JP2005091861A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately prevent out of color registration, in particular, to prevent going out of color registration from occurring between a color near to a position for detecting the speed of an intermediate transfer belt and a color shift therefrom. <P>SOLUTION: The intermediate transfer belt 10 is held between four photoreceptors 40 and intermediate transfer rollers 62, and also trained about a driving roller 14, a driven roller 15 and a secondary transfer roller 16. When the driving roller 14 is driven by a belt driving motor 90 controlled by a motor control part 94, the intermediate transfer belt 10 is rotated in a direction shown by an arrow. Belt speed detection sensors 93 are respectively arranged between the respective photoreceptors 40, and the control part 94 controls the belt driving motor 90, based on the mean value of the speed detected by the respective speed detection sensors 93. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a tandem type image forming apparatus that has a plurality of photoconductors and forms a color image.

2. Description of the Related Art Today, in electrophotographic apparatuses, color products such as color copiers and color printers are increasing in accordance with market demands.
A color electrophotographic apparatus is provided with a developing device of a plurality of colors around one photoconductor, and a toner is attached by these developing devices to form a composite toner image on the photoconductor, and the toner image is transferred. A so-called one-drum type that records a color image on a sheet, and a plurality of photoconductors are arranged side by side, each photoconductor is provided with a developing device, and a single color toner image is formed on each photoconductor, and those single colors There is a so-called tandem type in which toner images are sequentially transferred to record a composite color image on a sheet.

Comparing the 1-drum type and the tandem type, the former has the advantage that it can be made relatively small and cost can be reduced because there is only one photoconductor, but it can be reduced more than once (usually 4 times). ) Since image formation is repeated to form a full-color image, it is difficult to speed up image formation. The latter, on the contrary, has the advantage that it is easy to increase the speed of image formation, although it has the disadvantage of increasing the size and cost.
Recently, tandem type has been attracting attention because the demand for full-color monochrome speed is desired.

  As shown in FIG. 19, the tandem type electrophotographic apparatus includes a direct transfer system that sequentially transfers an image on each photoconductor 1 onto a sheet s conveyed by a sheet conveying belt 3 by a transfer apparatus 2. As shown in FIG. 20, the image on each photoconductor 1 is sequentially transferred to an intermediate transfer body (intermediate transfer belt) 4 by a primary transfer device 2 and then the image on the intermediate transfer body 4 is subjected to secondary transfer. There is an indirect transfer type in which the apparatus 5 performs batch transfer onto the sheet s. The transfer device 5 is a transfer conveyance belt, but there is also a roller-shaped method.

Comparing the direct transfer method of FIG. 19 with the indirect transfer method of FIG. 20, the former is a paper feed device 6 on the upstream side of a tandem type image forming apparatus T in which a plurality of photoconductors 1 are arranged. The fixing device 7 must be disposed on the downstream side, and there is a drawback that the size of the fixing device 7 increases in the sheet conveyance direction.
On the other hand, since the latter can set the secondary transfer position relatively freely, the paper feeding device 6 and the fixing device 7 can be arranged so as to overlap the tandem image forming device T, and the size can be reduced. There is an advantage that becomes possible.

Further, in order not to increase the size of the former in the sheet conveying direction, the fixing device 7 is disposed close to the tandem type image forming apparatus T. Therefore, the fixing device 7 cannot be disposed with a sufficient margin that the sheet s can bend, and an impact when the leading edge of the sheet s enters the fixing device 7 (particularly with a thick sheet), There is a drawback in that the fixing device 7 tends to affect image formation on the upstream side due to the speed difference between the sheet conveying speed when passing through the fixing device 7 and the sheet conveying speed by the transfer conveying belt.
On the other hand, since the latter can arrange the fixing device 7 with a sufficient margin that the sheet s can be bent, the fixing device 7 can hardly affect the image formation.
In view of the above, recently, an indirect transfer type of tandem type electrophotographic apparatus has attracted attention.

  Further, in this type of color electrophotographic apparatus, the transfer residual toner remaining on the photoconductor after the primary transfer is removed by the photoconductor cleaning device to clean the surface of the photoconductor so as to prepare for the image formation again. Yes. Further, the transfer residual toner remaining on the intermediate transfer member after the secondary transfer is removed by an intermediate transfer member cleaning device to clean the surface of the intermediate transfer member so as to prepare for the image formation again.

On the other hand, in the conventional color image forming apparatus, when colors are superimposed, the position is deviated from the target position, and a problem such as color misregistration on the image may occur. As one of the causes of this color misregistration, it has been found that, in an image forming apparatus using an intermediate transfer belt (intermediate transfer member), this is due to uneven speed of the intermediate transfer belt.
As a solution to this, a scale is provided on the intermediate transfer belt, the belt speed is detected by reading the scale with a sensor, the belt drive motor is controlled based on the detection result, and the color deviation is reduced by reducing the speed unevenness. It has been proposed to prevent (see, for example, Patent Document 1).

However, in this method, when the position of the sensor for reading the scale is arranged away from the transfer position at which the toner image on the photoconductor is transferred to the intermediate transfer belt, the detected belt speed is set between the photoconductor and the intermediate transfer belt. The transfer position speed may not always match. This is because the belt is an elastic body and partially expands and contracts before and after the drive roller, and also due to a thickness deviation of the belt. In this case, color misregistration cannot be prevented no matter how accurately and at high speed the motor is controlled from the sensor detection result. Further, in the correction control by one sensor, a color shift may occur between the color close to the sensor and the color far from the sensor at the transfer position from the photoconductor to the intermediate transfer belt.
Japanese Patent Laid-Open No. 11-024507

  An object of the present invention is to prevent color misregistration with high accuracy, and in particular, to prevent color misregistration from occurring between a color close to the intermediate transfer belt speed detection position and a distant color.

  The image forming apparatus according to claim 1 is a tandem color image forming apparatus in which a plurality of photoconductors are arranged and images formed on the photoconductors are sequentially transferred to an intermediate transfer belt conveyed by a motor. In the above, a scale provided on the intermediate transfer belt, a plurality of belt speed detecting means for detecting the speed of the intermediate transfer belt by reading the scale, and an average value of the speeds detected by the plurality of belt speed detecting means The control means for controlling the speed of the motor based on the above is provided.

  According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the plurality of belt speed detecting means are arranged between the transfer positions from the photoconductors to the intermediate transfer belt. To do.

  According to a third aspect of the present invention, in the image forming apparatus according to the second aspect, the plurality of belt speed detecting means are arranged in the vicinity of the transfer position.

  According to the first aspect of the present invention, since the motor is controlled from the average value of the detection results of the plurality of belt speed detection means, the speed between the color closer to the speed detection and the farther color than that controlled by one speed detection output. The unevenness can be suppressed and the occurrence of color misregistration can be prevented with high accuracy.

  According to the second aspect of the present invention, the plurality of belt speed detecting means are arranged between the transfer positions from the plurality of photoconductors to the intermediate transfer belt, so that the speed between the transfer positions is detected and the average thereof is detected. Since the motor is controlled by the value, the belt speed to be controlled is equal to the average value of the transfer positions of each photoconductor and the intermediate transfer belt, and the unevenness of the speed at each transfer position is suppressed, resulting in more accurate color misregistration. Can be prevented.

  According to the invention described in claim 3, the plurality of belt speed detecting means are arranged in the vicinity of the transfer positions from the plurality of photoconductors to the intermediate transfer belt, the speed at each transfer position is detected, and the average value of the motor is detected. Therefore, the belt speed to be controlled is equal to the average value of the transfer positions of the respective photosensitive members and the intermediate transfer belt, and the unevenness of the speed at each transfer position is suppressed to prevent the occurrence of color misregistration with higher accuracy. be able to.

  In the present invention, a plurality of belt speed detection means are used, and the motor is controlled based on the average value of the detection results.

FIG. 4 is a block diagram showing a configuration example of a tandem type intermediate transfer type electrophotographic apparatus as a conventional image forming apparatus to which the present invention can be applied.
In FIG. 4, 100 is a copying apparatus main body, 200 is a paper feed table on which the copying apparatus main body 100 is placed, 300 is a scanner mounted on the copying apparatus main body 100, and 400 is an automatic document feeder mounted on the scanner 300. (ADF).

  The copying machine main body 100 is provided with an endless belt-like intermediate transfer member (intermediate transfer belt) 10 in the center. As shown in FIG. 5, the intermediate transfer member 10 includes, for example, a base layer 11 made of a material that hardly stretches, such as a fluorine resin having a small elongation or a rubber material having a large elongation, and a material such as a fluorine rubber or acrylonitrile on the base layer 11. -An elastic layer 12 made of butadiene copolymer rubber or the like, and the surface of the elastic layer 12 is covered with, for example, a fluorine-based resin and a smooth coat layer 13.

  As shown in FIG. 4, the intermediate transfer member 10 is sandwiched between a primary transfer device (intermediate transfer roller) 62 and the photosensitive member 40, and is stretched over three support rollers 14, 15, and 16 to be shown in the figure. It can be rotated around. In the illustrated example, an intermediate transfer body cleaning device 17 that removes residual toner remaining on the intermediate transfer body 10 after image transfer is provided on the left side of the second support roller 15 of the three support rollers. On the intermediate transfer member 10 stretched between the first support roller 14 and the second support roller 15, Y (yellow), C (cyan), M (magenta), The tandem image forming apparatus 20 is configured by arranging four B (black) image forming units 18 side by side. An exposure device 21 is provided on the tandem image forming apparatus 20.

On the other hand, a secondary transfer device 22 is provided on the side opposite to the tandem image forming apparatus 20 with the intermediate transfer body 10 interposed therebetween. The secondary transfer device 22 includes a secondary transfer conveyance belt 24, which is an endless belt, between two rollers 23 in the drawing, and is pressed against the third support roller 16 via the intermediate transfer body 10. The image on the intermediate transfer body 10 is transferred to a sheet.
A fixing device 25 for fixing the transfer image on the sheet is provided beside the secondary transfer device 22. The fixing device 25 is configured by pressing a pressure roller 27 against a fixing belt 26 that is an endless belt.

The secondary transfer device 22 described above is also provided with a sheet transport function for transporting the sheet after image transfer to the fixing device 25. Of course, a transfer roller or a non-contact charger may be disposed as the secondary transfer device 22, and in this case, it is difficult to provide this sheet conveyance function together.
In the illustrated example, a sheet reversing device 28 for reversing the sheet to record images on both sides of the sheet is provided below the secondary transfer device 22 and the fixing device 25 in parallel with the tandem image forming device 20 described above.

  When copying using this color electrophotographic apparatus, the original is set on the original table 30 of the automatic original conveying apparatus 400. Alternatively, the automatic document feeder 400 is opened, a document is set on the contact glass 32 of the scanner 300, the automatic document feeder 400 is closed, and the document is pressed. When a start switch (not shown) is pressed, when the original is set on the automatic document feeder 400, the original is transported and moved onto the contact glass 32, and then the original is set on the contact glass 32. Immediately drives the scanner 300 to cause the first traveling body 33 and the second traveling body 34 to travel. Then, the first traveling body 33 emits light from the light source, and the reflected light from the document surface is further reflected toward the second traveling body 34 and reflected by the mirror of the second traveling body 34, and the imaging lens 35. Through the sensor 36 to read the original content.

  When the start switch is pressed, one of the support rollers 14, 15 and 16 is rotationally driven by a drive motor (not shown), the other two support rollers are driven to rotate, and the intermediate transfer member 10 is rotated and conveyed. At the same time, the image forming means 18 rotates the photoconductor 40 to form black, yellow, magenta, and cyan single color toner images on the photoconductor 40. Then, along with the conveyance of the intermediate transfer member 10, the single color images are sequentially transferred to form a composite color image on the intermediate transfer member 10.

  On the other hand, when the start switch is pressed, one of the paper feed rollers 42 of the paper feed table 200 is selectively rotated, the sheet is fed out from one of the paper cassettes 44 provided in multiple stages in the paper bank 43, and the separation roller 45 1 The sheets are separated one by one and put into the paper feed path 46, transported by the transport roller 47, guided to the paper feed path 48 in the copying machine main body 100, and abutted against the registration roller 49 and stopped. Alternatively, the sheet feed roller 50 is rotated to feed out the sheets on the manual feed tray 51, separated one by one by the separation roller 52, put into the manual feed path 53, and abutted against the registration roller 49 and stopped. Then, the registration roller 49 is rotated in synchronization with the composite color image on the intermediate transfer member 10, the sheet is fed between the intermediate transfer member 10 and the secondary transfer device 22, and transferred by the secondary transfer device 22. A color image is recorded on the sheet.

  The image-transferred sheet is conveyed by the secondary transfer device 22 and sent to the fixing device 25. The fixing device 25 applies heat and pressure to fix the transferred image, and then the switching roller 55 is used to switch the discharge roller. The paper is discharged at 56 and stacked on the paper discharge tray 57. Alternatively, it is switched by the switching claw 55 and put into the sheet reversing device 28, where it is reversed and guided again to the transfer position, an image is recorded also on the back surface, and then discharged onto the discharge tray 57 by the discharge roller 56.

On the other hand, the intermediate transfer body 10 after the image transfer removes residual toner remaining on the intermediate transfer body 10 after the image transfer by the intermediate transfer body cleaning device 17, and prepares for the image formation by the tandem image forming apparatus 20 again.
In general, the registration roller 49 is often used while being grounded, but a bias voltage may be applied to remove paper dust from the sheet.

  FIG. 6 shows a detailed configuration of each image forming unit 18 in the tandem image forming apparatus 20. As shown in FIG. 6, around the drum-shaped photoconductor 40, a charging device 60, a developing device 61, a primary transfer device (intermediate transfer roller) 62, a photoconductor cleaning device 63, a charge removal device 64, and the like are provided. Yes. L indicates laser light.

As the intermediate transfer belt (intermediate transfer member) 10, a fluorine-based resin, a polycarbonate resin, a polyimide resin or the like has been conventionally used. However, in recent years, an elastic belt using an entire belt or a part of the belt as an elastic member is used. Has been used. The transfer of a color image using a resin belt has the following problems.
A color image is usually formed with four colored toners. One to four toner layers are formed on one color image. The toner layer receives pressure by passing through the primary transfer (transfer from the photoreceptor to the intermediate transfer belt) and the secondary transfer (transfer from the intermediate transfer belt to the sheet), and the cohesive force between the toners increases. . When the cohesive force between the toners increases, the phenomenon of missing characters in the characters or missing images in the solid portion image tends to occur. Since the resin belt has high hardness and does not deform in accordance with the toner layer, the toner layer is easily compressed, and the character dropout phenomenon is likely to occur.

  Recently, there is an increasing demand for forming full-color images on various papers, for example, Japanese paper, intentionally irregularities, and forming images on paper. However, a paper with poor smoothness is liable to generate a gap with a toner image during transfer, and transfer loss easily occurs. If the transfer pressure at the secondary transfer portion is increased to improve the adhesion, the condensing power of the toner layer is increased, and the above-described character void is generated.

The elastic belt is used for the following purposes.
The elastic belt is deformed corresponding to the toner layer and the paper having poor smoothness at the transfer portion. In other words, since the elastic belt deforms following local unevenness, it can obtain good adhesion without excessively increasing the transfer pressure with respect to the toner layer, and paper with poor flatness with no voids in characters. In contrast, a transfer image with excellent uniformity can be obtained.

FIG. 7 is a diagram illustrating a configuration example of the intermediate transfer unit when one belt speed detection sensor 93 is provided.
The image forming apparatus having the intermediate transfer portion according to this example is provided with a scale on the intermediate transfer belt 10, the scale is read by a belt speed detection sensor 93 provided above the scale, and the speed of the belt surface is detected from the output value, By calculating an ideal belt speed from the detected speed and feeding it back to the belt drive motor 90, a stable belt drive is obtained.

  In FIG. 7, the intermediate transfer belt 10 is sandwiched between each photoconductor 40 and an intermediate transfer roller (primary transfer device) 62, and also includes a drive roller 14, a driven roller 15, and a secondary transfer roller 16 (supports in FIG. 1). Rollers 14, 15, 16). The intermediate transfer member 10 is rotated in a predetermined direction indicated by an arrow by driving the driving roller 14 via a belt driving motor 94 controlled by a motor control unit 94. As described above, the toner images formed on the respective photoreceptors 40 are sequentially transferred onto the intermediate transfer belt 10. Further, the image transferred to the intermediate transfer belt 10 is further secondarily transferred to the transport belt 24.

  In this example, a belt scale 91 as shown in FIG. 8 is provided on the surface of the intermediate transfer member 10. The belt scale 91 is provided with a scale slit 92 as shown. Further, a belt speed detection sensor 93 is provided above the intermediate transfer member 10 between the driven roller 15 (support roller 15 in FIG. 4) and the photosensitive member 40. The belt speed detection sensor 93 is disposed at a position where the scale slit 92 can be read. In the illustrated example, they are arranged on the same side as the photoconductor 40, but may be anywhere as long as the belt scale 91 can be read.

FIG. 9 shows the relationship between the belt speed detection sensor and the 93 belt scale 91.
As an example, a reflective optical sensor having a pair of light emitting and receiving elements is used as the belt speed detection sensor 93, and the belt scale 91 is configured so that the reflectance is different between the scale slit 92 and other portions.

As the output value of the belt speed detection sensor 91, a binary signal of High or Low is output depending on the difference in the reflectance of the scale as shown in the figure. For example, if the sensor receives a light at the light receiving unit and outputs a high signal, in the example of FIG. 9, if the reflectance of the scale slit 92 is higher than that of the other parts, the belt speed detection sensor 93. The output signal from is output while the slit is passing through the sensor in the range of t. The sensor output is shown depending on the presence or absence of the slit passing through the detection range of the sensor as the intermediate transfer belt 10 rotates. As described above, High and Low are repeated.
Accordingly, the surface speed of the intermediate transfer belt 10 can be detected by obtaining the time T from the time when the signal changes from Low to High until the next signal changes from Low to High.
The above is an example, and if the belt scale can be detected, there is no problem even if the sensor type, scale type, detection method, and the like are different.

  In FIG. 7, the motor controller 94 first controls the belt drive motor 90 to rotate at the basic speed. As the motor rotates, the intermediate transfer member 10 rotates and the belt scale 91 also rotates. Therefore, the belt speed detection sensor 93 reads the scale and feeds back the value to the motor control unit 94. In the motor control unit 94, if the fed back speed is the same as the basic speed, the motor is controlled at the basic speed as it is. If there is a difference in speed, the motor is controlled by adding or subtracting the difference to the basic speed.

FIG. 10 shows a detailed control flowchart.
The motor control unit 94 first turns on the motor so as to rotate the belt drive motor 94 at the basic speed V (S1). Thereafter, the next control is repeated until the timing for turning off the motor is generated (S2, No).
The motor control unit 94 detects the feedback output from the belt speed detection sensor 93 and grasps the speed V ′ of the currently rotating belt surface (S3). The speed comparison between V ′ and V is performed (S4). If V = V ′ (S5, No), it can be determined that the belt surface is also rotating at the same speed as the basic speed, so the control is performed at the basic speed as it is. If V ≠ V ′ (S5, Yes), the belt surface speed is different from the differential speed. At this time, a difference V ″ between the basic speed V and the surface speed V ′ is calculated (S6). If V ″> 0 (S7, Yes), since it can be determined that the belt surface speed is slower than the basic speed, the motor is controlled at a speed obtained by adding V ″ to V (S8). If V ″ <0 (S7, No), it can be determined that the belt surface speed is faster than the basic speed, so the motor is controlled at a speed obtained by subtracting V ″ from the basic speed V (S9). . When the belt drive motor is instructed to be turned off (S10), the process ends.
As described above, the belt surface speed can be kept constant at the basic speed.

The speed fluctuation (difference between the basic speed and the surface speed) of the intermediate transfer belt 10 described above is caused by several factors. The fluctuation frequency of the speed fluctuation includes a high frequency component and a low frequency as shown in FIG. Ingredients are present. In FIG. 11, when the rotation time of the belt is taken on the horizontal axis, the speed fluctuation amount is taken on the vertical axis, and the target speed, which is the basic speed (ideal speed), is shown in the center, Therefore, a low-frequency component whose speed changes slowly and a high-frequency component whose speed changes instantaneously coexist. It is possible to correct the speed fluctuation of these two frequency components by the motor control described above.
However, in order to correct both the low frequency component and the high frequency component, it is necessary to adjust the correction frequency range to the high frequency side, and a highly accurate and complicated control circuit is required.
This is because the correction accuracy has a problem with its control loop cycle and sensor detection accuracy.

If the series of controls shown in FIG. 12 is a motor drive feedback loop period A, as shown in FIG. 13, if the control loop period A is sufficiently faster than the low frequency period C, a deviation control amount is detected as shown. It is possible to correct the amount of speed deviation at the end of one loop and return to the target speed. That is, A >> C is sufficient.
However, with respect to the high-frequency cycle B, which is a cycle faster than the control loop cycle A, the speed fluctuation cannot be detected, so that it cannot be corrected. That is, when B> A, control becomes impossible. Therefore, in order to correct the high-frequency cycle B, it is necessary to configure the control loop cycle with a faster cycle.

In general, in order to fall within the control range, it is necessary to perform correction with a period of several tens of times the target correction frequency. Therefore, in order to correct the high-frequency cycle B, the control loop cycle must be considerably shortened. For this purpose, it is necessary to suppress the accuracy and variation of the constituent parts. Along with this, it is necessary to improve the accuracy of the sensor to be detected, and furthermore, the scale on the belt also requires high resolution and high accuracy, which makes it difficult to process. Therefore, it becomes necessary to construct a high-cost system.
The image forming apparatus according to this example pays attention to this point and limits the correction target to the low frequency periodic component. In general, the high frequency period is several kHz or more, while the low frequency period is several tens of Hz or less, so that a sufficiently low-cost system can be constructed.

Next, a description will be given below of not correcting the speed fluctuation in the high frequency cycle.
The tandem image forming apparatus cannot transfer toner to the same position on the intermediate transfer belt at the same time because of its configuration. That is, as shown in FIG. 14, since the two photoconductors 40 are arranged with a certain distance, the intermediate transfer belt 10 is transferred from the first photoconductor 40 and then transferred from the next photoconductor 40. There will be a time difference between For example, when four color dots are printed at the same position on the transfer paper, first, the toner transferred from the first photosensitive member onto the intermediate transfer belt is transferred to the next photosensitive member by the rotation of the intermediate transfer belt 10. When the transfer position is reached, the second color is first transferred onto the intermediate transfer belt. Thereafter, the third color and the fourth color are sequentially transferred with a time difference, and the four colors are transferred to the same position for the first time. At this time, if a speed fluctuation with a low frequency period slower than the transfer time difference between adjacent photoconductors occurs, the speed will reach the next transfer position slower or faster from the first transfer position by the speed fluctuation. Are different, and as a result, the image is affected as a color shift.

However, even if a speed fluctuation of a high frequency cycle with a period faster than the time difference from the first transfer position to the next transfer position occurs, if the speed returns to the original when the next transfer position is reached, the position shift will occur. Will not appear. Even if there is a slight misregistration, the color misregistration on the image is hardly noticeable.
In view of the above, it can be concluded that this image forming apparatus can sufficiently exhibit its effects.

  As a factor in which speed fluctuation appears as a low frequency cycle, a factor caused by one belt cycle is large. This is because the image forming apparatus is a tandem type, and it is necessary to increase the peripheral length of the intermediate transfer belt due to the structure of forming an image on a relatively large transfer paper such as A3 size. This is because the time required to do this is much longer than the time period of the belt speed control loop. For this reason, in general, factors such as an increase or decrease in speed during one rotation of the belt often affect the image due to a thickness deviation of the belt or a stacking tolerance of a mechanical layout. .

FIG. 15 shows the relationship between the fluctuation frequency and the speed fluctuation amount and the factors.
As a factor of the speed fluctuation of the high frequency cycle not caused by one round of the belt, there is a fluctuation of the tooth pitch of the gear for driving transmission. In this image forming apparatus, the target factor to be corrected is limited to the factor caused by one round of the belt. As a result, speed fluctuations in a high-frequency cycle that do not appear on the image can be ignored, and a low-cost system can be configured.

Next, in this image forming apparatus, attention is paid to the thickness unevenness of the intermediate transfer belt as a factor causing the speed fluctuation of the intermediate transfer belt, and the speed fluctuation caused by this is corrected. Hereinafter, the mechanism will be described.
FIG. 16 is a simplified diagram of the intermediate transfer unit when the belt has a thickness unevenness.
Here, in order to simplify the description, the roller is configured with two rollers, that is, a driving roller 14 and a driven roller 15, but the configuration is also the same with two or more rollers. In order to simplify the explanation, the thickness unevenness of the intermediate transfer belt 10 shown in the figure is one for each of the thick and thin portions, but the same is true in the case where there are a plurality of such unevennesses. is there. The intermediate transfer belt is held by a driving roller 14 and a driven roller 15. Further, the intermediate transfer belt 10 is rotated in the direction of the arrow in the drawing by the driving roller 14.

The point D on the surface of the intermediate transfer belt 10 is the thickest part of the belt, and the point E shows the thinnest part. The state of the belt when the point D is at the illustrated position on the drive roller side and the point E is at the illustrated position on the driven roller side is indicated by a solid line, and the belt rotates, and conversely, the D point is the illustrated position on the driven roller side. Further, the state of the belt when the point E is at the position shown in the figure on the drive roller side is indicated by a broken line. The thickness of the belt when the point D is on the driving roller side is X, and the thickness of the belt when the point E is on the driving roller side is x.
That is, X> x
Further, since the radius of the driving roller 14 does not change, the belt rotation radii from the center point of the driving roller 14 to the points D and E on the belt surface are R and r, respectively, and the difference between them is the same as X−x. become.
(R−r) = (X−x)

  Here, the belt surface speeds at the points D and E are different from each other in the radius of rotation as described above, so that the point D is faster than the point E. That is, when the belt rotates and the portion where the thickness of the belt is thicker than the other portions comes to the position of the driving roller 14, the surface speed of the belt becomes the highest, and then the belt further rotates gradually. The speed becomes slow, and the belt surface speed becomes slowest when the thinnest part of the belt comes to the drive roller. Therefore, this difference appears as belt speed unevenness. In general, it is impossible to make all the belt thickness uniform from the standpoint of belt manufacturing and process. For this reason, this speed unevenness always occurs. Further, since the thickness unevenness has a relatively small number of irregularities, the belt speed unevenness appears as a low frequency cycle. Therefore, as described above, the position shift occurs and appears as color unevenness on the image.

  This belt thickness unevenness is usually several Hz or less in the manufacturing process. Therefore, by focusing attention on this factor, it is possible to configure a lower cost system. In addition, by limiting the frequency to be corrected, the phase detection range in the feedback control can naturally be limited. As a result, the phase comparison with respect to the target speed and the amount of speed deviation detection can be performed with higher accuracy, resulting in a more stable speed. Control can be performed.

Next, in this image forming apparatus, attention is paid to the eccentricity of the driving roller 14 of the intermediate transfer belt as a factor causing the speed fluctuation of the intermediate transfer belt 10, and the speed fluctuation caused by this is corrected. Hereinafter, this mechanism will be described.
FIG. 17 is a simplified diagram of the intermediate transfer unit when the drive roller 14 of the intermediate transfer belt 10 has an eccentricity.
Here, in order to simplify the description, the roller has two configurations of the driving roller 14 and the driven roller 15, but the same applies to the configuration of two or more rollers. The intermediate transfer belt 10 is held by a driving roller 14 and a driven roller 15. Further, the intermediate transfer belt is rotated in the direction of the arrow in the drawing by the driving roller.

  In FIG. 17, when the drive roller 14 is eccentric, the radius from the center to the belt contact surface when the drive roller 14 swells most in the direction of the belt contact surface is R, and on the opposite side to the belt contact surface Let r be the radius from the center of the drive roller to the belt contact surface when it swells. Assuming that the thickness of the belt is uniform (X = x), the belt surface speed will cause a speed difference of (R−r), and the belt surface speed fluctuation will be generated accordingly. In general, the radius of the drive roller is often large, and therefore, this speed fluctuation appears in a relatively low frequency cycle and appears as uneven color on the image. Since this low frequency period is generally several Hz to several tens of Hz, paying attention to this factor and correcting it, the correction error due to the correction frequency limitation can be reduced at a lower cost and more stable speed. Control can be realized.

There are a variety of factors that induce belt speed fluctuations. Among them, the correction for the low frequency period is as described above. At this time, what can actually be a factor of the low frequency cycle depends on the system configuration at that time, for example, belt material, belt circumference, number of rollers, photosensitive member pitch, component accuracy, etc. , Each will be different. Therefore, it may be difficult to correct each factor. In general, speed fluctuations often appear on an image at low frequencies, and it is sufficient that these can be corrected to about 100 Hz.
FIG. 18 shows a conceptual diagram of the relationship between the speed fluctuation amount and the influence on the image with respect to the fluctuation frequency.
As shown in this figure, if the correction range is limited to about 100 Hz, the configuration of the feedback system can be realized at low cost.

Next, an embodiment of the image forming apparatus according to the present invention made based on the above-described matters will be described.
FIG. 1 is a configuration diagram of an intermediate transfer unit of the image forming apparatus according to the first embodiment. Parts corresponding to those in FIG.
In FIG. 1, three belt speed detection sensors 93 are arranged between four photoreceptors 40. That is, each belt speed detection sensor 93 is disposed at a transfer position between the four photoconductors 40 (a position where the toner image on the photoconductor 40 is transferred to the intermediate transfer belt 10).

FIG. 2 is a block diagram showing the configuration of the motor control unit 94.
In FIG. 2, the speeds detected by the three belt speed detection sensors 93 are input to an average value calculation circuit 95 to obtain an average value of the speed values. An error between the average value and the target belt position information 96 is obtained by the arithmetic circuit 97, and the error is input to the correction controller 98, and the output processed by the internal filter unit 99 and the proportional gain unit 101 is calculated. The circuit 102 corrects the target speed (reference drive frequency) 103 of the motor. In this example, since a pulse motor is employed as the belt drive motor 90, the corrected PWM signal (pulse width modulation signal) output is input to the drive circuit 104, and the pulse motor is driven while controlling the speed.

The correction controller 98 includes a filter unit 99 and a proportional gain unit 101. The filter unit 99 sets a filter coefficient according to the control band and filters the error signal. Actually cuts over a certain frequency. The proportional gain unit 101 sets a proportional gain according to the control band and determines the magnitude of the correction amount. Actually, the higher the control band, the faster it is necessary to correct the fluctuation, so the gain is increased.
In this example, the correction controller 98 is configured by the filter unit 99 and the proportional gain unit 101, but includes controllers for all algorithms having parameters. Further, the type of the belt drive motor 90 is not limited to the pulse motor, and a motor capable of speed control such as a DC servo motor is a target.

According to the present embodiment, since the motor is controlled from the average value of the detection results of the plurality of belt speed detection sensors 93, the speed unevenness between the color close to the sensor and the color far from the sensor is controlled rather than controlling by one sensor output. It is possible to suppress color misregistration with high accuracy.
In addition, a plurality of sensors are arranged between the transfer positions from the four photosensitive members 40 to the intermediate transfer belt 10, and the speed is detected between the transfer positions and the motor is controlled by the average value. The belt speed becomes equal to the average value of the transfer positions of the respective photoconductors 40 and the intermediate transfer belt 10, and the unevenness of the speed at each transfer position can be suppressed to prevent color deviation with higher accuracy.

FIG. 3 is a block diagram of the intermediate transfer unit according to the second embodiment. Parts corresponding to those in FIG.
In FIG. 3, four belt speed detection sensors 93 are arranged in the vicinity of the transfer position on the downstream side with respect to the belt rotation direction indicated by the arrows of the four photoconductors 40.
The configuration of the motor control unit 94 is a configuration using four belt speed detection sensors 93 in FIG. 2, and the average value calculation circuit 95 calculates the average value of the four detected speed values.
In the case of FIG. 1, four belt speed detection sensors 93 may be provided.

  According to this embodiment, a plurality of sensors are arranged in the vicinity of the transfer positions from the four photoconductors 40 to the intermediate transfer belt 10, the speed at each transfer position is detected, and the motor is controlled by the average value. The belt speed to be controlled becomes equal to the average value of the transfer positions of the respective photoconductors and the intermediate transfer belt, and the unevenness of the speed at each transfer position can be suppressed to prevent color deviation with higher accuracy.

FIG. 3 is a configuration diagram of an intermediate transfer unit of the image forming apparatus according to the first embodiment of the present invention. It is a block diagram which shows the Example of a motor control part. FIG. 6 is a configuration diagram of an intermediate transfer unit of an image forming apparatus according to a second embodiment of the present invention. 1 is a configuration diagram of an image forming apparatus to which the present invention is applicable. FIG. 3 is a configuration diagram of an intermediate transfer belt. FIG. 3 is a configuration diagram of an intermediate transfer unit. FIG. 6 is a configuration diagram of an intermediate transfer unit when one belt speed detection sensor is provided. FIG. 3 is a configuration diagram illustrating an intermediate transfer belt and a belt scale. It is a block diagram which shows the relationship between a belt speed detection sensor, a scale slit, and a sensor output. It is a flowchart which shows a motor control operation. It is a characteristic view which shows a speed fluctuation frequency component. It is a block diagram which shows a motor control loop period. It is a characteristic view which shows the relationship between a speed fluctuation frequency component and a control loop period. FIG. 6 is a configuration diagram illustrating a transfer time difference between photoconductors. It is a characteristic view which shows the relationship between a speed fluctuation frequency, speed fluctuation amount, and the factor. FIG. 3 is a configuration diagram of an intermediate transfer unit when there is unevenness in the thickness of a belt. FIG. 6 is a configuration diagram of an intermediate transfer unit when a drive roller has an eccentricity. It is a characteristic view which shows the speed fluctuation amount with respect to a speed fluctuation frequency, and the influence on an image. FIG. 3 is a configuration diagram of an intermediate transfer unit using a direct transfer method. It is a block diagram of the intermediate transfer part by an indirect transfer system.

Explanation of symbols

Reference Signs List 10 intermediate transfer belt 14 drive roller 15 driven roller 16 secondary transfer roller 40 photoconductor 62 intermediate transfer roller (primary transfer device)
DESCRIPTION OF SYMBOLS 90 Belt drive motor 93 Belt speed detection sensor 94 Motor control part 95 Average value calculation circuit 96 Target belt position 98 Correction control controller 103 Target speed

Claims (3)

In a tandem color image forming apparatus in which a plurality of photoconductors are arranged and an image formed on each photoconductor is sequentially transferred to an intermediate transfer belt conveyed by a motor.
A scale provided on the intermediate transfer belt;
A plurality of belt speed detecting means for detecting the speed of the intermediate transfer belt by reading the scale;
An image forming apparatus comprising: control means for controlling the speed of the motor based on an average value of speeds detected by the plurality of belt speed detecting means.   The image forming apparatus according to claim 1, wherein the plurality of belt speed detecting units are arranged between transfer positions from the photoconductors to the intermediate transfer belt.   The image forming apparatus according to claim 2, wherein the plurality of belt speed detection units are arranged in the vicinity of the transfer position.

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