Patent Application
20090297222
1. Field of the Invention
The present invention relates to a technique for preventing color misalignment in a color-image forming apparatus.
2. Description of the Related Art
In recent years, color-image forming apparatuses that adopt electrophotography, such as color printers and color copying machines have been required to output high-quality images.
As factors for determining the quality of an output image, there are misalignment of an image-writing position on a recording medium, recording accuracy typified by image expansion/contraction, and color misalignment, that is, the overlaying accuracy of color toner images which influences on the color of the image.
In particular, with the electrophotographic color-image forming apparatuses, degradation of recording accuracy and changes in color due to color misalignment are caused by environmental changes or variable factors of device components due to long use, thus degrading the quality of output images.
An example of the causes of such changes, for example, in an image forming apparatus that adopts an intermediate transfer belt as an endless belt, is speed fluctuations of the intermediate transfer belt.
Thus, for example, a method disclosed in Japanese Patent Application Laid-Open No. 01-142567 is used. Specifically, color toner patches are formed on an intermediate transfer belt, the positions of the toner patches are detected by a registration sensor, and the time at which the color-toner images are written to the intermediate transfer belt is changed using the detection results, thereby preventing color misalignment. Here, the toner patches are unfixed toner images for detecting color misalignment.
However, even if the known color misalignment correction using the registration sensor is executed, color misalignment occurs when the color toner images are actually transferred to a recording medium after the correction.
This is because the peripheral speed of the intermediate transfer belt when the positions of the toner patches on the intermediate transfer belt are detected by the registration sensor and the peripheral speed of the belt during actual-image formation differ. Here, generation of the difference in the peripheral speed of the intermediate transfer belt will be described in an orderly manner.
A belt driving torque T at that time is expressed by the following Eq. (1):
T=Tb+μF×4 Eq. (1)
where Tb is a torque that moves only the intermediate transfer belt 30 and μF is a frictional force that is generated due to the contact of the intermediate transfer belt 30 and the drums 26, where μ is the friction coefficient between the belt 30 and the drums 26, and F is a transfer pressure. Here, the contact means a state in which the intermediate transfer belt 30 and the photosensitive drums 26 are in contact to generate pressure, irrespective of the presence of a toner layer between the intermediate transfer belt 30 and photosensitive drums 26.
Next, as shown in
T=Tb−μF×4 Eq. (2)
Here, changes in torque after the belt 30 is driven from its halted state until it is halted again through image formation will be described with reference to Eq. (1).
First, when the friction coefficient μ between the belt 30 and the drums 26 is defined as the following two, changes in the torque T after the belt 30 is driven from its halted state until it is halted again through image formation are expressed by the following Eqs. (3) to (7). Changes in load torque applied on the belt 30 are shown in
In the drawings, reference numeral 26 denotes photosensitive drums, numeral 54 denotes developing rollers, numeral 52 denotes primary transfer rollers, and numeral 30 denotes an intermediate transfer belt. Reference character Y indicates yellow, character M indicates magenta, character C indicates cyan, and character Bk indicates black. Here, the friction coefficient μ between the belt and the drum is defined as the following two: a friction coefficient μ1 when there is no toner between the belt and the drum; and a friction coefficient μ2 when there is toner between the belt and the drum.
T=Tb+μ1F×4 Eq. (3) (see FIG. 13)
T=Tb+(μ1F×3+μ2F) Eq. (4) (see FIG. 14)
T=Tb+(μ1F×2+μ2F×2) Eq. (5) (see FIG. 15)
T=Tb+(μ1F+μ2F×3) Eq. (6) (see FIG. 16)
T=Tb+μ2F×4 Eq. (7) (see FIG. 17)
Hereinafter, see Eq. (6) (see FIG. 18)→Eq. (5) (see FIG. 19)→Eq. (4) (see FIG. 20)→Eq. (3) (see
The two friction coefficients μ1 and μ2 generally have a relationship, μ1>μ2. The load (torque) applied to the belt is decreased when the developing roller 54 comes into contact therewith and is increased when the developing roller 54 comes out of contact therewith.
The mechanism of decreasing the load will be described in more detail. For example,
Next, with reference to the above description, a case in which toner patches are detected by the registration sensor will be described. A belt driving torque when toner patches on the belt 30 are detected by the registration sensor is constant in the state in Eq. (7), and the peripheral speed of the belt 30 is also constant. On the other hand, as has been described with reference to the foregoing Eqs. (3) to (7) and
On the other hand, it is known that the belt-drive transmission system constituted of a gear train for driving a belt is elastically deformed in proportion to stress generated from its load torque, as expressed by Hooke's law. Elastic deformation according to the generation of the load temporarily changes the transmission speed of the drive transmission system. In other words, it temporarily changes the peripheral speed of the belt. More specifically, the elastic deformation also has continuity, and therefore, also the position of the belt temporarily changes gradually due to the continuous elastic deformation. The temporary positional change of the belt causes fluctuations in the belt peripheral speed.
That is, the belt peripheral speed changes when the individual states in Eqs. (3) to (7) shift to the next states. For example, when the load torque applied on the belt changes from small to large, the belt peripheral speed slows down, and in contrast, when it changes from large to small, the belt peripheral speed increases. The fluctuations in belt peripheral speed here can also be regarded as following changes in belt position and can also be considered as changes in belt position due to temporary load fluctuations.
Even if toner patches on the belt are detected by the registration sensor, with no fluctuations in belt peripheral speed, and correction is made on the basis of the result, color misalignment (transfer position displacement) will occur because the belt peripheral speed fluctuates during actual image formation.
To eliminate the fluctuations in belt speed, there are the following three typical methods: first, eliminating elastic deformation of the belt-drive transmission system by increasing the rigidity thereof; secondly, eliminating fluctuations of the friction coefficient μ between the belt and the drum; and thirdly, executing image formation after the state of Eq. (7) has been achieved.
The first method will be described. In general, increasing the rigidity of the belt-drive transmission system can reduce the elastic deformation described above. For example, if the material of gears, which are elements of the drive transmission system, is changed from resin, such as polyacetal, to metal, such as brass, the rigidity can be increased. It has been confirmed by our experiment that speed fluctuations can be improved by increasing the rigidity using metal gears.
However, the metal gears have excessively high rigidity, which causes vibrations due to engagement, thus posing the adverse effect of applying the vibrations to an image. Moreover, since the metal gears are formed by cutting, the cost thereof is considerably higher than that of resin gears formed by injection molding, so that they are not practical.
The second method will be described. Theoretically, setting the friction coefficients μ1 and μ2 equal can reduce fluctuations of the friction coefficient μ. However, the surface layers of the present photosensitive drums are so smooth that they are prone to adhere to the belt, thus causing a significantly large frictional force. Although microscopic unevenness may be provided on the surfaces of the photosensitive drums to decrease contact areas, degradation of image quality can occur, so that it is not practical. Moreover, fluctuations in friction cannot be made zero because there is an attraction force due to transfer bias, in addition to the presence/absence of toner.
The third method will be described. The third method is technically feasible by turning ON/OFF the charging, developing, and transfer processes of the image-formation processing unit, which are the causes of generating load fluctuations, except when transferring a visible image from the photosensitive drums to the intermediate belt.
However, although this provides a high-quality image in which color misalignment is reduced, the ON/OFF of the charging, developing, and transfer processes of the processing unit is performed except when transferring a visible image from the photosensitive drums to the intermediate belt. This increases the processing time, such as charging and developing, thus decreasing the productivity of the apparatus. In other words, this has the problem of decreasing the life of the processing unit. In particular, when frequently printing a small number of pages, the influence is negligible. That is, this not only causes the user to frequently replace the processing unit but also increases its running cost.
According to an aspect of the present invention, a color-image forming apparatus includes a plurality of photosensitive members corresponding to individual colors and disposed along the moving direction of a transfer member to which images are to be transferred (hereinafter simply referred to as a transfer member), in which the individual photosensitive members are brought into contact with the transfer member so that toner images are transferred to the transfer member in sequence, thereby performing image formation of an input job. The color-image forming apparatus includes a control unit configured to control the peripheral speed of a photosensitive member, which is not forming a toner image on the transfer member, so that a load during forming of a toner image on the transfer member with part of the photosensitive members at least comes close to a reference load generated between all the photosensitive members and the transfer member while all the photosensitive members are forming toner images on the transfer member.
Further features and aspects of the present invention will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings.
Exemplary embodiments of the present invention will now be illustrated. The individual embodiments described below will be helpful in understanding a variety of concepts of the present invention from the generic to the more specific. Further, the technical scope of the present invention is defined by the claims, and is not limited by the following individual embodiments.
An image forming apparatus according to a first embodiment will be described. Here, as an example of the image forming apparatus that employs an electrophotographic system, a four-drum full-color-image forming apparatus using an intermediate transfer belt is shown.
As shown in
Here, the individual process cartridges P have memory tags (not shown) and are configured to determine the remaining lives and the replacement conditions thereof through communication with the main body 2.
The individual process cartridges P have photosensitive drums 26Y, 26M, 26C, and 26Bk, which are image bearing members (photosensitive members), and are arranged along the moving direction of the intermediate transfer belt 30, which is a transfer member to which images are to be transferred. The photosensitive drum 26Y corresponds to the uppermost stream photosensitive drum, and the photosensitive drum 26Bk corresponds to the lowermost stream photosensitive drum in the moving direction of the transfer member. Each of the process cartridges P integrally includes a primary charger 50 serving as a charging member, a developing unit 51 serving as a developing member, and a cleaner 53 serving as a cleaning member around the individual photosensitive drums 26. The process cartridges P are arranged in parallel along the intermediate transfer belt 30.
In each of the process cartridges P, the primary charger 50 is disposed on the outer circumference of the photosensitive drum 26 and uniformly charges the surface of the photosensitive drum 26. The developing units 51 develop electrostatic latent color images formed on the surfaces of the individual photosensitive drums 26 with laser beams from laser exposure units (exposure units) 28Y, 28M, 28C, and 28Bk using toners of corresponding colors (yellow, magenta, cyan, and black). The developing roller 54 in the developing unit 51 is configured to prevent degradation of developer by moving out of contact with the photosensitive drum 26 to stop the rotation together with the developing unit 51. That is, the developing roller 54 can come into and out of contact with the photosensitive drum 26 together with the developing unit 51. After toner images are transferred in sequence, the cleaners 53 remove the remainder of the transfer toner adhering to the surfaces of the photosensitive drums 26.
A primary transfer roller 52 that forms a primary transfer unit together with the photosensitive drum 26 is disposed at a position where it clamps the intermediate transfer belt 30 with the photosensitive drum 26.
On the other hand, the intermediate transfer belt unit 31 includes the intermediate transfer belt 30 and three rollers, that is, a driving roller 100, a tension roller 105, and a secondary-transfer counter roller 108, which stretch the intermediate transfer belt 30. The intermediate transfer belt 30 is rotationally run by rotationally driving the driving roller 100 with a belt drive motor (not shown).
The tension roller 105 is configured to be able to move horizontally in
Furthermore, two registration sensors 90 for detecting toner patches on the intermediate transfer belt 30 are provided in the vicinity of both ends of the driving roller 100 in the longitudinal direction. The longitudinal direction is the axial direction of the tension roller 105 and the widthwise direction perpendicular to the belt running direction.
Furthermore, a secondary transfer roller 27 that forms a secondary transfer unit together with the secondary-transfer counter roller 108 is disposed at a position where it clamps the intermediate transfer belt 30 with the secondary-transfer counter roller 108. The secondary transfer roller 27 is held by a transfer conveying unit 33.
A feeding unit 3 that feeds a recoding medium Q to a secondary transfer unit is disposed below the main body 2. The feeding unit 3 includes a cassette 20 accommodating a plurality of the recoding mediums Q, a feeding roller 21, a retarding roller pair 22 for preventing double feeding, conveying roller pairs 23a and 23b, a registration roller pair 24, etc.
Discharge roller pairs 61, 62, and 63 are provided along the conveying path downstream of the fixing unit 25.
Furthermore, the color-image forming apparatus 1 is configured for duplex printing, in which the recoding medium Q whose first surface is subjected to image formation is discharged from the fixing unit 25 and is conveyed toward reversing roller pairs 70 and 71 by switching a switching member 69. After the rear end of the recoding medium Q passes through a switching member 72, the switching member 72 is switched, and at the same time, the reversing roller pair 71 is reversed to introduce the recoding medium Q to a duplex conveying path 73.
Then, duplex-conveying-path roller pairs 74, 75, and 76 are rotationally driven to feed the recoding medium Q again, thereby allowing printing of the second surface.
Referring next to
The main body 2 shown in
An image-processing control unit (control unit) 11 converts received and input data to CMYK signals, makes corrections of the gray level and density, and thereafter generates an exposure signal for the laser exposure units 28 (28Y, 28M, 28C, 28Bk). An image-formation control unit 12 controls the overall image forming operation, described below and controls the main body 2 during correction in the image-forming operation using the registration sensors 90 serving as patch detectors and a mark sensor 91 serving as a mark detector.
The image-formation control unit 12 includes a CPU 121 that controls processing by the image-formation control unit 12, a ROM 122 that stores programs etc. executed by the CPU 121, and a RAM 123 that stores various data during control operations by the CPU 121.
As shown in
A main drive motor 14 is a driving unit for rotationally driving the intermediate transfer belt 30 and all the photosensitive drums 26 at a predetermined speed in accordance with an instruction of the image-formation control unit 12.
A registration sensor unit 16 detects toner patches on the intermediate transfer belt 30 using the registration sensors 90.
A mark sensor unit 17 detects a position indicating mark provided on the intermediate transfer belt 30 using the mark sensor 91.
Referring to
When the image forming operation is started, the recoding mediums Q in a cassette 20 are fed by the feeding roller 21, are thereafter separated into individual sheets by the retarding roller pair 22, and are then conveyed to the registration roller pair 24 through the conveying roller pairs 23a and 23b etc. Here, at that time, the rotation of the registration roller pair 24 is halted, and the recording mediums Q abut against the nip of the registration roller pair 24, so that the skewing of the recoding mediums Q is corrected.
On the other hand, in parallel with the conveying operation for the recoding mediums Q, for example, in the yellow process cartridge PY, first, the surface of the photosensitive drum 26Y is uniformly negatively charged by the primary charger 50 and is then exposed to light by the laser exposure unit 28Y. Thus, an electrostatic latent image corresponding to a yellow image component of image signals is formed on the surface of the photosensitive drum 26Y.
Next, the developing roller 54 in the developing unit 51 comes into contact with the photosensitive drum 26Y while being rotationally driven, and the electrostatic latent image is developed by the developing unit 51 using the negatively charged yellow toner to be visualized as a yellow toner image. Thus-obtained yellow toner image is primarily transferred onto the intermediate transfer belt 30 by the primary transfer roller 52 to which bias voltage is supplied. At that time, the intermediate transfer belt 30 and the photosensitive drum 26 are in contact. The contact here indicates that the intermediate transfer belt 30 and the photosensitive drum 26Y are in contact to generate pressure irrespective of the presence/absence of a toner layer between the intermediate transfer belt 30 and the photosensitive drum 26y.
After the toner image is transferred, remaining toner that adheres to the surface of the photosensitive drum 26Y is removed by the cleaner 53.
Such a series of toner-image forming operations is also performed in sequence at a predetermined timing for the other process cartridges PM, PC, and PBk. The developing rollers 54 come into contact with the photosensitive drums 26 in sequence while rotating in order to prevent the degradation of the developer even if the upstream process cartridge is performing primary transfer directly before the image forming operation is started. Color toner images formed on the individual photosensitive drums 26 are primarily transferred in layers in sequence on the intermediate transfer belt 30 at the individual primary transfer unit. After completion of the developing operation, the developing rollers 54 are separated from the photosensitive drums 26 in sequence, and the rotation thereof is stopped in order to prevent the degradation of the developer even if the downstream process cartridge is performing primary transfer.
Next, the four-color toner image that is transferred in layers on the intermediate transfer belt 30 in this way is moved to the secondary transfer unit with the rotation of intermediate transfer belt 30 in the direction of the arrow.
Furthermore, the recoding mediums Q whose skewing is corrected by the registration roller pair 24 are fed to the secondary transfer unit in timing with the image on the intermediate transfer belt 30.
Thereafter, the four-color toner image on the intermediate transfer belt 30 is secondarily transferred onto the recoding mediums Q by the secondary transfer roller 27 that is in contact with the intermediate transfer belt 30 with the recoding mediums Q therebetween. The recoding mediums Q on which the toner image is transferred are conveyed to the fixing unit 25, where the toner image is fixed by application of heat and pressure, and are thereafter output and stacked on the upper surface of the main body 2 by the discharge roller pairs 61, 62, and 63.
The remaining toner on the surface of intermediate transfer belt 30 in which the secondary transfer is completed is removed by a belt cleaner (not shown) disposed in the vicinity of the tension roller 105.
Next, fluctuations in an intermediate-transfer-belt driving torque T when the peripheral speed Vd of the photosensitive drums 26 and the peripheral speed Vb of the intermediate transfer belt 30 are the same will be described. In the description below, the photosensitive drums 26 are sometimes simply referred to as drums, and the intermediate transfer belt 30 is sometimes simply referred to as a belt.
The relationship between the difference between the drum peripheral speed and the belt peripheral speed and the belt driving torque will be described in detail using the results of validation of measurements of an actual image forming apparatus.
In the above-configured image forming apparatus, the results of measurement of fluctuations in the rotation torque of the driving roller 100 when three sheets of LTR paper are continuously printed are shown in
In the measurement, a peripheral speed difference is intentionally generated between the intermediate transfer belt 30 and the photosensitive drums 26 by changing the steady rotational speed of the photosensitive drums 26.
As shown in
Specifically, the torque fluctuations begin when the developing unit 51 comes into contact with the yellow photosensitive drum 26Y while the developing roller 54 in the developing unit 51 is being rotationally driven, and when fogged toner enters the primary-transfer nip portion. When the downstream developing rollers 54 come into contact with the photosensitive drums 26 in sequence, the torque decreases gradually and settles at some point in time. When the developing units 51 of the individual colors come into contact with their corresponding photosensitive drums one by one, the torque decreases continuously, not step by step. When the primary transfer of the upstream yellow color is completed and the developing roller 54 is separated from the photosensitive drum 26Y, so that fogged toner no longer enter the primary-transfer nip portion between the photosensitive drum 26Y and the intermediate transfer belt 30, the torque increases again. For the other colors, the torques increase gradually as the developing units are separated.
Here, the torque fluctuations will be described in more detail. In this embodiment, there is a relation, photosensitive drum speed<intermediate transfer belt speed, and when the contact of the developing unit 51 is started, the intermediate-transfer-belt driving torque decreases. It was confirmed that after the developing units 51 come into contact, fogged color toners reach the primary-transfer nip portions one by one, and the frictional force between the drums and the belt decreases, so that a reaction force from the drums, which applies load on the belt, is decreased.
Furthermore, it was confirmed that the presence/absence of toner at the nip portions between the photosensitive drums and the intermediate transfer belt depends not only on image forming toner applied at actual latent image formation but also on the contact/separation of the developing rollers 54 of the developing units 51. Furthermore, it was confirmed that load fluctuations generated at the primary-transfer unit due to arrival of fogged toner to the primary transfer nip portions does not cause further fluctuations due to arrival of image forming toner during latent image formation. That is, arrival of toner, irrespective of whether fogged toner or latent-image forming toner, to the primary transfer unit reduces a given load as compared with a case in which no toner reaches.
On the other hand, when the developing units 51 begin to separate as the primary transfer is completed from the upstream yellow toner at the end of the image formation, the supply of toner to the primary-transfer nip portion decreases. Therefore, the drums begin to apply loads on the belt, thus increasing the belt driving torque. (Results of measurement of color misalignment under no control)
Here, the horizontal axis shows a distance from the leading end of the recording medium to the trailing end in the moving direction when a toner image is transferred, where the leading end is set at zero. That is, it shows the distance from the leading end of LTR paper in portrait format to the trailing end. In the drawing, this is referred to as “distance in paper feeding direction”. On the other hand, the vertical axis shows that a case in which yellow shifts to the trailing end of the paper with respect to black on the image is positive. The reason why attention is given on the color misalignment between black and yellow is that the color misalignment taken here occurs significantly between yellow, the first color in order of transfer, and black, the last color.
Referring to the measurement of the first page in
The color misalignment of the first sheet is due to the fact that the belt speed during the primary transfer of yellow, which is the first color in transfer order, gradually increases as the belt driving torque is decreased with the start of contact of the developing units 51, shown in
For the second sheet that is subjected to the primary transfer with no torque fluctuations, little color misalignment occurs. Although not discussed here, magenta and cyan also have color misalignment, but it is not so noticeable as yellow and black.
It is known that the torque fluctuations shown in
Thus, this embodiment takes measures to reduce the difference in peripheral speed between the drums and the belt, which is the cause of torque fluctuations.
Here, the difference in peripheral speed between the photosensitive drums and the intermediate transfer belt can be reduced to a certain extent when the manufacturing tolerance of components that determine the individual speeds is reduced. However, reducing dimension errors of the components will inevitably increase manufacturing costs. Accordingly, to reduce the torque fluctuations shown in
For the measures, first, the configuration of relevant components will be described, and thereafter, the control sequence thereof will be described hereinbelow.
The first drive motor 80ab is configured to rotationally drive the photosensitive drum 26Y for yellow (first station) and the photosensitive drum 26M for magenta (second station). The second drive motor 80c is configured to rotationally drive the photosensitive drum 26C for cyan (third station). The third drive motor 80d is configured to rotationally drive the photosensitive drum 26Bk for black (fourth station) and the intermediate transfer belt 30.
The intermediate transfer belt 30 is configured to transmit driving from a gear 89 via a gear train 88 to a gear (not shown) on the shaft of the driving roller 100.
Furthermore, the intermediate transfer belt 30 is configured to transmit driving to photosensitive-drum drive gears 82 (82a, 82b, 82c, and 82d) (see
To prevent color misalignment due to the gear accuracy of the drive gears 82 (82a, 82b, 82c, and 82d), the drive gears 82 (82a, 82c, and 82d) are provided with cylindrical flanges 85 (85a, 85c, and 85d) having slits 84 (84a, 84c, and 84d). The slits 84 (84a, 84c, and 84d) are detected by photointerupters 86 (86a, 86c, and 86d) provided at the driving unit, and the phases of the drive gears 82 (82a, 82c, and 82d) relative to the image position are aligned for the individual colors.
Here, the drive gears 82 (82a and 82b) of the first station and the second station are assembled, with their relative phases aligned when the gear train is assembled. All the drive gears 82 (82a, 82b, 82c, and 82d) are made of the same molded cabinets so that the fluctuation profile of one rotation cycle can be made equal to allow the profiles of the colors to be made equal by aligning the phases, thereby reducing color misalignment. The phase aligning operation for the drive gears 82 (82a, 82c, and 82d) is performed at the timing other than during an image forming operation, such as the end or start of a print job, and is achieved by accelerating or decelerating the object drive gear 82 corresponding to a reference color.
In the driving unit, described with reference to
On the other hand, “TLy end”, “TLm end”, “TLc end”, and “TLBk end” indicate the timing at which image formation of the last pages of the individual colors ends. The timing at which image formation ends here indicates the timing at which the primary transfer ends. Actually, the developing rollers 54 have already been separated from the photosensitive drums 26 directly after or directly before the primary transfer ends. Accordingly, the image-formation end timing may generally be set at the timing at which the developing rollers 54 are separated from corresponding photosensitive drums 16.
In the description below, to discriminate the photosensitive drums 26 of Y, M, C, and K, they are sometimes referred to as a first photosensitive drum (first photosensitive member), a second photosensitive drum (second photosensitive member), a third photosensitive drum (third photosensitive member), and a fourth photosensitive drum (fourth photosensitive member). On the other hand, when forming a toner image with one photosensitive drum, this photosensitive drum may be discriminated as a first photosensitive drum, and a photosensitive drum that is increased or decreased in speed to reduce load fluctuations may be discriminated as a second photosensitive drum; various ways of discrimination are assumed. In the description below, a description in which the four photosensitive drums are discriminated will first be made.
First, although the order is reversed, the third drive motor 80d will be described. Changing the peripheral speed (moving speed) of the intermediate transfer belt 30 during an image forming operation will cause color misalignment. Accordingly, the third drive motor 80d that drives both the photosensitive drum 26Bk for black and the intermediate transfer belt 30 is set to normally rotate steadily at a predetermined speed during an image forming operation. Next control in the individual intervals included in
In the interval (i), as indicated by the two-dot chain line in
On the other hand, the second drive motor 80c drives the photosensitive drum 26C for cyan. When performing an image forming operation for the first page, the photosensitive drum 26C disposed in a position other than the uppermost stream rotates 0.25% higher than usual from “T1y start” to directly before “T1c start”. This speed control is performed according to an instruction of the CPU 121.
Since the peripheral speed of the photosensitive drum 26C is increased by 0.25% relative to a reference speed, the load (brake) applied to the intermediate transfer belt is decreased under the relation of photosensitive drum speed<intermediate transfer belt speed, thus solving the problem of significant load fluctuations. In other words, the total load generated between the intermediate transfer belt and the individual photosensitive drums can be controlled so as to at least come close to a reference load (the reference load will be defined later).
The speed increasing control will be described in more detail. As shown in
Although the above interval (i) is described using an example in which the speed of the second drive motor 80c is increased substantially at the same time as “T1y start”, the invention is not limited thereto. At least, the speed of a photosensitive drum that is not forming a toner image should be controlled to a reference load while forming a toner image on the intermediate transfer belt by the photosensitive drum 26Y, which is the first photosensitive member.
In the interval from “T1c start” before “TLm end”, control for increasing or decreasing the peripheral speeds (moving speeds) of the individual photosensitive drums is not performed. That is, the control is performed by the CPU 121 so that the individual photosensitive drums are rotated at a normal rotation speed.
The load generated between all the photosensitive drums (photosensitive members) and the intermediate transfer belt (transfer member) while all the photosensitive drums are forming toner images on the intermediate transfer belt in the interval (ii) are the target reference load to be performed in the other intervals. The state in which this reference load is generated corresponds to the state in
(iii) TLm End<T≦TLBk End
In this embodiment, the first drive motor 80ab drives the photosensitive drums 26Y and 26M for yellow and magenta, disposed at positions other than the lowermost stream. The photosensitive drums 26Y and 26M rotate while increasing in speed 0.15% higher than the reference during the interval other than the foregoing interval (i), that is, directly after “TLm end” at which the primary transfer of magenta of the image of the last page ends to “TLBk end” at which the primary transfer of black ends. This speed control is performed according to an instruction of the CPU 121. This interval also corresponds to an interval in which a toner image is formed on the intermediate transfer belt 30 with part of the photosensitive drums, as in the interval (i). Also in this case, the total load generated between the intermediate transfer belt and the individual photosensitive drums can be controlled so as to at least come close to the reference load.
Here, the speed increasing control will be described in more detail. As shown in
To cope with it, by increasing the peripheral speeds of the photosensitive drums 26Y and 26M for yellow and magenta by 0.15% relative to the reference, the torque on the driving roller shaft can be maintained at a steady torque until completion of the image forming operation. Thus, the image forming operation can be performed without influence of the presence of toner.
Although the above interval (iii) is described using an example in which the speed of the first drive motor 80ab is increased substantially at the same time as “TLm end”, the invention is not limited thereto. At least, the speed of a photosensitive drum that is not forming a toner image should be controlled to the reference load (at least to come close thereto) while forming a toner image on the intermediate transfer belt by the photosensitive drums 26C and 26Bk, which is the third and fourth photosensitive members.
Thus, driving torque fluctuations of the intermediate transfer belt due to the start of contact and separation of the developing units can be prevented by controlling the peripheral speeds (moving speeds) of the photosensitive drums 26 by the CPU 121 so as to reduce device costs and not to waste the life of the components. Accordingly, driving torque fluctuations of the intermediate transfer belt during image formation are small. Therefore, the belt peripheral speed can be held constant, and as a result, a high-quality image without color misalignment can be output.
Of the photosensitive drums 26 that are not performing image formation, a photosensitive drum whose peripheral speed is controlled is located as far as possible from photosensitive drums (for example, 26Y and 26M) in which fluctuations in the presence of toner occur at the primary-transfer nip portion. This can prevent vibrations at the primary-transfer nip portion. The vibrations may cause variations in the state of primary transfer to cause banding and density variations.
Since the amount of change in peripheral speed is a known value, it is confirmed that the phase shift of the drive gears, described above, is a value that has little influence (in this embodiment, about 2°), so that the color misalignment is not worsened.
However, for continuous printing in which the drive motors 80 are not stopped so that the contact/separation operation of the developing units is repeated, color misalignment can be prevented from worsening by performing control to return the phases of the drive gears 82 during the contact/separation operation.
Since the phase shift amount is known, it is also effective to execute phase alignment considering the known phase shift amount of the drive gears 82 according to this embodiment.
In the above-described embodiment, a configuration in which three drive motors are disposed as the driving source for the photosensitive drums 26 and the intermediate transfer belt has been described. Here, another form for controlling the speed of a photosensitive drum that is not forming a toner image so as to apply a reference load during forming a toner image on the intermediate transfer belt will be described.
In this embodiment, four drive motors 80 are provided, and one of the drive motors 80 drives both the photosensitive drum 26 and the intermediate transfer belt 30, and the other drive motors 80 individually control the peripheral speeds of the other three photosensitive drums 26. This can reduce the difference in peripheral speed between the photosensitive drums 26 and the intermediate transfer belt 30 to prevent the occurrence of color misalignment and to improve the recording accuracy of an output image. This will be described below in detail.
Since the configuration of the image forming apparatus is the same as the configuration of the image forming apparatus 1 shown in
The first drive motor 80a is configured to rotationally drive a photosensitive drum 26Y for yellow (first station). The second drive motor 80b is configured to rotationally drive a photosensitive drum 26M for magenta (second station). The third drive motor 80c is configured to rotationally drive a photosensitive drum 26C for cyan (third station). The fourth drive motor 80d is configured to rotationally drive a photosensitive drum 26Bk for black (fourth station) and the intermediate transfer belt 30.
The intermediate transfer belt 30 is configured to transmit driving to gears (not shown) on the shaft of a driving roller 100 via a gear train 88.
Furthermore, it is configured to transmit driving from the drive motors 80 (80a, 80b, 80c, and 80d) via reduction gears 81 (81a, 81b, 81c, and 81d) to photosensitive drum drive gears 82 (82a, 82b, 82c, and 82d) (see
All the drive gears 82 (82a, 82b, 82c, and 82d) are made of the same molded cabinets so that the fluctuation profile of one rotation cycle can be made equal to allow the profiles of the colors to be made equal by aligning the phases, thereby reducing color misalignment. The phase aligning operation for the drive gears 82 (82a, 82b, 82c, and 82d) is performed at the timing other than during an image forming operation, such as the end or start of a print job, and is achieved by accelerating or decelerating the object drive gear 82 corresponding to a reference color.
Changing the peripheral speed of the intermediate transfer belt 30 during an image forming operation will cause color misalignment. Accordingly, the fourth drive motor 80d that drives both the photosensitive drum 26Bk for black and the intermediate transfer belt 30 is set to normally rotate steadily at a predetermined speed during the image forming operation.
The second drive motor 80b drives the photosensitive drum 26M for magenta. The third drive motor 80c drives the photosensitive drum 26C for cyan. This interval corresponds to an interval in which a toner image is formed on the intermediate transfer belt 30 with part of the photosensitive drums. When performing an image forming operation for the first page, the photosensitive drum 26M disposed in a position other than the uppermost stream rotates 0.25% higher than usual from “T1y start” to directly before “T1c start”. This speed control is performed according to an instruction of the CPU 121. Thus, the total load generated between the intermediate transfer belt and the individual photosensitive drums can be controlled so as to at least come close to a reference load. This also applies to intervals (iii) to (v) below.
From “T1y start” to directly before “T1c start”, the photosensitive drum 26C disposed at a position other than the uppermost stream rotates 0.25% higher than usual. This speed control is also performed according to an instruction of the CPU 121. Although both the second and third drive motors 80b and 80c are increased in speed by 0.25%, they may be increased by 0.15% depending on the characteristics of the photosensitive drums 26M and 26C and the intermediate transfer belt 30.
In the interval after “T1c start” before “TLy end”, control for increasing or decreasing the peripheral speeds (moving speeds) of the individual photosensitive drums is not performed. That is, the control is performed by the CPU 121 so that the individual photosensitive drums are rotated at a normal rotation speed.
(iii) TLy End≦T<TLm End
The first drive motor 80a drives the photosensitive drum 26Y for yellow. In the interval after “TLy end” to before “TLm end” in which the image on the last page has completed primary transfer of yellow, the photosensitive drum 26Y disposed at a position other than the lowermost stream rotates 0.15% higher than usual. This speed control is performed according to an instruction of the CPU 121. This interval also corresponds to an interval in which a toner image is formed on the intermediate transfer belt 30 by part of the photosensitive drums.
In the interval after “TLm end” to before “TLc end”, the drive motors 80a and 80b corresponding to the photosensitive drums 26Y and 26M rotate 0.15% higher than usual. This speed control is also performed according to an instruction of the CPU 121. This interval also corresponds to an interval in which a toner image is formed on the intermediate transfer belt 30 by part of the photosensitive drums.
In the interval after “TLc end” to before “TLBk end”, the drive motors 80a, 80b, and 80c corresponding to the photosensitive drums 26Y, 26M, and 26C disposed in positions other than the lowermost stream rotate 0.15% higher than usual. This speed control is also performed according to an instruction of the CPU 121. This interval also corresponds to an interval in which a toner image is formed on the intermediate transfer belt 30 by part of the photosensitive drums.
In the second embodiment, the relationship between changes in the peripheral speed of the photosensitive drums by speed control and load on the intermediate transfer belt is omitted because it is the same as that of the first embodiment.
Thus, driving torque fluctuations of the intermediate transfer belt due to the start of contact and separation of the developing units can be prevented by controlling the peripheral speeds (moving speeds) of the photosensitive drums 26 by the CPU 121, thereby providing the same advantages as the first embodiment.
In the above embodiments, the color-image forming apparatus has been described on the assumption that photosensitive drum speed<intermediate transfer belt speed. However, the invention is not limited thereto. The invention can be applied also to a color-image forming apparatus in which photosensitive drum speed>intermediate transfer belt speed.
In the case of photosensitive drum speed>intermediate transfer belt speed, the photosensitive drum speed acts to increase the rotation of the intermediate transfer belt. Therefore, this creates a need for speed control to contrarily slow down the photosensitive drum speed so that the photosensitive drums do not excessively increase the rotation of the intermediate transfer belt before a sufficient amount of fogged toner reaches the primary-transfer nip portions.
Accordingly, by decreasing the amount of increased photosensitive-drum peripheral speed, described in the first and second embodiments, the same operations and advantages, that is, the advantage of decreasing color misalignment can be provided during image formation for the first and last pages of an input job.
In the above embodiments, speed control of photosensitive members that are not forming toner images of both of the first and last pages has been described. However, from the viewpoint of not decreasing the productivity of the apparatus, at least from conventional ones, the following forms can also be considered.
That is, when forming a toner image of the first page, the operation shift shown in
The fourth embodiment can also provide the same advantages, to some extent, as in the first to third embodiments.
In the foregoing embodiments, a case in which the drive motor 80d drives both the photosensitive drum 26Bk for black and the intermediate transfer belt 30 has been described. However, the invention is not limited thereto. For example, both the photosensitive drum 26y and the intermediate transfer belt 30 may be driven, and the photosensitive drum 26Bk and the intermediate transfer belt 30 may be driven by different drive motors.
In the case of
In the case in
Thus, also by driving the image forming apparatus in this way, the same advantages as in the foregoing embodiments can be offered.
Although the foregoing embodiments are configured such that the peripheral speeds of the plurality of photosensitive drums are changed, the invention is not limited thereto. For example, control of changing the peripheral speed of only one photosensitive drum that is not performing image formation even during an image forming operation can offer the same advantages, and therefore, the speed control of only one photosensitive drum can be performed.
In the foregoing embodiments, four-color-image forming apparatuses equipped with the first to fourth photosensitive drums have been described; however, the invention may also be applied to, for example, six-color-image forming apparatus equipped with first to sixth photosensitive drums. In this case, the foregoing fourth photosensitive drum should be used as the sixth photosensitive drum, and the foregoing third photosensitive drum should be used as any of the third to fifth photosensitive drums (photosensitive members).
As described above, a photosensitive drum that is forming a toner image may be referred to as a first photosensitive drum, and a photosensitive drum that is increased or decreased in speed so as to reduce load fluctuations may be discriminated as a second photosensitive drum. That is, the first photosensitive drum (first photosensitive member), the second photosensitive drum (second photosensitive member), the third photosensitive drum (third photosensitive member), and the fourth photosensitive drum (fourth photosensitive member) may be roughly classified into two, such as a first photosensitive drum and a second photosensitive drum.
For example, in the foregoing embodiments, at least one of the photosensitive drums 26M, 26C, and 26Bk, whose peripheral speed is to be controlled during image formation while the developing roller 54 is brought into contact with only the photosensitive drum 26Y, can be used as the second photosensitive drum. In this case, the photosensitive drum 26Y corresponds to the first photosensitive drum.
In the foregoing embodiments, at least one of the photosensitive drums 26Y, 26M, and 26C, whose peripheral speed is to be controlled during image formation while the developing roller 54 is brought into contact with only the photosensitive drum 26Bk, can be used as the second photosensitive drum. In this case, the photosensitive drum 26Bk corresponds to the first photosensitive drum.
Also with the six-color-image forming apparatus, as in the above description, it is apparent that a photosensitive drum that is forming a toner image and a photosensitive drum whose speed is to be increased or decreased can be classified as the first photosensitive drum and the second photosensitive drum.
Although the foregoing embodiments are configured such that the peripheral speed of the photosensitive drum is changed to a predetermined value, the invention is not limited thereto; for example, control for changing the peripheral speed step by step in accordance with the profile of load fluctuations.
In the foregoing embodiments, an image forming apparatus is used by way of example which adopts photosensitive drums as image bearing members and adopts an intermediate transfer belt as an intermediate transfer member; however, the invention is not limited thereto. In the case where elastic deformation of the driving system is the main factor, load fluctuations cause the same phenomenon in both of a case where the driven side is a belt and a case where the driven side is drums. For example, the invention may be an image forming apparatus that adopts a photosensitive belt as an image bearing member and adopts an intermediate transfer drum as an intermediate transfer member. In this case, the speed of the photosensitive belt can be corrected by the same speed correction sequence. That is, the foregoing embodiments can be applied to speed control of various photosensitive members.
In the foregoing embodiments, process cartridges that are detachably mounted to an image-forming-apparatus main body are used by way of example, each of which integrally includes a photosensitive drum and a charging member, a developing member, and a cleaning member which serve as processors that act on the photosensitive drum. However, the process cartridges are not limited thereto. It may be process cartridges that each integrally include, in addition to the photosensitive drum, one of the developing member, the developing unit, and the cleaning member.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2008-138047 filed May 27, 2008, and No. 2009-099066 filed Apr. 15, 2009, which are hereby incorporated by reference herein in their entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-138047 | May 2008 | JP | national |
| 2009-099066 | Apr 2009 | JP | national |
