JP2006154575A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus with which speed fluctuation is hindered by switching from correction control by a sensor output to another control when outputs from a plurality of sensors are abnormal. <P>SOLUTION: The image forming apparatus comprises: a detecting sensor 80 for marks formed at regular intervals along the entire circumference of an intermediate transfer belt; a switching circuit 120 for switching between a signal detected and an output signal from an encoder 128 provided for a belt drive means 81 for driving the intermediate transfer belt; a comparator 121 for comparing the period of a signal switched by a switching circuit 120 and a reference belt period 122, thereby detecting an error; a control section 130 for determining the switching control of the switching circuit 120 and the error level of the comparator 121; a correcting controller 123 for generating a drive frequency based on the error level of the comparator 121; a target speed generator 124 for generating a reference drive frequency; adder 125 for adding the value of the correcting controller 123 to the target speed generator 124; and a drive circuit 126 for producing drive power for a motor. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus, and more particularly to a technique for correcting the moving speed accuracy of an intermediate transfer belt in a tandem color image forming apparatus including an intermediate transfer belt.

In a conventional color image forming apparatus, when colors are superimposed, the position is shifted from a target position, and a problem such as color unevenness may occur on an image. As one of the causes of the color misregistration, it has been found that, in an image forming apparatus using an intermediate transfer (such as a belt or a drum as a medium), it is caused by uneven speed of the intermediate transfer medium.
As a solution to this problem, Patent Document 1 provides a scale on an intermediate transfer belt, reads the scale with a sensor, detects the speed of the intermediate transfer belt, and controls the belt driving motor based on the detection result to prevent speed unevenness. It has been proposed to prevent color misregistration by reducing it.
Further, in Patent Document 2, as a control method for keeping the belt surface speed constant, a mark, a sensor for detecting the mark, and an encoder are provided on the belt, and a sensor signal, an index signal and an encoder that are generated every time the drive roll makes one rotation. It is assumed that a signal is generated. Thus, the encoder pulse from the index signal to the sensor signal is counted, the value Ci and the time ti during which the mark passes the sensor are obtained, and the belt speed fluctuation is calculated from the relationship. When the belt speed varies, the variation pattern is stored in the memory, and the speed is controlled based on the variation pattern. Furthermore, a system is disclosed in which the timing of image writing is changed by detecting the tilt when the belt is mounted tilted with respect to the traveling direction.
JP 11-24507 A Japanese Patent Laid-Open No. 06-263281 Japanese Patent Application No. 2003-326110

However, in the conventional techniques disclosed in Patent Documents 1 and 2, when the position of the sensor for reading the scale is arranged away from the transfer position between the photoconductor and the intermediate transfer belt, the detected belt speed is the same as that of the photoconductor. The speed of the transfer position with the intermediate transfer belt may not always match. This is because the belt is an elastic body and partially expands and contracts before and after the drive roller, or due to a thickness deviation of the belt. In this case, it is difficult to prevent color misregistration 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 a color near the sensor and a color far from the sensor at the transfer position from the photoconductor to the intermediate transfer belt.
As a countermeasure to this problem, in Patent Document 3 filed by the same applicant, by controlling the motor from the average value of the detection results of a plurality of sensors, the color closer to the sensor than that controlled by one sensor output is obtained. It has been proposed to prevent the occurrence of color misregistration by suppressing speed unevenness between distant colors. However, there is no mention of abnormality processing when a sensor output abnormality occurs due to sensor or dirt on the toner, sensor failure, or scale damage when a plurality of sensors are used.
In view of such a problem, the present invention uses the output of a plurality of sensors as a control parameter, and in the case of control in which each sensor output has a great influence on the control accuracy, when the sensor output is abnormal, the correction control by the sensor output is changed to another control. An object of the present invention is to provide an image forming apparatus in which the influence on the speed unevenness is suppressed by switching.

In order to solve the above problems, the present invention provides a tandem color image forming apparatus including an intermediate transfer belt that transfers a toner image of each color formed on a photoconductor to synthesize a color image. Mark detection means for detecting marks formed at regular intervals over the entire circumference of the intermediate transfer belt, an encoder provided in a signal detected by the mark detection means and a drive means for driving the intermediate transfer belt Switching means for switching the output signal, error detection means for detecting an error by comparing the period of the signal switched by the switching means with a reference period, switching control of the switching means, and error level of the error detection means Control means for determining the movement speed error of the intermediate transfer belt based on the error detected by the error detection means, and the detection result Therefore, the control unit determines that the mark detection unit is abnormal when the error detected by the error detection unit exceeds a predetermined value. The switching means is controlled to switch from the control by the mark detection means to the control by the encoder.
The present invention comprises means for switching a signal from the mark detection means and a signal from the encoder, and means for detecting an error by comparing the reference belt period and the period of the mark detection means, and the error is larger than a predetermined value. Thus, it is possible to detect that the mark detection means is abnormal, and thereby the control is performed by correcting the intermediate transfer belt speed by switching to the encoder signal.
According to a second aspect of the present invention, there is provided a tandem color image forming apparatus including an intermediate transfer belt for transferring a color toner image formed on a photosensitive member to synthesize a color image, over the entire periphery of the intermediate transfer belt. A plurality of mark detection means for detecting marks formed at regular intervals, an average value calculation circuit for calculating an average value of signal periods output from each mark detection means, and a calculation result output by the average value calculation circuit And a switching means for switching an output signal of an encoder provided in the driving means for driving the intermediate transfer belt, and an error detection means for detecting an error by comparing the period of the signal switched by the switching means with a reference period And switching means for controlling the switching means and control means for determining an error level of the error detecting means, and based on the error detected by the error detecting means, An image forming apparatus that detects a transfer belt moving speed error and corrects and controls the intermediate transfer belt speed based on the detection result, wherein the control means detects that the error detected by the error detecting means has a predetermined value. If exceeded, it is determined that at least one of the plurality of mark detection means is abnormal, and the switching means is controlled to switch from control by the mark detection means to control by the encoder. Features.
The present invention includes a plurality of mark detection means and an arithmetic circuit for calculating an average value of the mark detection means. That is, the signals from the plurality of mark detection means are averaged by the arithmetic circuit. The signal is used as a signal for the mark detection means. Other operations are the same as those in the first aspect.

According to a third aspect of the present invention, there is provided a tandem color image forming apparatus including an intermediate transfer belt that transfers a toner image of each color formed on a photosensitive member to synthesize a color image, over the entire periphery of the intermediate transfer belt. Two main and sub mark detecting means for detecting marks formed at a constant interval, a signal correcting means for correcting the output of the main mark detecting means by a signal output from the sub mark detecting means, and a signal correcting means A switching unit that switches between an output signal and an output signal of an encoder provided in a driving unit that drives the intermediate transfer belt, and an error that detects an error by comparing a cycle of the signal switched by the switching unit with a reference cycle Detection means, and control means for determining the switching level of the switching means and the error level of the error detection means, and based on the error detected by the error detection means An image forming apparatus that detects a moving speed error of the intermediate transfer belt and corrects and controls the intermediate transfer belt speed based on the detection result, wherein the control means detects an error detected by the error detecting means. When the predetermined value is exceeded, it is determined that the main mark detection means is abnormal among the two main and sub mark detection means, and the switching means is controlled to switch from the control by the mark detection means to the control by the encoder. It is characterized by doing.
The present invention comprises two main and sub mark detection means, and a signal correction means for correcting the signal of the main mark detection means with the signal of the sub mark detection means. For example, a logical sum of signals from the main and sub mark detection means is taken as a correction means, and when the main mark detection means fails, it is replaced with the sub mark detection means and the signal is used as a signal of the mark detection means It is. Other operations are the same as those in the first aspect.

According to the first aspect of the present invention, when the error detected by the error detecting unit exceeds a predetermined value, the control unit determines that the mark detecting unit is abnormal, and the switching unit controls the control by the mark detecting unit. Since the control is switched to the control by the encoder, it is possible to minimize the unevenness of the speed of the intermediate transfer belt even if the mark detection means breaks down.
According to a second aspect of the invention, there is provided an average value calculation circuit for calculating an average value of the signal periods output from the respective mark detection means, and the control means has a plurality of errors when the error detected by the error detection means exceeds a predetermined value. Of the mark detection means, it is determined that at least one mark detection means is abnormal, and the switching means switches from control by the mark detection means to control by the encoder, so that an average value of a plurality of mark detection means is detected. As a result, the detection error can be minimized, and even if the mark detection unit fails, the speed unevenness of the intermediate transfer belt can be minimized.
According to a third aspect of the invention, there is provided signal correction means for correcting the output of the main mark detection means with a signal output from the sub mark detection means, and the control means has an error detected by the error detection means exceeding a predetermined value. In this case, it is determined that the main mark detection means is abnormal among the two main and sub mark detection means, and the switching means switches from the control by the mark detection means to the control by the encoder. Even if a failure occurs, it can be replaced with a sub-mark detecting means to minimize the speed unevenness of the intermediate transfer belt.

Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings. However, the components, types, combinations, shapes, relative arrangements, and the like described in this embodiment are merely illustrative examples and not intended to limit the scope of the present invention only unless otherwise specified. .
FIG. 1 is a configuration diagram of an image forming apparatus of a tandem type intermediate transfer system. The image forming apparatus 900 includes a copying apparatus main body 500, a paper feed table 600 on which the copying apparatus main body 500 is placed, a scanner 700 mounted on the copying apparatus main body 500, and an automatic document feeder (ADF) 800 mounted thereon. It is configured. The copying apparatus main body 500 is provided with an endless belt-shaped intermediate transfer member 10 in the center. As shown in FIG. 2, the intermediate transfer member 10 is formed by forming a base layer 11 made of a base material 11 made of a material that hardly stretches, such as a canvas made of a fluorine resin with a small elongation or a rubber material with a large elongation. 12 is provided. The elastic layer 12 is made of, for example, fluorine-based rubber or acrylonitrile-butadiene copolymer rubber. The surface of the elastic layer 12 is coated with, for example, a coating layer 13 having good smoothness by coating a fluorine resin.
As shown in FIG. 1, in the illustrated example, it is wound around three support rollers 14, 15, 16 and can be rotated and conveyed clockwise in the figure. In this illustrated example, an intermediate transfer body cleaning device 17 for removing residual toner remaining on the intermediate transfer body 10 after image transfer is provided on the left of the second support roller 15 among the three. Further, among the three images, four images of yellow, cyan, magenta, and black are arranged on the intermediate transfer member 10 stretched between the first support roller 14 and the second support roller 15 along the conveyance direction. The tandem image forming apparatus 20 is configured by arranging the forming units 18 side by side. An exposure device 21 is further provided on the tandem image forming apparatus 20 as shown in FIG.

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. In the illustrated example, the secondary transfer device 22 is configured by spanning a secondary transfer belt 24, which is an endless belt, between two rollers 23, 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 arranged as the secondary transfer device 22, and in such a case, it is difficult to provide this sheet conveying function together.
In FIG. 1, under such a secondary transfer device 22 and a fixing device 25, a sheet reversing device 28 for reversing the sheet so as to record images on both sides of the sheet in parallel with the tandem image forming device 20 described above. Is provided.
Further, when making a copy using this color electrophotographic apparatus, the original is set on the original table 30 of the automatic original conveying apparatus 800. Alternatively, the automatic document feeder 800 is opened, a document is set on the contact glass 32 of the scanner 700, and the automatic document feeder 800 is closed and pressed. When a start switch (not shown) is pressed, when the document is set on the automatic document feeder 800, the document is transported and moved onto the contact glass 32, and then the document is set on the other contact glass 32. At that time, the scanner 700 is immediately driven to travel the first traveling body 33 and the second traveling body 34. The first traveling body 33 irradiates light from the light source, and further reflects reflected light from the document surface toward the second traveling body 34 and reflects by the mirror of the second traveling body 34 to form the imaging lens 35. Through the sensor 36 to read the original content.

When a start switch (not shown) 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 body 10 is rotated and conveyed. To do. At the same time, the individual image forming means 18 rotates the photoconductor 40 to form black, yellow, magenta, and cyan monochrome images on each photoconductor 40. Then, along with the conveyance of the intermediate transfer member 10, those single color images are sequentially transferred to form a composite color image on the intermediate transfer member 10.
On the other hand, when a start switch (not shown) is pressed, one of the paper feed rollers 42 of the paper feed table 200 is selectively rotated, and the sheet is fed out from one of the paper feed cassettes 44 provided in multiple stages in the paper bank 43, and the separation roller 45. Are separated one by one into the paper feed path 46, conveyed by the conveyance roller 47, guided to the paper feed path 48 in the copying machine main body 500, 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 sheet after the image transfer is conveyed by the secondary transfer device 22 and sent to the fixing device 25, and heat and pressure are applied by the fixing device 25 to fix the transferred image. 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 led again to the transfer position, and 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 is removed by the intermediate transfer body cleaning device 17 to remove residual toner remaining on the intermediate transfer body 10 after the image transfer, so that the tandem image forming apparatus 20 can prepare for another image formation. Here, the registration roller 49 is generally used while being grounded, but it is also possible to apply a bias for removing paper dust from the sheet.

FIG. 3 is a diagram showing individual image forming means of the image forming apparatus 20. In the tandem image forming apparatus 20 described above, the individual image forming means 18 includes a charging device 60, a developing device 61, and a primary transfer device 62 around a drum-shaped photoreceptor 40 as shown in detail in FIG. And a photoconductor cleaning device 63, a charge removal device 64, and the like. Conventionally, a fluorine-based resin, a polycarbonate resin, a polyimide resin, or the like has been used for the intermediate transfer belt, but in recent years, an elastic belt in which the entire belt layer or a part of the belt is used as an elastic member has been used. The transfer of a color image using a resin belt has the following problems.
That is, a color image is usually formed with four color toners. On one color image, toner layers of 1 to 4 layers are formed. 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 and missing edges in the solid portion image tends to occur. Since the resin belt has a high hardness and does not deform according to 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 toner and voids at the time of transfer, and transfer loss is likely to occur. When 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 irregularities, it is possible to obtain a good adhesion without excessively increasing the transfer pressure with respect to the toner layer, and a paper with poor flatness that has no void in characters. The transfer image with excellent uniformity can be obtained.

FIG. 4 is a diagram illustrating an indirect transfer method in a tandem type image forming apparatus. As shown in FIG. 4, the images on the respective photosensitive members 1 are once transferred sequentially to the intermediate transfer member 4 by the primary transfer device 2, and then the images on the intermediate transfer member 4 are transferred to the sheet s by the secondary transfer device 5. There is an indirect transfer system that performs batch transfer, and the transfer device 5 is a transfer conveyance belt, but there are also roller shapes and systems.
FIG. 5 is a diagram illustrating a direct transfer method in a tandem type image forming apparatus. As shown in FIG. 5, there is a direct transfer type in which images on the respective photoreceptors 1 are sequentially transferred onto a sheet s conveyed by a sheet conveying belt 3 by a transfer device 2.
Comparing the direct transfer type and the indirect transfer type, the former arranges the sheet feeding device 6 on the upstream side of the tandem image forming apparatus T on which the photoconductors 1 are arranged, and the fixing device 7 on the downstream side. There is a drawback in that the size increases in the sheet conveying direction. On the other hand, the latter can set the secondary transfer position relatively freely. The sheet feeding device 6 and the fixing device 7 can be arranged so as to overlap the tandem type image forming apparatus T, and there is an advantage that downsizing is possible.
In the former, in order not to increase the size in the sheet conveyance direction, the fixing device 7 is disposed close to the tandem type image forming apparatus T. For this reason, the fixing device 7 cannot be disposed with a sufficient margin that allows the sheet s to bend, and an impact when the leading edge of the sheet s enters the fixing device 7 (particularly with a thick sheet) or fixing. Due to the speed difference between the sheet conveying speed when passing through the apparatus 7 and the sheet conveying speed by the transfer conveying belt, there is a drawback that the fixing device 7 tends to affect the image formation on the upstream side. On the other hand, in the latter case, the fixing device 7 can be disposed with a sufficient margin that the sheet s can be bent, so that the fixing device 7 can hardly affect the image formation.
In this type of color electrophotographic apparatus, as shown in FIG. 4, the transfer residual toner remaining on the photoreceptor 1 after the primary transfer is removed by the photoreceptor cleaning device 8 to clean the surface of the photoreceptor 1. In preparation for another image formation. Further, the transfer residual toner remaining on the intermediate transfer body 4 after the secondary transfer is removed by the intermediate transfer body cleaning device 9 to clean the surface of the intermediate transfer body 4 to prepare for image formation again.

Next, a scale is provided on the intermediate transfer belt used in the present invention, the scale is read by a scale sensor provided on the scale, the speed of the belt surface is detected from the output value, and the ideal speed is detected from the detected speed. An example of an image forming apparatus that obtains stable belt driving by calculating a belt speed to be fed and feeding back to a belt driving motor will be described.
FIG. 6 is a schematic view of an example of an intermediate transfer unit according to the image forming apparatus of the present invention. The same components will be described with the same reference numerals. In this intermediate transfer portion, the belt drive motor 81 drives the drive roller 14 to rotate the intermediate transfer belt 10 in a predetermined direction. As described above, a plurality of photoconductors 40 are arranged on the intermediate transfer belt 10 and images are sequentially transferred onto the intermediate transfer belt 10. A mark sensor (mark detection means) 80 is provided on the transfer belt 10. On the other hand, a belt scale (mark detecting means) 84 is provided on the surface of the intermediate transfer belt 10 as shown in FIG. The belt scale 84 is provided with a scale slit 83. The mark sensor 80 is laid out at a position where the scale slit 83 can be read. In FIG. 6, the mark sensor 80 is laid out in the same direction as the photoconductor 40, but this is an example, and any position where the scale slit 83 can be read may be used.

FIG. 8 is a diagram showing the relationship between the mark sensor 80 and the belt scale 84. As an example, a reflective optical sensor having a pair of light emitting element 80a and light receiving element 80b is used as the mark sensor 80, and the belt scale 84 is configured so that the reflectance is different between the slit part 83 of the scale and other parts. ing. As shown in the figure, the sensor output 80c is a binary signal of High or Low depending on the difference in the reflectance of the scale. For example, if the mark sensor 80 is a type that outputs a High signal when receiving light to the light receiving element 80b, in the example of FIG. 9, if the reflectance of the slit portion 83 is higher than that of other parts, The output signal from the sensor 80 is output during the range of t while the slit portion 83 passes through the mark sensor 80, and the slit portion 83 passes through the detection range of the mark sensor 80 as the intermediate transfer belt 10 rotates. Depending on the presence or absence, the output of the mark sensor 80 repeats as shown in High and Low. Therefore, the surface speed of the intermediate transfer belt 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. These are merely examples, and as long as the belt scale 84 can be detected, there is no problem even if the sensor type, scale type, detection method, and the like are different.
FIG. 9 is a motor speed control block diagram. The motor controller 82 first controls the belt drive motor 81 to rotate at the basic speed 82a. When the belt drive motor 81 is rotated, the intermediate transfer belt 10 is rotated and the belt scale 84 on the surface of the intermediate transfer belt 10 is also rotated. The value is fed back to the motor control unit 82. In the motor control unit 82, if the fed back speed is the same as the basic speed 82a, the control is performed at the basic speed 82a. If there is a difference in speed, the difference is calculated and added to the basic speed 82a. The motor is controlled with the specified value.

FIG. 10 is a detailed control flowchart for explaining the motor control operation. First, the intermediate transfer drive motor control unit turns on the motor so that the motor rotates at the basic speed V (S1). At that time, the basic speed V is output (S2). Next, it is checked whether or not the motor is OFF (S3). If it is OFF, the belt drive motor 81 is turned OFF and the process ends (S12). If the motor is ON in step S3, the output of the mark sensor 80 is detected (S4), and the actual speed V1 on the belt surface is grasped (S5). Then, the basic speed V is compared with the speed (S6). As a result, if V = V1, the process returns to step S3 and repeats. If V and V1 are not equal, V2 = V−V1 is obtained (S8). As a result, if V2> 0, V = V1 + V2 ( S10) Return to step S3 and repeat. If V2 <0 in step S9, V = V1-V2 (S11) and return to step S3 to repeat. By repeating this operation, the belt surface speed can be made constant at the basic speed.
The aforementioned speed fluctuation (difference between basic speed and surface speed) of the intermediate transfer belt is caused by several factors. The fluctuation frequency of the speed fluctuation includes a high frequency component 85 and a low frequency component as shown in FIG. 86 exists. That is, as shown in FIG. 11, when the horizontal axis indicates the rotation time of the intermediate transfer belt 10 and the vertical axis indicates the speed fluctuation amount, the target speed that is the basic speed (ideal speed) is indicated at the center. While the belt 10 makes one revolution, a low frequency component 86 whose speed changes relatively slowly and a high frequency component 85 whose speed changes instantaneously coexist. With the conventional image forming apparatus described above, it is possible to correct the speed fluctuation of these two frequency components. However, in order to correct both the low frequency component 86 and the high frequency component 85, 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 the control loop period and the detection accuracy of the sensor.

  FIG. 12 is a diagram schematically showing motor speed control. That is, the belt scale 84 is read by the mark sensor 80 with reference numeral 90, the phase is detected (91), the phase is compared with the target value (92), and the amount of deviation is detected (93). Based on the amount of deviation, both control values are calculated (94), the value is added to the target value (95), and motor control is performed (96). If the series of controls shown in FIG. 12 is the motor drive feedback loop period A, as shown in FIG. 13, if the control loop period A is sufficiently earlier than the low frequency period C, as shown in FIG. Can be detected, and at the end of one loop, the amount of speed deviation can be corrected and returned to the target speed. That is, A >> C is sufficient. However, since the speed fluctuation cannot be detected for the high frequency period B that is earlier than the control loop period A, it cannot be corrected. That is, when B> A, control is impossible. Therefore, in order to correct the high-frequency cycle B, it is necessary to configure the control loop cycle with a cycle earlier than that. In general, in order to be 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, and as a result, 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. Further, the scale on the belt also requires high resolution and high accuracy, which makes it difficult to process. To achieve this, a need arises to construct a high cost system.

The image forming apparatus of the present invention pays attention to this point, and proposes to limit the correction target to those having a low frequency period. 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 it is possible to construct a system at a sufficiently low cost. 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, the photoconductors 100 and 102 are arranged with a certain distance, and therefore, after being transferred from the first photoconductor 100, until being transferred from the next photoconductor 102. In addition, a transfer time difference 106 is generated. For example, when four color dots are printed at the same position on the transfer paper, first, the toner 101 transferred from the first photoconductor 100 onto the intermediate transfer belt 105 is moved to the next by the rotation of the intermediate transfer belt 105. The second color is transferred onto the intermediate transfer belt 105 for the first time when it arrives at the transfer position (overlapping position 104) of the photoconductor 102.
This means that there is a time difference from the first transfer to the next transfer. 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 cycle slower than the transfer time difference 106 between adjacent photoconductors occurs, the next transfer position is reached later or earlier from the first transfer position by the speed fluctuation. The transfer position is 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, It will not appear as a misalignment. Therefore, even if there is a slight positional shift, the color shift is hardly noticeable on the image. In view of the above, it can be concluded that the effect can be sufficiently exhibited in the image forming apparatus of the present invention.
That is, 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, a factor that induces an increase or decrease in speed during one rotation of the belt may affect the image due to a thickness deviation of the belt or a stacking tolerance of a mechanical layout. Many.

  FIG. 15 is a diagram showing the relationship between factors of fluctuation frequency and speed fluctuation amount. As can be seen from this figure, the speed fluctuation amount is large when the fluctuation frequency is a low frequency component. Incidentally, as a factor of the speed fluctuation of the high frequency cycle not caused by one round of the belt, there is a pitch fluctuation of the gear teeth for driving transmission. In view of the above, the image forming apparatus of the present invention proposes that the target factor to be corrected is limited to the factor caused by one round of the belt. As a result, speed fluctuations in the high frequency cycle that do not appear on the image can be ignored, and the system can be configured at low cost. In the image forming apparatus of the present invention, 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 it is proposed to correct the speed fluctuation caused by this. Hereinafter, this mechanism will be described.

FIG. 16 is a simplified diagram of the intermediate transfer unit when the belt has thickness unevenness. Here, in order to simplify the description, the configuration of the roller is two configurations of the driving roller 111 and the driven roller 110, but the configuration is the same for the configuration of two or more rollers. In order to simplify the explanation, the thickness unevenness of the intermediate transfer belt 112 shown in the figure is assumed to be one thick portion and one thin portion, but the case where there are a plurality of these unevennesses is theoretically the same. is there. The intermediate transfer belt 112 is held by a driving roller 111 and a driven roller 110. Further, the intermediate transfer belt 112 is rotated in the direction of the arrow in the drawing by the driving roller 111. The point D on the belt surface 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 driving roller 111 side and the point E is at the illustrated position on the driven roller 110 side is indicated by a solid line, and the belt rotates, and conversely, the point D is on the driven roller 110 side. The state of the belt when the point E is at the position shown in the figure on the drive roller 111 side is indicated by a broken line.
The belt thickness when the point D is on the drive roller 111 side is X, and the belt thickness when the point E is on the drive roller 111 side is x. That is, X> x. Further, since the radius of the driving roller 111 does not change, the belt rotation radii from the center point of the driving roller 110 to the points D and E that are the belt surface are R and r, respectively, and the difference between them is X−x Will be the same. That is, (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 111, the surface speed of the belt becomes the highest, and then the belt further rotates. The speed gradually decreases, and the belt surface speed becomes the slowest when the thinnest part of the belt reaches the driving 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 manufacture and process. For this reason, this speed unevenness always occurs. Furthermore, 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 is shifted and appears as uneven color on the image. In addition, the belt thickness unevenness is usually several Hz or less in the manufacturing process. Therefore, by focusing attention on this factor, it becomes possible to configure the system at a lower cost. Also, 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 can be detected with higher accuracy, resulting in a more stable speed. Control can be performed.

Next, in the image forming apparatus of the present invention, it is proposed to pay attention to the eccentricity of the driving roller 111 of the intermediate transfer belt 112 as a factor causing the speed fluctuation of the intermediate transfer belt 112 and to correct the speed fluctuation caused by this. Yes. Hereinafter, this mechanism will be described.
FIG. 17 is a simplified diagram of the intermediate transfer unit when the intermediate transfer belt 112 drive roller 111 has an eccentricity. Here, in order to simplify the description, the configuration of the roller is two configurations of the driving roller 111 and the driven roller 110, but the configuration is the same for the configuration of two or more rollers. The intermediate transfer belt 112 is held by a driving roller 111 and a driven roller 110. Further, the intermediate transfer belt is rotated in the direction of the arrow in the drawing by the driving roller 111.
As shown in FIG. 17, when the driving roller 111 is eccentric, the radius from the center to the belt contact surface when the drive roller 111 swells most in the direction of the belt contact surface is R, and conversely, the opposite side from the belt contact surface Let r be the radius from the center of the drive roller to the belt contact surface when it swells most. Assuming that the belt thickness is uniform (X = x), the belt surface speed will produce a speed difference of (R−r). As a result, belt surface speed fluctuations occur. In general, the radius of the drive roller is often large, so that this speed fluctuation appears in a relatively low frequency cycle. As a result, color irregularities appear on the image. Since this low frequency period is generally about several Hz to several tens of Hz, paying attention to this factor and correcting this, it is possible to reduce the correction error due to lower cost and correction frequency limitation, and more Stable speed control can be realized. There are a variety of factors that induce belt speed fluctuations. Among them, correction of factors caused by a low frequency cycle is as described above. At this time, what can actually be a factor of the low frequency cycle is the system configuration at that time, for example, belt material, belt circumference, number of constituent rollers, photoreceptor pitch, component accuracy, etc. It will be different for each. Therefore, it may be difficult to correct each factor. In general, speed fluctuations appear on an image due to many low-frequency factors, and it is sufficient that these can be corrected to about 100 Hz.
FIG. 18 is a conceptual diagram showing 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.

FIG. 19 is a block diagram of the motor speed control unit according to the first embodiment of the present invention. This is a mark sensor (mark detecting means) 80 for detecting marks formed at regular intervals over the entire periphery of the intermediate transfer belt (not shown), and a signal detected by the mark sensor 80 and the intermediate transfer belt are driven. A switching circuit (switching means) 120 that switches an output signal of the encoder 128 provided in the belt driving motor (driving means) 81, a cycle of the signal switched by the switching circuit 120, and a reference belt cycle (reference cycle) 122. Based on a comparator (error detection means) 121 that detects an error by comparison, a switching control of the switching circuit 120 and a control unit (control means) 130 that determines an error level of the comparator 121, and an error level of the comparator 121. A correction control controller 123 for generating a drive frequency, a target speed generator 124 for generating a reference drive frequency, and a correction control An adder 125 for adding the target speed generator 124 to the value of the controller 123, and includes a drive circuit 126 for generating a driving power of the motor. The correction controller 123 includes a filter unit 123a and a proportional gain unit 123b, and the filter unit 123b sets a filter coefficient in accordance with the control band and filters the error signal. Actually cuts over a certain frequency. The proportional gain unit 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 control controller 123 includes the filter unit 123a and the proportional gain unit 123b, but includes controllers for all algorithms having parameters. Further, the type of motor is not limited to a DC servo motor, and a speed-controllable motor such as a pulse motor is a target.
FIG. 20 is a flowchart for explaining control of the belt drive motor 81 provided in the image forming apparatus according to the first embodiment of the present invention. The control unit 30 switches the switching circuit 120 to the mark sensor 80 side in advance (S21). When the belt drive motor 81 rotates and a signal is generated from the mark sensor 80, it is determined whether or not the cycle of the signal is within a predetermined cycle (S22). If it is within the predetermined period in step S22 (YES route in S22), the process returns to step S21 and is repeated. In this state, since almost no error signal is generated in the comparator 121, no value to be added is generated from the correction controller 123, and a frequency close to the target speed is supplied to the drive circuit. When it is out of the predetermined cycle in step S22 (NO route in S22), the control unit 30 switches the switching circuit 120 to the encoder 128 side (S23). As a result, the signal cycle generated from the encoder 128 is input to the comparator 121, the cycle is compared with the reference belt cycle 122, and the same operation is performed thereafter. That is, at this time, the mark sensor 80 is determined to be abnormal for some reason and is disconnected from the circuit.
As described above, the present embodiment includes the switching circuit 120 that switches the signal from the mark sensor 80 and the signal from the encoder 128, and the comparator 121 that compares the reference belt period 122 with the period of the mark sensor 80 to detect an error. In addition, when the error is larger than a predetermined value, it is possible to detect that the mark sensor 80 is abnormal, thereby switching to the encoder 128 and correcting and controlling the intermediate transfer belt speed. Thereby, even if the mark sensor 80 breaks down, the speed unevenness of the intermediate transfer belt can be minimized.

FIG. 21 is a block diagram of a motor speed control unit according to the second embodiment of the present invention. FIG. 21 differs from FIG. 19 in that a plurality of mark sensors 80 are provided, and the output of the average value calculation circuit 131 that calculates the average value of the signal period output from each mark sensor 80 (ac) is the switching circuit 120. It is a point connected to. When the error detected by the comparator 121 exceeds a predetermined value, the control unit 130 determines that at least one of the mark sensors 80 (ac) is abnormal, The switching circuit 120 is controlled so as to switch from the control by the mark sensor 80 (ac) to the control by the encoder 128. Other operations are the same as those in FIG.
FIG. 22 is a configuration diagram of a motor speed control unit according to the second embodiment of the present invention. The same components will be described with the same reference numerals. In this configuration, three mark sensors 80 a, 80 b, and 80 c are arranged between the respective photoreceptors 40. Other configurations are the same as those in FIG.
As described above, this embodiment includes a plurality of mark sensors 80 and includes an average value calculation circuit 131 that calculates an average value of the mark sensors 80. That is, signals from the plurality of mark sensors 80 are signals averaged by the average value calculation circuit 131. The signal is used as a mark sensor signal. Accordingly, it is possible to minimize the detection error by detecting the average value of the plurality of mark sensors, and it is possible to minimize the speed unevenness of the intermediate transfer belt even if the mark sensor fails.

FIG. 23 is a block diagram of a motor speed control unit according to the third embodiment of the present invention. The same components will be described with the same reference numerals. FIG. 23 differs from FIG. 19 in that a main mark sensor 80d and a sub mark sensor 80e are provided as the mark sensor 80, and a sensor signal correction circuit that corrects the output of the main mark sensor 80d with a signal output from the sub mark sensor 80e. (Signal correction means) The output of 132 is connected to the switching circuit 120. When the error detected by the comparator 121 exceeds a predetermined value, the control unit 130 determines that the main mark sensor 80d is abnormal among the main mark sensor 80d and the sub mark sensor 80e, and the main mark sensor 80d. The switching circuit 121 is controlled to switch from the control by the sub mark sensor 80e to the control by the encoder 128.
As described above, the present embodiment includes two main and sub mark sensors, and includes a sensor signal correction circuit 132 that corrects the signal of the main mark sensor 80d with the signal of the sub mark sensor 80e. For example, a logical sum of signals from the main and sub mark sensors is taken as correction means, and when the main mark sensor 80d fails, the sub mark sensor 80e is replaced and the signal is used as a signal of the mark sensor. is there. Thereby, even if the main mark sensor 80d breaks down, the speed unevenness of the intermediate transfer belt can be minimized.

1 is a configuration diagram of an image forming apparatus of a tandem type intermediate transfer system. FIG. 3 is a cross-sectional configuration diagram of an intermediate transfer member. 2 is a diagram showing individual image forming means of the image forming apparatus 20. FIG. FIG. 3 is a diagram illustrating an indirect transfer method in a tandem type image forming apparatus. 3 is a diagram illustrating a direct transfer method in a tandem type image forming apparatus. FIG. FIG. 3 is a schematic diagram illustrating an example of an intermediate transfer unit according to the image forming apparatus of the present invention. FIG. 4 is a diagram of a belt scale formed on an intermediate transfer belt. FIG. 6 is a diagram showing a relationship between a mark sensor 80 and a belt scale 84. It is a motor speed control block diagram. It is a detailed control flowchart explaining a motor control operation. It is a figure which shows the relationship between rotation time and speed variation. It is a figure showing typically speed control of a motor. It is a figure explaining operation | movement of a deviation | shift control amount. It is a figure explaining the process of a color superimposition. It is a figure which shows the relationship of the factor of a fluctuation frequency and a speed fluctuation amount. FIG. 6 is a simplified diagram of an intermediate transfer unit when a belt has a thickness unevenness. FIG. 6 is a simplified diagram of an intermediate transfer unit when the intermediate transfer belt 112 drive roller 111 is eccentric. It is a conceptual diagram which shows the relationship between the speed fluctuation amount with respect to a fluctuation frequency, and the influence on an image. It is a block diagram of the motor speed control part which concerns on the 1st Embodiment of this invention. 4 is a flowchart illustrating control of a belt drive motor 81 provided in the image forming apparatus according to the first embodiment of the present invention. It is a block diagram of the motor speed control part which concerns on the 2nd Embodiment of this invention. It is a block diagram of the motor speed control part which concerns on the 2nd Embodiment of this invention. It is a block diagram of the motor speed control part which concerns on the 3rd Embodiment of this invention.

Explanation of symbols

  80 mark sensor, 81 belt drive motor, 120 switching circuit, 121 comparator, 122 reference belt cycle, 123 correction control controller, 124 target speed generator, 125 adder, 126 drive circuit, 128 encoder, 130 control unit

Claims (3)

In a tandem type color image forming apparatus provided with an intermediate transfer belt for transferring a color toner image formed on a photoconductor to synthesize a color image,
Mark detection means for detecting marks formed at regular intervals over the entire circumference of the intermediate transfer belt, an encoder provided in a signal detected by the mark detection means and a drive means for driving the intermediate transfer belt Switching means for switching the output signal, error detection means for detecting an error by comparing the period of the signal switched by the switching means with a reference period, switching control of the switching means, and error level of the error detection means Control means for determining
An image forming apparatus that detects a moving speed error of the intermediate transfer belt based on an error detected by the error detecting unit, and corrects and controls the intermediate transfer belt speed based on the detection result,
The control means determines that the mark detection means is abnormal when the error detected by the error detection means exceeds a predetermined value, and switches from control by the mark detection means to control by the encoder. An image forming apparatus that controls a switching unit. In a tandem type color image forming apparatus provided with an intermediate transfer belt for transferring a color toner image formed on a photoconductor to synthesize a color image,
A plurality of mark detection means for detecting marks formed at constant intervals over the entire circumference of the intermediate transfer belt, an average value calculation circuit for calculating an average value of signal periods output from the mark detection means, Switching means for switching between a calculation result output by the average value calculation circuit and an output signal of an encoder provided in a driving means for driving the intermediate transfer belt, a period of the signal switched by the switching means, and a reference period Error detection means for comparing the error detection means, and switching means for controlling the switching means and control means for determining the error level of the error detection means,
An image forming apparatus that detects a moving speed error of the intermediate transfer belt based on an error detected by the error detecting unit, and corrects and controls the intermediate transfer belt speed based on the detection result,
When the error detected by the error detection unit exceeds a predetermined value, the control unit determines that at least one of the plurality of mark detection units is abnormal, and the mark detection unit An image forming apparatus that controls the switching unit to switch from control to control by the encoder. In a tandem type color image forming apparatus provided with an intermediate transfer belt for transferring a color toner image formed on a photoconductor to synthesize a color image,
The main and sub two mark detection means for detecting marks formed at regular intervals over the entire circumference of the intermediate transfer belt, and the output of the main mark detection means is corrected by a signal output from the sub mark detection means A signal correcting means for switching, a switching means for switching between an output signal of the signal correcting means and an output signal of an encoder provided in a driving means for driving the intermediate transfer belt, a cycle and a reference of the signal switched by the switching means An error detection means for detecting an error by comparing with a period; and a control means for determining a switching level of the switching means and an error level of the error detection means,
An image forming apparatus that detects a moving speed error of the intermediate transfer belt based on an error detected by the error detecting unit, and corrects and controls the intermediate transfer belt speed based on the detection result,
When the error detected by the error detection unit exceeds a predetermined value, the control unit determines that the main mark detection unit is abnormal among the main and sub two mark detection units, and the mark detection unit An image forming apparatus that controls the switching unit to switch from control to control by the encoder.

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