus formed is developed with toner by the developing unit 4. In other words, in this embodiment, the developing unit 4 is configured to be detachably attached to the support frame 40 that is rotatably provided about the shaft center, a rotation drive unit that is not shown, and the support frame 40, and each color toner. Are provided with a yellow developing device 4Y, a cyan developing device 4C, a magenta developing device 4M, and a black developing device 4K. The developing unit 4 is controlled by the developing device controller 104 as shown in FIG. Based on the control command from the developing device controller 104, the developing unit 4 is driven to rotate, and the developing devices 4Y, 4C, 4M, and 4K are selectively positioned at positions facing the photoconductor 2. At the same time, a toner of a selected color is applied to the surface of the photoreceptor 2 by applying a developing bias described later. As a result, the electrostatic latent image on the photoreceptor 2 is visualized with the selected toner color.
[0031]
These developing units 4Y, 4C, 4M, and 4K all have the same structure. Therefore, the configuration of the developing device 4K will be described in more detail with reference to FIG. 3, but the structures and functions of the other developing devices 4Y, 4C, and 4M are the same. FIG. 3 is a cross-sectional view showing a developing device of the image forming apparatus. In the developing device 4K, a supply roller 43 and a developing roller 44 are axially attached to a housing 41 that accommodates toner T therein. When the developing device 4K is positioned at a predetermined position, the "toner" A developing roller 44 functioning as a “bearing member” is positioned opposite to the photosensitive member 2 with a predetermined gap therebetween, and these rollers 43 and 44 are engaged with a rotation driving unit (not shown) provided on the main body side. Rotate in a predetermined direction. The developing roller 44 is formed in a cylindrical shape from a metal or alloy such as copper, aluminum, and stainless steel so that a developing bias described later is applied. Then, the two rollers 43 and 44 rotate while being in contact with each other, whereby the black toner is rubbed against the surface of the developing roller 44 and a toner layer having a predetermined thickness is formed on the surface of the developing roller 44.
[0032]
In the developing device 4K, a regulating blade 45 for regulating the thickness of the toner layer formed on the surface of the developing roller 44 to a predetermined thickness is disposed. The regulation blade 45 is composed of a plate-like member 451 such as stainless steel or phosphor bronze and an elastic member 452 such as rubber or resin member attached to the tip of the plate-like member 451. The rear end portion of the plate member 451 is fixed to the housing 41, and the elastic member 452 attached to the front end portion of the plate member 451 is the rear end portion of the plate member 451 in the rotation direction D3 of the developing roller 44. It arrange | positions so that it may be located in the upstream rather than. Then, the elastic member 452 elastically contacts the surface of the developing roller 44, and the toner layer formed on the surface of the developing roller 44 is finally restricted to a predetermined thickness.
[0033]
Each toner particle constituting the toner layer on the surface of the developing roller 44 is charged by being rubbed with the supply roller 43 and the regulating blade 45, and here, the toner will be negatively charged. Toner that is positively charged by appropriately changing the potential of each part of the apparatus can also be used. FIG. 4 shows an example in which a toner layer composed of almost two layers of toner particles is formed, but the thickness of the toner layer is not limited to this.
[0034]
The toner layer thus formed on the surface of the developing roller 44 is conveyed to a developing position facing the photoreceptor 2 by the rotation of the developing roller 44. FIG. 4 is a diagram showing the development position in this embodiment. FIG. 5 is a diagram illustrating an example of the waveform of the developing bias. In this apparatus, the developing roller 44 provided in one developing device (yellow developing device 4Y in the example of FIG. 1) disposed at a position facing the photosensitive member 2 and the photosensitive member 2 form a gap G at the developing position DP. Oppositely arranged apart.
[0035]
Here, the radius of the developing roller 44 is 0.32 times the radius of the photoreceptor 2. Further, the peripheral speed ratio of the developing roller 44 to the photosensitive member 2, that is, the ratio of the moving speeds (peripheral speeds) of both surfaces is set to 1.6. Therefore, in this embodiment, “the length corresponding to the circumferential length of the developing roller 44 as the toner carrying member” is a value obtained by multiplying the circumferential length of the actual developing roller 44 by the circumferential speed ratio 1.6. Further, in this embodiment, the developing roller 44 rotates exactly five times during one rotation of the photosensitive member 2 from the radius ratio and the peripheral speed ratio.
[0036]
Then, a developing bias is applied to the developing roller 44 from the developing device controller 104 as “bias applying means”. As shown in FIG. 5A, this developing bias is an alternating voltage having a waveform in which a rectangular wave voltage having an amplitude Vpp is superimposed on the DC component Vavg, and this developing bias is applied to the developing roller 44. Then, an alternating electric field is generated at the developing position DP sandwiched between the developing roller 44 and the photosensitive member 2. Due to the action of the electric field, a part of the toner T carried on the developing roller 44 is released from the developing roller 44 and flies to the developing position DP to reciprocate (reference numeral T1). The toner thus flying adheres to each part of the photoconductor 2 according to the surface potential, whereby the electrostatic latent image on the photoconductor 2 is developed with the toner.
[0037]
In the developing bias, the frequency of the rectangular wave is 3 kHz and the amplitude Vpp is 1400V. Further, as will be described later, in this embodiment, the DC component of the developing bias (hereinafter referred to as “DC developing bias”) Vavg can be changed as a density control factor so that the patch image density becomes a predetermined target density. A desired image density is obtained by adjusting the DC developing bias Vavg. The variable range of the DC developing bias Vavg is (−110) V to (−330) V.
[0038]
These numerical values and the like are not limited to the above, and should be changed as appropriate according to the apparatus configuration. Further, the waveform of the alternating voltage as the developing bias is not limited to this, and for example, a sine wave or a triangular wave may be superimposed on the DC component. For example, as shown in FIG. 5B, a waveform whose duty ratio is not 50% may be used.
[0039]
Returning to FIG. 1, the description of the device configuration will be continued. The toner image developed by the developing unit 4 as described above is primarily transferred onto the intermediate transfer belt 71 of the transfer unit 7 in the primary transfer region TR1. The transfer unit 7 includes an intermediate transfer belt 71 stretched between a plurality of rollers 72 to 75, and a drive unit (not shown) that rotates the intermediate transfer belt 71 in a predetermined rotation direction D2 by rotationally driving the roller 73. It has. Further, a secondary transfer roller 78 configured to be able to contact and separate from the surface of the belt 71 is provided at a position facing the roller 73 with the intermediate transfer belt 71 interposed therebetween. When a color image is transferred to the sheet S, the color toner images formed on the photoreceptor 2 are superimposed on the intermediate transfer belt 71 to form a color image, and the color image is taken out from the cassette 8 to be intermediate. The color image is secondarily transferred onto the sheet S conveyed to the secondary transfer region TR2 between the transfer belt 71 and the secondary transfer roller 78. Further, the sheet S on which the color image is formed in this way is conveyed via the fixing unit 9 to a discharge tray portion provided on the upper surface portion of the apparatus main body. Thus, in this embodiment, the intermediate transfer belt 71 functions as the “intermediate body” of the present invention.
[0040]
Further, a cleaner 76, a density sensor 60, and a vertical synchronization sensor 77 are disposed in the vicinity of the roller 75. Among these, the cleaner 76 can be moved toward and away from the roller 75 by a cleaner driving unit (not shown). Then, the blade of the cleaner 76 abuts on the surface of the intermediate transfer belt 71 that is stretched over the roller 75 while moving to the roller 75 side, and the toner that remains on the outer peripheral surface of the intermediate transfer belt 71 after the secondary transfer. Remove. The vertical synchronization sensor 77 is a sensor for detecting the reference position of the intermediate transfer belt 71 and is used to obtain a synchronization signal output in association with the rotational drive of the intermediate transfer belt 71, that is, a vertical synchronization signal Vsync. Functions as a vertical sync sensor. In this apparatus, the operation of each part of the apparatus is controlled based on the vertical synchronization signal Vsync in order to align the operation timing of each part and accurately superimpose the toner images formed in the respective colors. Further, the density sensor 60 is provided to face the surface of the intermediate transfer belt 71, and measures the optical density of the patch image formed on the outer peripheral surface of the intermediate transfer belt 71 as will be described later.
[0041]
In FIG. 2, reference numeral 113 denotes an image memory provided in the main controller 11 for storing an image signal given from an external device such as a host computer via the interface 112. Reference numeral 127 is executed by the CPU 101. This is a memory for storing a calculation program, control data for controlling the engine unit EG, and the like.
[0042]
In the image forming apparatus configured as described above, the CPU 101 executes a predetermined optimization process based on a program stored in the memory 127 immediately after the power is turned on or when a predetermined number of images are formed. The image development is improved and stabilized by optimizing the DC developing bias Vavg as a density control factor. More specifically, for each toner color, a toner image of a predetermined pattern is formed as a patch image each time the DC developing bias Vavg is changed, and the toner density of the patch image thus formed is detected, and the detection result The optimum value of the DC developing bias Vavg is obtained based on the above. Hereinafter, “formation of patch image” and “determination of optimum development bias” which are main parts of the optimization processing will be described in more detail.
[0043]
FIG. 6 is a flowchart showing the patch image forming operation in this embodiment. In this embodiment, the DC developing bias Vavg is changed and set in six stages from V0 having the smallest absolute value | Vavg | to V5 having the largest absolute value | Vavg |, and a patch image is formed at each stage. First, one of the four colors, for example, a yellow color, is selected, and the developing unit 4 is rotated so that the developing roller 44 provided in the developing device 4Y corresponding to the color is disposed at a position facing the photoconductor 2. (Step S1). Next, the count value n of the internal counter of the CPU 101 is reset (step S2). Then, the DC developing bias Vavg is set to Vn (here, since n = 0, Vn = V0) (step S3). Here, it is determined whether or not the count value n is 5 (step S4). In this case, since n = 0, the process proceeds to step S5, and a patch image composed of four patch pieces Pf1 to Pf4 shown in FIG. P0 is formed. FIG. 7 is a view showing a patch image transferred to the surface of the intermediate transfer belt in this embodiment. Note that an arbitrary image pattern of the patch image, for example, a solid image or a halftone image can be used. The reason why the patch image has such a shape will be described in detail later.
[0044]
Then, the count value n is incremented (step S6), the process returns to step S3, and the processes of steps S3 to S6 are repeated until the count value n becomes 5.
[0045]
On the other hand, when the count value n is 5 in step S4, the process proceeds to step S7 to form a patch image P5 consisting only of the patch piece Pf1, and then the developer is switched (step S8). More specifically, the developing unit 4 shown in FIG. 1 is rotated 90 degrees counterclockwise. By doing so, the cyan developing device 4C is positioned to face the photosensitive member 2 instead of the yellow developing device 4Y.
[0046]
As a result of forming a patch image with each developing bias in this way, each of the five stages of DC developing bias Vn (n = 0, 1,..., 4) on the intermediate transfer belt 71 along the moving direction D2. A patch formed of four patch pieces Pf1 to Pf4 and formed of four patch pieces Pn (n = 0, 1,..., 4) and a DC development bias V5 and formed of one patch piece Pf1. The image P5 is arranged in order. The total number of patch pieces is 21. FIG. 7 representatively shows only a patch image Pn formed by one DC developing bias Vn and including four patch pieces Pf1 to Pf4.
[0047]
Here, the reason why the patch image Pn at each DC developing bias Vn is shaped as described above will be described with reference to FIGS. FIG. 8 is a diagram showing the eccentricity of the photosensitive member and the developing roller and the fluctuation of the gap between them based on the eccentricity. FIG. 9 is a diagram showing the density variation of the patch image that occurs according to the gap variation. As described above, in this type of image forming apparatus, fluctuations in image density may occur in synchronization with the rotation cycles of the photoreceptor 2 and the developing roller 44. As an example of the cause of such density fluctuation, here, a case where the photosensitive member 2 and the developing roller 44 are eccentric is considered. In addition to the eccentricity of the photosensitive member 2 and the developing roller 44, the cause of the periodic density fluctuation is, for example, deformation due to wear, scratches / dirt generated on the surface thereof, and further the photosensitive member. The variation in sensitivity in the plane of 2 is considered. Although the degree of density fluctuations caused by these changes, the fluctuations vary depending on the rotation cycle of the photosensitive member 2 and the developing roller 44. Therefore, these effects should be considered in the same manner as in the case of the eccentricity described below. Can do.
[0048]
When the photoreceptor 2 is eccentric, the radius of the portion facing the developing position DP is periodically increased or decreased with time t as shown in FIG. 8A, for example, in synchronization with the rotation period To. The eccentric amount of the photosensitive member 2 here is a difference between the radius of the photosensitive member 2 on the line segment connecting the central axes of the photosensitive member 2 and the developing roller 44 and the average radius thereof. On the other hand, since the developing roller 44 rotates five times while the photosensitive member 2 makes one rotation, the rotation cycle Td is ５ of the rotation cycle To of the photosensitive member 2. Therefore, the variation in radius due to the eccentricity is as shown in FIG. 8B, for example. As a result, the gap G between the photosensitive member 2 and the developing roller 44 at the developing position DP (FIG. 4) varies in a complicated manner as shown in FIG. 8C.
[0049]
In the non-contact development type image forming apparatus, the amount of toner flying in the gap G varies depending on the electric field strength of the alternating electric field generated in the gap G. Therefore, such a gap variation causes a change in image density. That is, as shown by a curve a in FIG. 9, the density of the formed image changes periodically according to the fluctuation of the gap G. For this reason, the density of the patch image formed as an index for optimizing the density control factor also changes depending on the formation position, and this density variation may affect the optimization process. For example, even when the DC developing bias Vavg as the density control factor is set to a certain value, the image density between the patch image formed at the position A and the patch image formed at the position B shown in FIG. If the optimum value of the DC developing bias Vavg is obtained based on the image density, the result is greatly different.
[0050]
Therefore, in this apparatus, in view of the fact that the density fluctuation appears in synchronization with the rotation cycle of the photosensitive member 2 and the developing roller 44, as shown in FIG. The patch image Pn is determined by the value of Vavg), and is composed of four patch pieces Pf1 to Pf4. The patch pieces Pf1 and the like are arranged at equal intervals in a section corresponding to the circumferential length Lo of the photosensitive member 2 and have lengths corresponding to the circumferential length of the developing roller 44 (that is, the circumferential speed ratio to the circumferential length of the developing roller 44). A value obtained by multiplying by 1.6) is formed so as to cover four detection regions Rd having Ld. More specifically, the patch pieces Pf1 to Pf4 are formed as rectangles that are slightly larger than the detection region Rd in consideration of positional deviations in image formation or toner density detection. By doing so, density fluctuations in the rotation period of the developing roller 44 appear as density fluctuations in each patch piece, while density fluctuations in the rotation period of the photosensitive member 2 appear as density differences between the patch pieces. Thus, these can be processed individually. The detection region Rd is virtually provided to determine a region where the toner density is detected by the density sensor 60, and requires some special configuration on the surface of the photoreceptor 2 or the intermediate transfer belt 71. It is not a thing.
[0051]
In the patch pieces Pf1 to Pf4 formed in this way, density fluctuations as shown in FIG. That is, for example, in the patch piece Pf1, the image density varies between the highest density d1max and the lowest density d1min depending on the position. In the density fluctuation, a phenomenon caused by the photosensitive member 2 (curve b in FIG. 9) and a phenomenon caused by the developing roller 44 are superimposed. Among these, the periodic density fluctuation caused by the developing roller 44 can be canceled by averaging the length Ld corresponding to the circumference. That is, if the average image density d1avg for the length Ld of the patch piece Pf1 is obtained, the average value d1avg is a curve representing the density fluctuation caused by the photoreceptor 2 as shown by the circle Q in FIG. It is a value that rides on b.
[0052]
Similarly, if the average image density is obtained for the length Ld of the other patch pieces Pf2, Pf3 and Pf4, the density fluctuations in the rotation cycle of the developing roller 44 are canceled, and the circles shown in FIG. As indicated by the marks, these values substantially represent density fluctuations in the rotation cycle of the photosensitive member 2. Then, by averaging the four values of the average image density obtained for each patch piece Pf1 to Pf4, the average image density davg (n of patch image Pn from which the influence of density fluctuation in the rotation cycle of the photoreceptor 2 is eliminated. ).
[0053]
On the other hand, the patch image P5 formed with the maximum value V5 in the variable range of the DC developing bias Vavg is composed of one patch piece Pf1. This is because when the DC development bias Vavg is increased, the density fluctuation decreases as the image density increases. Therefore, the influence of the density fluctuation is small in the region where the DC development bias Vavg is large as described above, and the patch is not necessarily applied as described above. This is because it is not necessary to construct an image. In this embodiment, when the DC developing bias Vavg is set to the maximum value V5, the toner consumption is reduced by forming a patch image P5 composed of only one patch piece.
[0054]
As described above, in this embodiment, the patch image Pn including the four patch pieces Pf1 to Pf4 is formed at the five-stage bias values V0 to V4 in which the image density is lower among the six-stage DC current biases V0 to V5. The image forming conditions obtained by setting the DC developing bias Vavg to any one of these values V0 to V4 correspond to the “selected image forming conditions” of the present invention. Which one of the multi-stage image forming conditions is set as the selected image forming condition is not limited to the above, but is arbitrary. However, as described above, since the density variation appears remarkably in the condition where the image density is relatively low, It is desirable to configure the patch image as described above at least on the low density side image forming conditions where the image density is lowest.
[0055]
Next, based on the above discussion, a method for determining the optimum developing bias while eliminating the influence of the density variation of the patch image will be described. FIG. 10 is a flowchart showing the operation for determining the optimum developing bias in this embodiment. Regarding the total of 21 patch pieces formed as described above, the toner density is measured at the timing when each patch piece moves to a position facing the density sensor 60 as the intermediate transfer belt 71 moves. (Step S11). At this time, since the CPU 101 samples the output signal from the density sensor 60 at a constant period, the toner density of each patch piece is detected at a plurality of detection positions which are different from each other in the patch length direction D2. Will be.
[0056]
Then, while increasing the count value n of the internal counter of the CPU 101 by 1 from 0 to 4 (steps S12 and S15), the average toner density d1avg for each of the four patch pieces Pf1 to Pf4 formed with each developing bias Vn. -D4avg is obtained (step S13). More specifically, for example, the average value of the data detected in a range corresponding to the length Ld corresponding to the circumferential length of the developing roller 44 among the toner density data sampled at a plurality of locations of the patch piece Pf1 is the patch. The average toner density d1avg of the piece Pf1 is assumed. Similarly, the average toner density d2avg and the like are obtained for the other patch pieces Pf2 and the like.
[0057]
Next, the average values of the average toner densities d1avg to d4avg of the respective patch pieces Pf1 to Pf4 thus obtained are obtained, and this is set as the average toner density davg (n) of the patch image Pn (step S14). The above steps S13 and S14 are repeated while incrementing the count value n until it is determined that n = 5 in step S16, whereby the average of the patch images P0 to P4 formed with the DC developing biases V0 to V4, respectively. The toner concentrations davg (0) to davg (4) are obtained.
[0058]
On the other hand, for the patch image P5 formed only by the patch piece Pf1 formed by the DC developing bias V5, the average toner density of the patch piece Pf1 is set as the average toner density davg (5) of the patch image P5 (step S17).
[0059]
Then, the optimum value Vop of the DC developing bias Vavg is obtained from the average toner density davg (n) of each patch image Pn thus obtained based on the principle shown in FIG. 11, for example (step S18). FIG. 11 is a diagram in which the toner density davg (n) is plotted for the patch image Pn formed with each DC developing bias Vn. By obtaining the average toner density davg (n) of each patch image Pn as described above, the relationship between the DC developing bias Vavg and the patch image density is obtained. If the DC developing bias at which the toner density becomes the predetermined target density dt is obtained from this result, it is the optimum value Vop of the DC developing bias Vavg. In the example shown in FIG. 11, the target density dt is between the density davg (2) of the patch image P2 formed with the DC developing bias V2 and the density davg (3) of the patch image P3 formed with the DC developing bias V3. Therefore, interpolate between these two plots with a straight line or other appropriate function, and the optimum value of the DC development bias corresponding to the intersection (marked with x) with the straight line representing the density dt The value Vop can be determined.
[0060]
In this way, when the optimum value Vop of the DC developing bias Vavg for obtaining a desired image density for one toner color is obtained, the value is stored in the memory 127, and stored in the memory 127 in the subsequent image formation. The developing bias set based on the set value is applied to the developing roller 44.
[0061]
Then, by executing the above processing for each of the four toner colors, the optimum value Vop of the DC developing bias Vavg for each toner color is obtained, and by performing image formation under the image forming conditions thus optimized, In this image forming apparatus, it is possible to stably form a toner image with good image quality. As shown in FIG. 1, a position (primary transfer region TR1) where a toner image as a patch image is formed on the intermediate transfer belt 71 and a position where the toner density is detected (position facing the density sensor 60). Since the two processes of patch image formation and toner density detection can be performed independently of each other, it is possible to simultaneously execute two processes in parallel at these two positions. is there. Therefore, for example, if the processing for each toner color is performed in parallel, for example, the patch image formation for the next cyan color is performed during the density detection of the patch image formed for the yellow color, The time required for this process can be shortened.
[0062]
As described above, in the image forming apparatus according to this embodiment, the DC developing bias Vavg is functioned as a density control factor, and patch images are formed while variously changing the DC developing bias Vavg, and the toner density is detected. The optimum value Vop of the DC developing bias Vavg is obtained based on the detection result. Each patch image is composed of a plurality of patch pieces arranged at equal intervals in an area corresponding to the circumferential length Lo of the photosensitive member 2 on the surface of the intermediate transfer belt 71, and each patch piece is developed. The roller 44 has a length Ld corresponding to the circumferential length of the roller 44. Then, the toner density detected for each patch piece formed in this way is averaged, and the toner density of each patch piece is averaged to obtain the toner density of each patch image. Therefore, it is possible to cancel the influence of the periodic density fluctuation caused by the configuration of the photosensitive member 2 and the developing roller 44. As a result, the DC developing bias Vavg is set to an optimum state based on the patch image density, and the image quality is improved. Can be formed stably.
[0063]
The present invention is not limited to the above-described embodiment, and various modifications other than those described above can be made without departing from the spirit of the present invention. For example, in the above-described embodiment, the patch image Pn including four patch pieces Pf1 to Pf4 is formed. However, the number of patch pieces constituting one patch image is not limited to this, and the photosensitive member is not limited thereto. It may be determined as appropriate according to the dimensional ratio between the developing roller and the developing roller and according to the degree of density fluctuation that appears in each rotation cycle. However, in order to accurately extract the density fluctuation in the rotation cycle of the photoconductor, it is desirable to provide at least two detection areas for one rotation of the photoconductor.
[0064]
Furthermore, for example, it may be a patch image as a strip-shaped continuous image that covers a plurality of detection areas at once. FIG. 12 is a diagram illustrating an example of a patch image configured as a continuous image. In the present invention, the patch image Pn is configured to cover the entire surface of the plurality of detection regions Rd, but may have an arbitrary configuration for the other regions. Accordingly, as shown in FIG. 12, the patch image Pn may be formed as a continuous image that covers all of the plurality of detection regions Rd at once, and some of the detection regions Rd, for example, two each You may make it form the patch piece which covers.
[0065]
Here, when the two types of patch images shown in FIGS. 7 and 12 are compared, the amount of toner consumed as the patch image is larger in the case shown in FIG. Therefore, for example, when the interval between the detection regions Rd is large because the size ratio between the developing roller and the photosensitive member is large or the number of patch pieces to be formed may be small, as shown in FIG. It is possible to reduce toner consumption by forming a patch image composed of a plurality of patch pieces. On the other hand, when the distance between the detection areas is relatively small, there are few advantages, such as accuracy of alignment between the patch image forming position and the toner density detection position, detection error due to density unevenness at the edge of the image, etc. In view of this, it is preferable to use a continuous image as shown in FIG.
[0066]
In the above-described embodiment, the peripheral speed ratio between the photosensitive member 2 and the developing roller 44 is 1.6, that is, the developing roller 44 rotates at a peripheral speed 1.6 times that of the photosensitive member 2. However, the peripheral speed ratio between them may be other values. However, in this case, the length of each patch piece Pf1 etc. needs to be increased or decreased according to the peripheral speed ratio. For example, in a device in which the peripheral speed ratio is 1, that is, both rotate at the same peripheral speed, the “length corresponding to the peripheral length of the developing roller” is equal to the peripheral length of the developing roller. Therefore, in this case, the length of each detection region Rd may be the same as the circumferential length of the developing roller.
[0067]
Further, in the above-described embodiment, the circumferential length of the developing roller 44 is configured to be 0.32 times the circumferential length Lo of the photoreceptor 2, but the dimensional ratio between them is not necessarily a value other than this. Also good.
[0068]
Further, for example, in the above-described embodiment, the density sensor 60 is disposed so as to face the surface of the intermediate transfer belt 71 and the density of the patch image carried on the intermediate transfer belt 71 is detected. For example, a density sensor may be arranged facing the surface of the photoreceptor 2 to detect the density of the patch image developed on the photoreceptor 2.
[0069]
Further, for example, in the above-described embodiment, the density sensor 60 is configured by a reflective photosensor that irradiates light toward the surface of the intermediate transfer belt 71 and detects the amount of light reflected from the surface. In addition to this, for example, the light emitting element and the light receiving element of the density sensor may be installed so as to face each other with the intermediate transfer belt interposed therebetween, and the amount of light transmitted through the intermediate transfer belt may be detected.
[0070]
Further, for example, in the above-described embodiment, in order to obtain the average toner density of each patch piece, the average value of the toner density data sampled plurally at different positions of each patch piece is obtained, but the present invention is not limited to this. Instead, for example, the output voltage from the density sensor 60 may be detected continuously in each detection region Rd, and the average toner density may be obtained from the integrated value.
[0071]
Further, for example, in the above-described embodiment, the bias value when the image density coincides with the target density dt is obtained as the optimum value Vop of the DC developing bias Vavg. However, the present invention is not limited to this. For example, the bias value that is closest to the target density among the bias values that can be set in multiple steps and discretely may be set as the optimum value.
[0072]
In the above-described embodiment, the DC developing bias is used as a density control factor. In addition, the developing bias amplitude Vpp, the charging bias applied to the charging unit 3, the energy density of the exposure beam L, and the like are controlled. It can function as a factor.
[0073]
Further, the above-described embodiment is a non-contact developing type image forming apparatus in which the photosensitive member 2 and the developing roller 44 are arranged to face each other with a gap G therebetween. The present invention can also be applied to a system apparatus. In the contact development type apparatus, there is no problem of fluctuation of the gap G as in the above embodiment, but the contact pressure between the photosensitive member and the developing roller may periodically fluctuate due to their eccentricity, etc. Regarding variations in body characteristics, etc., there are problems similar to those of the non-contact developing system described above. Therefore, periodic density fluctuations may appear in the contact developing type image forming apparatus as well, and the influence can be eliminated by applying the present invention.
[0074]
The above-described embodiment is an image forming apparatus having the intermediate transfer belt 71 as an intermediate medium that temporarily carries the toner image developed on the photosensitive member 2, but other intermediates such as a transfer drum and a transfer roller. The present invention also relates to an image forming apparatus having a medium and an image forming apparatus configured to directly transfer a toner image formed on the photoreceptor 2 having an intermediate medium onto a sheet S as a final transfer material. Can be applied.
[0075]
In the above-described embodiment, the image forming apparatus is configured to be capable of forming a full-color image using toners of four colors of yellow, cyan, magenta, and black. For example, the present invention can be applied to an apparatus that forms a monochrome image using only black toner.
[0076]
Furthermore, in the above-described embodiment, the present invention is applied to a printer that executes an image forming operation based on an image signal from the outside of the apparatus. However, in response to a user's image formation request, for example, a copy button push, The present invention can also be applied to a copying machine that creates an image signal and executes an image forming operation based on the image signal, and a facsimile machine that executes an image forming operation based on an image signal given through a communication line. Needless to say.
[0077]
【The invention's effect】
As described above, according to the image forming apparatus or the image forming method according to the present invention, among the density fluctuations appearing in the patch image, those caused due to the structure and characteristics of the image carrier, and the structure of the toner carrier And those caused by characteristics and the like can be individually extracted.
[0078]
Then, by obtaining the toner density of the patch image while eliminating the influence of these density fluctuations and controlling each part of the apparatus based on the result, it is possible to stably form a toner image with good image quality.
[Brief description of the drawings]
FIG. 1 is a diagram showing an embodiment of an image forming apparatus according to the present invention.
2 is a block diagram showing an electrical configuration of the image forming apparatus of FIG.
FIG. 3 is a cross-sectional view illustrating a developing device of the image forming apparatus.
FIG. 4 is a diagram showing a development position in this embodiment.
FIG. 5 is a diagram illustrating an example of a waveform of a development bias.
FIG. 6 is a flowchart showing a patch image forming operation in this embodiment.
FIG. 7 is a diagram showing a patch image transferred to the surface of the intermediate transfer belt in this embodiment.
FIG. 8 is a diagram showing the eccentricity of the photosensitive member and the developing roller and the fluctuation of the gap based on the eccentricity.
FIG. 9 is a diagram illustrating a density variation of a patch image that occurs according to a gap variation.
FIG. 10 is a flowchart showing an operation for determining an optimum developing bias in this embodiment.
FIG. 11 is a graph plotting the toner density of a patch image formed with each DC developing bias.
FIG. 12 is a diagram illustrating an example of a patch image configured as a continuous image.
[Explanation of symbols]
2 ... Photoconductor (image carrier)
3 ... Charging unit
4. Development unit
4Y, 4C, 4M, 4K ... Developer
6 ... Exposure unit
10 ... Engine controller
11 ... Main controller
44. Developing roller (toner carrier)
71. Intermediate transfer belt (intermediate)
101 ... CPU
104... Developer control unit (bias applying means)
60 ... Concentration sensor
DP: Development position
EG ... Engine part
Ld: Length corresponding to the circumference of the developing roller
Lo: Perimeter of photoconductor
Pf1 to Pf4 ... patch pieces
Pn ... Patch image (with DC development bias Vn)
Rd ... detection area

Claims (8)

An image carrier that is formed in an endless shape and moves in a predetermined direction to convey an electrostatic latent image carried on the surface thereof to a predetermined development position;
A toner carrier that transports the toner to the development position by rotating in a predetermined direction while carrying the toner on the surface, and moves the toner carried on the toner carrier to the image carrier. In the image forming apparatus for forming a toner image by visualizing the electrostatic latent image with toner,
By changing and setting the density control factor that affects the image density in multiple stages, while forming the toner image as a patch image under each image formation condition while changing the image formation conditions in multiple stages, the toner density is detected, Based on the detection result, the concentration control factor is optimized,
In at least one selected image forming condition among the multi-stage image forming conditions, the patch image includes a plurality of detection regions arranged on the outer circumferential surface of the image carrier so as to be different from each other in the circumferential direction. Each of the plurality of detection regions has a length corresponding to the circumferential length of the toner carrier in the patch length direction corresponding to the moving direction of the image carrier, An image forming apparatus , wherein an average value of toner density is obtained for each detection area, and a value obtained by further averaging the average value for each detection area is used as a toner density of the patch image . In the selected image forming condition, an average value of toner density at each detection position obtained based on detection results at a plurality of detection positions different from each other in one detection area of the plurality of detection areas is obtained from the patch image. The image forming apparatus according to claim 1, wherein an average value of toner density for the detection area is used. Wherein said patch image formed by the selected image forming conditions, the image forming apparatus according to claim 1 or 2 comprising a plurality of patches pieces corresponding to the respective detection region. 3. The image forming apparatus according to claim 1, wherein the patch image has a strip shape that continuously extends in the patch length direction and collectively covers the plurality of detection areas under the selected image forming condition. 5. The image forming apparatus according to claim 1, wherein the detection areas are arranged at equal intervals within a range corresponding to a circumferential length of the image carrier. Circumferential length of the image bearing member, an image forming apparatus according to any one of claims 1 configured so as to be an integral multiple of that length corresponds to the peripheral length of the toner carrier 5. The low density side image forming conditions by the image density is the lowest among the multiple stages of image forming conditions, the image forming apparatus according to one any of claims 1 to 6 which is of the selected image forming conditions. While forming an electrostatic latent image on the surface of the image carrier that is formed endlessly and moves around in a predetermined direction,
In an image forming method of forming a toner image by developing the electrostatic latent image with toner by moving the toner from a toner carrier rotating in a predetermined direction while carrying toner on the surface thereof to the image carrier ,
By changing and setting the density control factor that affects the image density in multiple stages, while forming the toner image as a patch image under each image formation condition while changing the image formation conditions in multiple stages, the toner density is detected, Based on the detection result, the concentration control factor is optimized,
At least one of the multi-stage image forming conditions has a length corresponding to the circumferential length of the toner carrier in the patch length direction corresponding to the moving direction of the image carrier, and The patch image is formed on the outer peripheral surface of the image carrier so as to cover all of the plurality of detection areas arranged at different positions in the circumferential direction, and the average value of the toner density is determined for each detection area. An image forming method characterized in that a toner density of the patch image is obtained by obtaining and averaging the average value for each detection area .

Images