JP2012123044A - Image forming apparatus - Google Patents

Abstract

An image forming belt is provided with an image-carrying belt in which a variation in a stretched state is suppressed, and a blur at a transfer portion is suppressed to maintain a good image forming state.
A photosensitive drum, an image forming unit that forms an image on the photosensitive drum, an intermediate transfer belt, and a primary transfer unit that transfers a toner image on the photosensitive drum to the intermediate transfer belt. A belt driving roller 12 that also serves as a tension roller for the intermediate transfer belt 11 and drives the intermediate transfer belt 11, a secondary transfer roller 20 that is in contact with the intermediate transfer belt 11 and has a conveying force, and a tension state of the intermediate transfer belt 11. A tension detection mechanism 50 for detecting and a control unit. The control unit uses the detection result of the tension detection mechanism 50 in a state where the tension state of the intermediate transfer belt 11 is substantially equal over the entire circumference as a control target value. The drive speed of the secondary transfer roller 20 is determined.
[Selection] Figure 1

Description

  The present invention relates to an image forming apparatus for transferring and recording an image formed on an image carrier such as an electrostatic recording system or an electrophotographic recording system onto a transfer material.

  Among electrophotographic full-color image forming apparatuses, there is an image forming apparatus using an intermediate transfer belt. A schematic configuration of the transfer unit will be described.

  The intermediate transfer belt is stretched by three stretching rollers. One of the stretching rollers is a driving roller for driving the intermediate transfer belt. The driving roller conveys the intermediate transfer belt from the inside. Further, one of the stretching rollers other than the driving roller is disposed at a position facing the secondary transfer roller. For this reason, the secondary transfer roller conveys the intermediate transfer belt from the outside. In this way, the intermediate transfer belt receives a conveying force at different positions by the drive roller and the secondary transfer roller.

  In this configuration, if there is even a slight speed difference between the surface speed of the intermediate transfer belt and the surface speed of the secondary transfer roller, a tangential force is generated in the secondary transfer portion. Due to this tangential force, a tension (tension) difference is generated on the intermediate transfer belt upstream and downstream of the secondary transfer portion. On the other hand, the secondary transfer roller is slightly deformed in the gear that drives the secondary transfer roller.

  When the transfer material passes through the secondary transfer nip portion in such a state, the conveyance force (tangential force) from the secondary transfer roller received by the intermediate transfer belt changes. Therefore, in the intermediate transfer belt, speed fluctuations and tension fluctuations (stretching fluctuations) occur, and problems such as color misregistration and image blurring at the primary transfer portion occur. On the other hand, in the secondary transfer roller, the slight deformation of the gear is eliminated, and a slight speed fluctuation is generated by the amount of backlash, which causes a problem of blurring in the secondary transfer portion. .

  In order to solve the above problems, a contact member is attached to the intermediate transfer belt, and the tension of the intermediate transfer belt at the time of conveyance is detected from the deformation amount of the contact member. There is a technique for performing control so as to match the detection result of time (see, for example, Patent Document 1). Here, the speed of the driving roller that governs the speed of the intermediate transfer belt and the speed of the secondary transfer roller facing it are controlled.

JP2008-145680

  However, in the above prior art, even when there is a speed difference between the intermediate transfer belt and the secondary transfer roller, control is performed to maintain the speed difference. For this reason, the tangential force of the secondary transfer portion is not lost. In other words, if there is a speed difference between the intermediate transfer belt and the secondary transfer roller at the normal time as a control target, the tension of the intermediate transfer belt is maintained by maintaining the tension difference between the upstream and downstream with respect to the secondary transfer portion. Fluctuation is suppressed. Therefore, although color shift and blurring at the primary transfer portion can be suppressed, blurring at the secondary transfer portion cannot be suppressed.

  It is possible to set speed control target values for the driving roller and the secondary transfer roller based on setting values such as the outer diameter of the driving roller, the outer diameter of the secondary transfer roller, and the film thickness of the intermediate transfer belt. However, a speed difference actually occurs between the intermediate transfer belt and the secondary transfer roller due to variations in processing, wear due to long-term use, and a slight slip between the driving roller and the intermediate transfer belt.

  An object of the present invention is to maintain a good image forming state by suppressing fluctuations in the tension state of the image bearing belt and suppressing blurring at the transfer portion.

In order to achieve the above object, a typical configuration of the present invention is as follows.
An image carrier;
An image forming unit for forming an image on the image carrier;
An image bearing belt;
A primary transfer portion for transferring a toner image on the image carrier to the image carrier belt;
A first driving member that doubles as a tension roller of the image bearing belt and drives the image bearing belt;
A second drive member in contact with the image bearing belt and having a conveying force;
A tension detection mechanism for detecting the tension state of the image bearing belt;
A control unit,
The control unit determines a driving speed of the second driving member by using, as a control target value, a detection result of the tension detection mechanism in a state where the tension state of the image bearing belt is substantially equal over the entire circumference. And

  With the above-described configuration, it is possible to make the tension state of the belt during image bearing substantially equal over the entire circumference. As a result, color shift and blurring at the primary transfer portion can be suppressed, and blurring at the secondary transfer portion can also be suppressed.

1 is a schematic diagram of an image forming apparatus according to a first embodiment. Explanatory drawing of the tension detection mechanism which concerns on 1st Embodiment. 1 is a schematic diagram of an optical distance measuring sensor according to a first embodiment. FIG. 5 is a relationship diagram between a detection distance and a voltage value in the optical distance measuring sensor according to the first embodiment. Explanatory drawing in connection with rotation control of the motor which concerns on 1st Embodiment. The figure which shows the relationship between the state and tension AD value in 1st Embodiment. The figure explaining the tangential force of 1st Embodiment. Explanatory drawing of the tension detection mechanism which concerns on 2nd Embodiment. The figure which shows the relationship between the state which concerns on 3rd Embodiment, and tension AD value. Explanatory drawing of the tension detection mechanism which concerns on 4th Embodiment. The figure which shows the relationship between the state which concerns on 5th Embodiment, and tension AD value. FIG. 10 is a relationship diagram between a secondary transfer driving motor and a tension AD value according to a fifth embodiment.

[First Embodiment]
The image forming apparatus according to the first embodiment will be described with reference to FIG. FIG. 1 is a schematic diagram of an image forming apparatus according to the first embodiment. In the description, the subscripts (yellow Y, magenta M, cyan C, black K) in the drawings are omitted as necessary.

(Schematic configuration of image forming unit)
As shown in FIG. 1, the image forming apparatus includes four photosensitive drums 2 (image carriers) for yellow, magenta, cyan, and black. A primary charger 7 and a developing unit 3 are arranged around each photosensitive drum 2 in order from the upstream side in the rotation direction.

  An intermediate transfer belt 11 (image bearing belt) to which a toner image formed on the surface of the photosensitive drum 2 is primarily transferred is disposed at a position facing the photosensitive drum 2. The intermediate transfer belt 11 is stretched by a belt driving roller 12 (first driving member), a secondary transfer counter roller 13, a tension roller 15, and a driven roller 14.

  The intermediate transfer belt 11 is rotationally driven by a belt drive motor 28. The tension roller 15 is urged in one direction by a tension spring 16 to apply a predetermined tension to the intermediate transfer belt 11. The intermediate transfer belt 11 is provided with a tension detection mechanism 50 for detecting tension.

As the intermediate transfer belt 11, an endless PVdF single-layer resin belt having a thickness of 100 microns and adjusted to a volume resistivity of about 10 ¹⁰ Ωcm by adding an ionic conductive agent was used. As the belt driving roller 12, a hollow aluminum tube having an outer diameter of 24 mm covered with EPDM rubber with a thickness of 0.5 mm and having an electric resistance of 10 ⁵ Ω or less was used.

  A primary transfer roller 4 is disposed at a position facing the photosensitive drum 2 with the intermediate transfer belt 11 interposed therebetween.

  The intermediate transfer belt 11 is provided with a cleaning roller 18 for removing toner (residual toner) adhering to the intermediate transfer belt 11. The cleaning roller 18 is driven to rotate by the intermediate transfer belt 11.

  A secondary transfer roller 20 (second drive member) having a conveyance force is disposed at a position facing the secondary transfer counter roller 13 with the intermediate transfer belt 11 interposed therebetween. That is, the secondary transfer roller 20 contacts the outer peripheral surface of the intermediate transfer belt 11 and conveys the intermediate transfer belt 11.

In the present embodiment, as the secondary transfer roller 20, a 6-mm-thick conductive foam rubber is coated on a SUS core, the hardness is 30 degrees (when Asker-C4.9N (500 gf) load), and the electrical resistance value is A roller of 1 × 10 ⁷ Ω with an outer diameter of 18 mm was used. The secondary transfer roller 20 is urged in one direction toward the intermediate transfer belt 11 by a spring (not shown), forms a secondary transfer nip portion T2 (contact portion), and is rotationally driven by the secondary transfer drive motor 29. Yes.

(Operation of image forming apparatus)
The image forming operation of the image forming apparatus will be described together with the configuration other than the image forming unit.

  When the image forming operation is started, first, the transfer material P in the cassette 30 is fed by the feeding roller 31 and then conveyed to the registration roller pair 33. At this time, the registration roller pair 33 stops rotating, and the transfer material P is abutted against the nip of the registration roller pair 33. Thereby, the skew of the transfer material P is corrected.

  On the other hand, in parallel with the transfer operation of the transfer material P, the image forming operation is performed in the image forming unit. This will be described by exemplifying a yellow image forming unit.

  First, the surface of the yellow photosensitive drum 2Y is uniformly negatively charged by the primary charger 7Y. Next, when image exposure is performed by the exposure unit 1, an electrostatic latent image is formed on the surface of the photosensitive drum 2Y. The electrostatic latent image is an image corresponding to the yellow image component of the image signal. Next, when the developing means 3Y comes into contact with the photosensitive drum 2Y, the electrostatic latent image is developed using a negatively charged yellow toner. In this way, the yellow toner image is visualized on the photosensitive drum 2. The yellow toner image thus obtained is primarily transferred onto the intermediate transfer belt 11 by the primary transfer roller 4Y supplied with the primary transfer bias.

  Such a series of toner image forming operations are sequentially performed with predetermined timing on the other photosensitive drums 2M, 2C, and 2K. The color toner images formed on the photosensitive drums 2 are primary-transferred on the intermediate transfer belt 11 in sequence at the primary transfer portions T1.

  The four-color toner images transferred superimposed on the intermediate transfer belt 11 move to the secondary transfer nip T2 as the intermediate transfer belt 11 moves. On the other hand, the transfer material P whose skew has been corrected by the registration roller pair 33 is sent to the secondary transfer nip portion T2 in timing with the image on the intermediate transfer belt 11. For this reason, at the secondary transfer nip T2, the four color toner images on the intermediate transfer belt 11 are secondarily transferred onto the transfer material P all at once. The transfer material P onto which the toner image has been primarily transferred in this way is conveyed to the fixing device 40. In the fixing device 40, the transfer material P is heated and pressurized, and the toner image is fixed to the transfer material P. Thereafter, the paper is discharged and stacked on the discharge tray 42 by the discharge roller pair 41.

  The intermediate transfer belt 11 that has finished the secondary transfer is subjected to removal of residual toner remaining on the surface by a cleaning roller 18 installed in the vicinity of the secondary transfer counter roller 13.

(Outline of tension detection mechanism)
The tension detection mechanism 50 of the intermediate transfer belt 11 will be described with reference to FIG. FIG. 2 is an explanatory diagram of the tension detection mechanism according to the first embodiment. (A), (b), (c) is a figure according to a state. Each state will be described later.

  The tension detection mechanism 50 is disposed downstream of the secondary transfer roller 20 with respect to the moving direction of the intermediate transfer belt 11 (A portion in FIG. 2). The tension detection mechanism 50 includes a tension detection roller 52 (movable member), a tension detection member 53, a compression spring 54, and an optical distance measuring sensor 55 (movable member detection unit). A distance measuring sensor is a sensor that measures distance.

  The tension detection roller 52 is movable. The tension detection roller 52 is urged to the inside of the intermediate transfer belt 11 with a constant force by the compression spring 54. The vertical position of the tension detection roller 52 is determined by the balance between the tension of the intermediate transfer belt 11 and the force of the compression spring 54.

  The tension detection member 53 moves up and down together with the tension detection roller 52. The optical distance measuring sensor 55 is a sensor that detects the position of the tension detecting member 53, and an infrared distance measuring sensor is used.

  The optical distance measuring sensor 55 will be described with reference to FIG. FIG. 3 is a schematic diagram of the optical distance measuring sensor according to the first embodiment.

  As shown in FIG. 3, the optical distance measuring sensor 55 includes an LED light emitting unit 60 and a position sensing device 61 (Position Sensitive Device: PSD).

  At the time of detection, first, infrared light 62 is emitted from the light emitting unit 60 to the tension detection member 53. The reflected light 62 diffused and reflected by the distance measuring object is narrowed down by the light receiving condensing means 63 disposed in front of the light receiving surface 61a of the position detecting element 61 and guided to the light receiving surface 61a. Then, the distance from the target object is measured by the triangulation method based on the position of the distribution center of the infrared ray 62 that has reached the light receiving surface 61a.

  Since the position of the distribution center of the infrared rays 62 reaching the light receiving surface 61a is detected and converted into a distance, even if the reflectance changes in the surface state of the target object, the distance data is not affected. Then, the position detected by the position detection element 61 is converted into a distance by the calculation IC and output as a voltage value.

  FIG. 4 is a relationship diagram between a detection distance and a voltage value in the optical distance measuring sensor according to the first embodiment. With reference to FIG. 4, in the present embodiment, the optical distance measuring sensor 55 is arranged so that the distance from the optical distance measuring sensor 55 to the tension detecting member 53 as a detection object is 4 mm to 10 mm.

(Operation of tension detection mechanism 50)
With reference to FIG. 2, how the tension detection mechanism 50 operates according to a change in the tension state (stretched state) of the intermediate transfer belt 11 will be described.

  FIG. 2A shows a state where the tension state is neutral. When there is no tangential force at the secondary transfer nip T2 in the intermediate transfer belt 11, the tension state of the intermediate transfer belt 11 becomes neutral. The neutral state is a state where the stretched state is substantially the same over the entire circumference of the intermediate transfer belt 11. That is, the tension of the A portion downstream of the secondary transfer roller 20 with respect to the movement direction of the intermediate transfer belt 11 and the B portion of the upstream side of the secondary transfer roller 20 with respect to the movement direction of the intermediate transfer belt 11. Are substantially equal.

  FIG. 2B shows a state in which the speed of the secondary transfer roller 20 has slowed and a tangential force in the direction of the arrow is generated in the secondary transfer nip portion T2 of the intermediate transfer belt 11. In this case, the tension of the A part is increased. At this time, in the tension detection roller 52, the balance position with the compression spring 54 changes as the tension of the A portion on the intermediate transfer belt 11 increases. Then, the tension detection roller 52 moves to the inside (in the direction of the arrow) of the intermediate transfer belt 11. As a result, the tension detection member 53 configured integrally with the tension detection roller 52 also moves in the same manner, and the distance between the optical distance measuring sensor 55 and the tension detection member 53 is reduced.

  FIG. 2C shows a state in which the speed of the secondary transfer roller 20 is increased and a tangential force in the arrow direction is generated in the secondary transfer nip portion T <b> 2 of the intermediate transfer belt 11. In this case, the tension at part A is low. At this time, in the tension detection roller 52, the balance position with the compression spring 54 changes as the tension of the portion A on the intermediate transfer belt 11 decreases. Then, the tension detection roller 52 moves to the outside (in the direction of the arrow) of the intermediate transfer belt 11. As a result, the tension detection member 53 integrally formed with the tension detection roller 52 moves in the same manner, and the distance between the optical distance measuring sensor 55 and the tension detection member 53 is increased.

  Thus, the tension of the intermediate transfer belt 11 can be detected based on the distance measured by the optical distance measuring sensor 55.

  In the present embodiment, the tension detection mechanism 50 is disposed on the downstream side of the secondary transfer roller 20 with respect to the moving direction of the intermediate transfer belt 11, which is indicated by “A” in FIG. 2. However, the present invention is not limited to this, and it may be arranged upstream of the secondary transfer roller 20 with respect to the moving direction of the intermediate transfer belt 11, which is indicated by B portion in FIG. 2.

  In the present embodiment, an example in which the optical distance measuring sensor 55 detects the position of the tension detection member 53 configured to move integrally with the tension detection roller 52 has been described. However, the present invention is not limited to this, and the position of the tension detection roller 52 can also be detected via a link.

(Control method of belt drive motor 28 and secondary transfer drive motor 29)
A method for controlling the belt drive motor 28 and the secondary transfer drive motor 29 will be described with reference to FIG. FIG. 5 is an explanatory diagram related to the rotation control of the motor according to the first embodiment. In addition to the CPU 71, the control unit includes a secondary transfer motor rotation speed setting unit 76 and an intermediate transfer belt drive motor control unit 77.

  The belt drive motor 28 is a DC brushless motor and controls the speed of the intermediate transfer belt 11. The intermediate transfer belt drive motor control unit 77 controls the drive of the belt drive motor 28. The intermediate transfer belt drive motor control unit 77 receives a rotation state signal from the belt drive motor 28 and controls the rotation speed to be suitable for image formation.

  The output voltage value of the optical distance measuring sensor 55 obtained according to the tension of the intermediate transfer belt 11 is converted into a digital signal by the AD conversion unit 73 of the CPU 71 provided in the controller 70 in the apparatus. With this configuration, an AD value of tension corresponding to the distance between the optical distance measuring sensor 55 and the tension detecting member 53 is obtained. The CPU 71 has a RAM 74 and stores a control target value that is a control target. The control target value will be described later.

  The secondary transfer drive motor rotation speed determination unit 72 in the CPU 71 uses the control target value stored in the RAM 74 and the AD value of the tension of the intermediate transfer belt 11 calculated by the CPU 71 to rotate the secondary transfer drive motor 29. Determine the number. The determined motor rotational speed information is sent to the secondary transfer motor rotational speed setting section 76 in the secondary transfer drive motor control section 75. Based on the rotational speed set by the secondary transfer motor rotational speed setting unit 76, the secondary transfer drive motor 29 is rotationally driven.

  In this embodiment, the motor rotation speed is controlled using PI control so that the tension AD value during operation converges to the control target value. The proportional gain of P control (proportional control) was determined in a range where the tension AD value did not cause overshoot and hunting with respect to the control target value.

  This is because, when overshoot or hunting occurs, tension variation occurs and color misregistration or the like occurs. Then, the integral control parameter is determined so that the deviation (offset) from the control target value that remains without reaching the control target value only by the proportional control is removed by the integral control. When integral control is added, output changes are accumulated as long as there is an offset, so that the offset gradually attenuates and can converge to the control target value.

(Correction mode for determining the control target value)
The correction mode for determining the control target value is performed by an operation different from the normal image forming operation. The operation in the correction mode will be described below.

  In the present embodiment, the required control target value is the tension AD value when the tension of the intermediate transfer belt 11 is substantially equal over the entire circumference. Explaining in FIG. 2, the tension AD value in a state where the tensions of the A part and the B part are substantially equal. This is because if the tensions of the A part and the B part are substantially equal, even if the transfer material P and the toner image (developer image) pass through the secondary transfer nip part during image formation, the tension fluctuation can be minimized. Because. As a result, if the tensions of the A part and the B part are substantially equal, the occurrence of color misalignment and image blur can be suppressed. At the same time, the secondary transfer roller 20 can also minimize the speed fluctuation and suppress the occurrence of transfer blur.

  On the other hand, if there is a difference in tension between A and B shown in FIG. 2, if the transfer material or toner image passes through the secondary transfer nip during image formation, the tension changes by the difference. To do. As a result, the tension fluctuation propagates to the primary transfer portion, causing problems such as color misregistration and image blurring. At the same time, the speed fluctuation of the secondary transfer roller also occurs and image blurring occurs.

  A correction mode in which the tension AD value in a state where the tensions of the A part and the B part are substantially equal to each other and detected as a control target value, which is an operation different from the normal image forming operation, will be described with reference to FIG. To do. FIG. 6 is a diagram illustrating the relationship between the state according to the first embodiment and the tension AD value.

  In FIG. 6, the vertical axis represents the tension AD value that is AD converted to an 8-bit digital value. The tension AD value decreases as the distance between the tension detection member 53 and the optical distance measuring sensor 55 increases. That is, in the present embodiment, the tension AD value decreases as the looseness of the tension increases, and conversely, the tension AD value increases as the tension tension increases.

  Therefore, a smaller value on the vertical axis means a state where the tension in the B part is higher than that in the A part (tensioned state). Conversely, a larger value on the vertical axis means a state where the tension in the B part is lower than that in the A part (loosening). In the figure, D state, E state, and F state represent the state of the apparatus during image formation, D is the pre-rotation, E is the state where the transfer material and the toner image are present in the secondary transfer nip T2, and F is the post-rotation. Indicates.

  FIG. 6 shows a case where the conveyance speed of the intermediate transfer belt 11 by the secondary transfer drive motor 29 is relatively faster than the conveyance speed of the intermediate transfer belt 11 by the belt drive motor 28. Specifically, it was set 0.3% faster than the speed setting of the secondary transfer drive motor 29 in which the surface speeds of the intermediate transfer belt 11 and the secondary transfer roller 20 in design are constant.

  When the secondary transfer drive motor 29 is fast, during the pre-rotation (D state), the tension of the A part is loosened and the tension of the B part is tensioned. At the timing when the transfer material and the toner image enter the secondary transfer nip portion T2 (start point of the E state), the looseness of the A portion and the tension of the B portion are gradually eliminated, and the tensions of the A portion and the B portion are substantially equal. Become. The state in which the tensions in the A part and the B part are substantially equal is maintained until the transfer material and the toner image pass through the secondary transfer nip part T2 (end point of the E state). In the post-rotation (F state), the tension of the A part is loosened and the tension of the B part is reversed and the process ends.

  In a state where the transfer material and the toner image are not present in the secondary transfer nip portion T2 (pre-rotation D, post-rotation F), the intermediate transfer belt 11 is moved to the secondary transfer roller 20 by the frictional force between the secondary transfer roller 20 and the intermediate transfer belt 11. A large tangential force is received from the transfer roller 20. For this reason, even with a slight speed difference, the tension is loosened and tensioned.

  On the other hand, in a state where the transfer material and the toner image are present in the secondary transfer nip portion T2 (E state), a slip effect (lubricating effect) is generated in the secondary transfer nip portion T2 due to the presence of the transfer material and the toner image. . As a result, even if the conveyance speed of the intermediate transfer belt 11 by the secondary transfer drive motor 29 is higher than the conveyance speed of the intermediate transfer belt 11 by the belt drive motor 28, the intermediate transfer belt 11 of the secondary transfer roller 20 The applied tangential force is extremely small. For this reason, during the secondary transfer, the tension tension state and the loose state are eliminated, and a substantially equal movement is observed. That is, the tension AD value when the transfer material and the toner image are present in the secondary transfer nip portion T2 (E state) becomes the control target value.

  The correction mode for calculating the control target value includes the timing when the image forming apparatus is first turned on, the regular timing after the power is turned on, and the timing when the intermediate transfer belt 11 and the secondary transfer roller 20 are replaced. It was set to be carried out.

  The purpose of executing the correction mode at the timing when the power of the image forming apparatus is first turned on is to correct the speed difference between the intermediate transfer belt 11 and the secondary transfer roller 20 due to the dimensions of parts incorporated in the image forming apparatus. It is said.

  The reason why the correction mode is performed at a regular timing after the power is turned on is that the tangential force by the secondary transfer roller 20 changes due to long-term use. Specifically, when the intermediate transfer belt 11, the belt driving roller 12, and the secondary transfer roller 20 are worn and the dimensions change or the friction coefficient changes, the speed of the intermediate transfer belt 11 decreases and the secondary transfer belt 11 decreases. The tangential force by the transfer roller 20 changes.

  The reason why the correction mode is performed at the timing when the intermediate transfer belt 11 and the secondary transfer roller 20 are replaced is that the speed of the intermediate transfer belt 11 and the secondary transfer roller 20 changes depending on the component dimensions of the replaced components. is there.

  This correction mode may be configured to be performed simultaneously with other corrections such as color misregistration correction and density correction, or may be configured to be arbitrarily performed by the user.

  Further, in a state (E state) where the transfer material and the toner image are present in the secondary transfer nip portion T2, a secondary transfer bias smaller than that in normal image formation is applied. Thus, the tangential force that the intermediate transfer belt 11 receives from the secondary transfer counter roller 13 can be reduced as much as possible in a state (E state) in which the transfer material and the toner image are present in the secondary transfer nip T2.

  Accordingly, the tension detection difference detected during the pre-rotation (D state) and the state where the transfer material and the toner image are present in the secondary transfer nip portion T2 (E state) becomes clear. That is, it is possible to easily determine the state in which the toner image is present in the secondary transfer nip portion T2 from the tension detection difference detected by the tension detection mechanism 50.

  In the present embodiment, the case where the conveyance speed of the intermediate transfer belt 11 by the secondary transfer drive motor 29 is relatively faster than the conveyance speed of the intermediate transfer belt 11 by the belt drive motor 28 is shown. However, the present invention is not limited to this, and conversely, even if the conveyance speed of the intermediate transfer belt 11 by the secondary transfer drive motor 29 is set relatively slower than the conveyance speed of the intermediate transfer belt 11 by the belt drive motor 28. I do not care.

  In this embodiment, the set speed by the secondary transfer drive motor 29 is set with respect to the speed setting of the secondary transfer drive motor 29 in which the surface speeds of the intermediate transfer belt 11 and the secondary transfer roller 20 in design are constant. An example of setting 0.3% faster or slower was shown. This is a speed setting for obtaining an appropriate control target value in the present embodiment. Accordingly, the material is optimized according to the material of the intermediate transfer belt 11, the stretched structure, the material of the secondary transfer roller 20, the nip force of the secondary transfer nip T 2, and the like, but is not limited thereto. .

(About transport force)
In this embodiment, the tension information obtained by the tension detection mechanism 50 is fed back to the driving of the secondary transfer roller 20 having a small conveying force instead of the belt driving roller 12 having a large conveying force, and the driving speed is determined. . The conveyance force disclosed in this embodiment is defined below.

The belt driving roller 12 disposed inside the intermediate transfer belt 11 and disposed so as to be wound around the intermediate transfer belt 11 has a high conveying force due to the winding portion. The conveying force F ₁ can be expressed as shown in FIG. 7 according to Euler's belt theory. FIG. 7 is a chart for explaining the tangential force of the first embodiment.

As shown in FIG. 7, the conveying force F ₁ is determined by Equation (1). Note that the coefficient of static friction between the front surface of the belt driving roller 12 and the back surface of the intermediate transfer belt 11 is μ ₁ , the winding angle of the intermediate transfer belt 11 is θ, and the tension on the belt surface of the intermediate transfer belt 11 is T.

On the other hand, the conveying force F ₂ of which is disposed on the outer side of the intermediate transfer belt 11 secondary transfer roller 20, the total pressure of the secondary transfer nip portion N, static friction coefficient of the secondary transfer roller 20 surface and the surface of the intermediate transfer belt 11 when was the μ _2, obtained by the formula (2).

Therefore, it is possible to determine the magnitude relationship between the conveying force F ₁ and the conveying force F _2. The friction coefficient of each member can obtain the effect of this embodiment in the test method defined in JIS-K7125.

In this embodiment, μ ₁ = 0.6, T = 30 (N), θ = 2.27 (rad) = 130 (deg), and μ ₂ = 0.5, T = 20 (N). Therefore, F ₁ and F ₂ respectively satisfy F ₁ > F ₂ as shown in FIG.

  Therefore, it is possible to determine that the driving of the secondary transfer roller 20 is a driving source having a small conveying force.

  As described above, in this embodiment, the tension detection mechanism 50 detects the tension state of the intermediate transfer belt 11. Then, the rotational drive of the secondary transfer drive motor 29 is controlled so that the tension AD value during operation becomes the control target value. This control target value is the tension AD value when the tension of the intermediate transfer belt 11 is substantially equal over the entire circumference. Thereby, it is possible to suppress the tension fluctuation and the belt speed fluctuation through the image formation, and also to suppress the speed fluctuation of the secondary transfer roller 20, thereby reducing the occurrence of color misregistration and transfer blur.

[Second Embodiment]
In the present embodiment, another embodiment of the tension detection mechanism 50 will be described. The rest is the same as in the first embodiment described above, and only the difference will be described.

(Outline of tension detection mechanism)
In the present embodiment, a configuration having two tension detection units will be described. FIG. 8 is an explanatory diagram of a tension detection mechanism according to the second embodiment.

  The tension detection mechanism 50 of the intermediate transfer belt 11 includes a first detection unit 51A that detects the tension of the intermediate transfer belt 11 in part A in FIG. 8 and a second detection unit 51B that detects the tension of the intermediate transfer belt 11 in part B. Consists of. A driven roller 14 is disposed between the portion B and the secondary transfer roller. The first detection unit 51A and the second detection unit 51B have the same configuration. With reference to FIG. 8, how the tension detection mechanism operates in accordance with a change in the tension state (tension) of the intermediate transfer belt 11 will be described.

  FIG. 8A shows a state where the intermediate transfer belt 11 has no tangential force at the secondary transfer nip portion T2 and the tension state of the intermediate transfer belt 11 is neutral. In this state, the distance from the optical distance measuring sensor 55A (first movable member detecting portion) to the tension detecting member 53A and the distance from the optical distance measuring sensor 55B (second movable member detecting portion) to the tension detecting member 53B. Are set to be the same.

  FIG. 8B shows a case where the speed of the secondary transfer roller 20 becomes slow and a tangential force in the direction of the arrow is generated in the secondary transfer nip portion T2 of the intermediate transfer belt 11. In this case, the tension of the A part increases, and on the contrary, the tension of the B part becomes loose. The tension detection roller 52A (first movable member) changes its balance position with the compression spring 54A as the tension of the A portion on the intermediate transfer belt 11 increases, and in the direction of the arrow shown in FIG. Moving. The tension detection member 53A configured integrally with the tension detection roller 52A also moves in the same manner, and the distance between the optical distance measuring sensor 55A and the tension detection member 53A is reduced.

  On the other hand, the tension detection roller 52B (second movable member) changes its balance position with the compression spring 54B as the tension of the portion B on the intermediate transfer belt 11 decreases, and the direction of the arrow shown in FIG. Move to. The tension detection member 53B configured integrally with the tension detection roller 52B also moves in the same manner, and the distance between the optical distance measuring sensor 55B and the tension detection member 53B is increased.

  FIG. 8C shows a case where the speed of the secondary transfer roller 20 is increased and a tangential force in the direction of the arrow is generated in the secondary transfer nip portion T2 of the intermediate transfer belt 11. The tension at part A becomes loose, and the tension at part B increases. The tension detection roller 52A moves in the direction of the arrow shown in FIG. 8B as the balance position with the compression spring 54A changes as the tension of the A portion on the intermediate transfer belt 11 decreases. The tension detection member 53A configured integrally with the tension detection roller 52A also moves in the same manner, and the distance between the optical distance measuring sensor 55A and the tension detection member 53A is increased.

  On the contrary, the tension detection roller 52B moves in the direction of the arrow shown in FIG. 8 (b) as the balance position with the compression spring 54B changes as the tension of the B portion on the intermediate transfer belt 11 increases. The tension detection member 53B integrally formed with the tension detection roller 52B moves in the same manner, so that the distance between the optical distance measuring sensor 55B and the tension detection member 53B is reduced.

  Thus, the tension of the intermediate transfer belt 11 can be detected based on the distance measured by the optical distance measuring sensor 55.

(Control method of belt drive motor 28 and secondary transfer drive motor 29)
A method for controlling the belt drive motor 28 and the secondary transfer drive motor 29 will be described with reference to FIG.

  The output voltage value of the optical distance measuring sensor 55A obtained according to the tension of the intermediate transfer belt 11 is converted by the AD conversion unit 73 of the CPU 71 provided in the controller 70 in the apparatus. Then, a tension AD value -A corresponding to the distance is obtained. Similarly, the output voltage value of the optical distance measuring sensor 55B is converted by the AD conversion unit 73 of the CPU 71 provided in the controller 70 in the apparatus, and a tension AD value -B corresponding to the distance is obtained. The tension AD value -A and the tension AD value -B are compared by the CPU 71 to calculate a tension AD value difference.

  The CPU 71 has a RAM 74 and stores a control target value that is a control target. The control target value is a tension AD value difference in a state where the tensions of the A part and the B part in FIGS. 2 and 8 are substantially equal.

  In the present embodiment, the distance between the optical distance measuring sensor 55A and the tension detecting member 53A and the distance between the optical distance measuring sensor 55B and the tension detecting member 53B are set to be the same. For this reason, the control target value becomes zero. Actually, a predetermined width with respect to zero is allowed.

  As described above, in this embodiment, the tension states of the A part and the B part are detected. Therefore, even if the length of the intermediate transfer belt 11 changes, the tension AD value difference does not change. For this reason, the tension can be detected with higher accuracy. Further, since the tension AD value difference when the tension states of the A part and the B part are substantially equal is substantially zero, the control target value is also substantially zero, and a correction mode for correcting the control target value is not necessary.

[Third Embodiment]
In the present embodiment, a case where a correction mode for determining a control target value is required in a configuration using two optical distance sensors 55 will be described. The rest is the same as the first embodiment and the second embodiment described above, and only the difference will be described.

  When there is an individual difference in the optical distance measuring sensor 55 described in the second embodiment, the output voltage values are different even if the optical distance measuring sensor 55A and the optical distance measuring sensor 55B detect the same distance. Even when there is no individual difference in the optical distance measuring sensor 55, the output voltage value may be different even when the tension state where the tensions of the A part and the B part are substantially equal is neutral. For example, when the distance between the optical distance measuring sensor 55A and the tension detecting member 53A is different from the distance between the optical distance measuring sensor 55B and the tension detecting member 53B, the output voltage values are different.

  In these cases, the tension AD value -A and the tension AD value -B are also different, and the tension AD value difference does not become zero. That is, the control target value cannot be made zero. Therefore, a correction mode in which a tension AD value difference in a state where the tension difference between the A part and the B part is substantially zero is detected and set as a control target value will be described with reference to FIG. FIG. 9 is a diagram illustrating the relationship between the state according to the third embodiment and the tension AD value.

  In FIG. 9, the vertical axis represents the tension AD value difference that is AD converted to an 8-bit digital value. The tension AD value difference was calculated as a value obtained by subtracting the tension AD value-A from the tension AD value-B. Therefore, a negative value on the vertical axis means that the tension in the B part is lower than that in the A part (slack state). On the contrary, a positive value on the vertical axis means that the tension of the B part is higher than the A part (tight state).

  As in the first embodiment, in the state where the transfer material and the toner image are interposed (E state), the tension tension state and the loose state are eliminated during the secondary transfer. That is, the tension AD value when the transfer material and the toner image are present in the secondary transfer nip portion T2 (E state) becomes the control target value.

  As described above, in this embodiment, the transfer material and the toner image are sent to the secondary transfer nip portion T2, and the tension AD value difference in the state where the tension states of the A portion and the B portion are substantially equal is detected, and this is detected. Control target value. Thereby, even when there is an individual difference in the optical distance measuring sensor 55, the control target value can be determined. Further, even when the distance between the optical distance measuring sensor 55A and the tension detecting member 53A and the distance between the optical distance measuring sensor 55B and the tension detecting member 53B are different in a neutral state, the control target value is set. Can be determined.

[Fourth Embodiment]
In the present embodiment, another embodiment of the configuration of the tension detection mechanism 50 will be described. The rest is the same as in the first embodiment described above, and only the difference will be described.

(Outline of tension detection mechanism)
FIG. 10 is an explanatory diagram of a tension detection mechanism according to the fourth embodiment. In this embodiment, the tension detection mechanism 50 has a connecting member 83 having two rollers 81 and 82 on the upper and lower sides. In addition, a tension detecting member 85 that is movable around the rotation center 84 according to the movement of the connecting member 83 is provided. In addition, the optical distance measuring sensor 55 using infrared rays for detecting the position of the tension detecting member 85 is provided.

  In the rollers 81 and 82 connected by the connecting member 83, the rollers 81 contact the A portion of the intermediate transfer belt 11 and the rollers 82 contact the B portion of the intermediate transfer belt 11. The rollers 81 and 82 are arranged in contact with the back side of the intermediate transfer belt 11, and the positions thereof are determined by the balance with the tension.

  FIG. 10A shows a balanced state of the tension of the intermediate transfer belt 11 and the connecting member 83 in a state where there is no tangential force at the secondary transfer nip portion T <b> 2 of the intermediate transfer belt 11.

  When the speed of the secondary transfer roller 20 becomes slow and a tangential force in the direction of the arrow in FIG. 10B is generated in the secondary transfer nip portion T2 of the intermediate transfer belt 11, it is the downstream portion of the secondary transfer roller 20. The tension in FIG. 10A is increased. On the other hand, the tension in the portion of FIG. 10B that is the upstream portion of the secondary transfer roller 20 is lowered.

  For this reason, the connecting member 83 is pushed down as shown in FIG. 10B due to the balance with the tension of the intermediate transfer belt 11. Then, in synchronization with the movement of the connecting member 83, the tension detecting member 85 is displaced, and the displacement amount can be detected using the optical distance measuring sensor 55.

  When the speed of the secondary transfer roller 20 is increased and a tangential force in the direction of the arrow in FIG. 10C is generated in the secondary transfer nip portion T2 of the intermediate transfer belt 11, it is the downstream portion of the secondary transfer roller 20. The tension in FIG. 10A becomes lower. On the other hand, the tension in the part shown in FIG. 10B, which is the upstream part of the secondary transfer roller 20, increases.

  Therefore, the connecting member 83 is pushed up as shown in FIG. 10C from the balance with the tension of the intermediate transfer belt 11. Then, in synchronization with the movement of the connecting member 83, the tension detecting member 85 is displaced, and the displacement amount can be detected using the optical distance measuring sensor 55.

  The tension detection mechanism 50 of the present embodiment performs tension detection at the position of a tension detection member 85 determined by the balance of tensions at two locations on the intermediate transfer belt 11. That is, tension detection is performed only by the tension of the intermediate transfer belt 11 without using a spring or the like. For this reason, detection with higher accuracy and higher responsiveness is possible.

  Also in the present embodiment, the detection result of the optical distance measuring sensor 55 corresponding to the position of the tension detection member 85 is fed back to the secondary transfer drive motor 29 as described in the first embodiment. At this time, in the state where the transfer material and the toner image are present in the secondary transfer nip portion T2, the tension AD value detected by the balance state of the tension of the intermediate transfer belt 11 and the connecting member 83 is set as the control target value.

  As described above, in the present embodiment, the tension can be detected by one optical distance measuring sensor 55 by the link mechanism as described above. Further, the tension is detected only by the tension of the intermediate transfer belt 11. This makes it possible to detect tension with higher accuracy and higher responsiveness.

[Fifth Embodiment]
In the present embodiment, another embodiment of the correction mode for determining the control target value will be described. The rest is the same as any of the above-described embodiments, and only the difference will be described.

  Depending on the nip pressure by the secondary transfer roller 20 and the material of the intermediate transfer belt 11, the tension difference between the A part and the B part generated during the pre-rotation causes the transfer material and the toner image to intervene in the secondary transfer nip part T2. Even if it is in a state, it may not be resolved. That is, the control target value cannot be made substantially zero. Even in such a case, a correction mode in which a tension AD value in which the tension difference between the A part and the B part is substantially zero is detected and set as a control target value will be described with reference to FIGS. FIG. 11 is a diagram illustrating the relationship between the state according to the fifth embodiment and the tension AD value.

  State S in FIG. 11 shows a case where the conveyance speed of the intermediate transfer belt 11 by the secondary transfer drive motor 29 is relatively faster than the conveyance speed of the intermediate transfer belt 11 by the belt drive motor 28. In this case, the tension AD value changes on the plus side.

  On the other hand, the state U in FIG. 11 shows a case where the conveyance speed of the intermediate transfer belt 11 by the secondary transfer drive motor 29 is relatively slower than the conveyance speed of the intermediate transfer belt 11 by the belt drive motor 28. In this case, the tension AD value changes on the minus side.

  Although the state S and the state U in FIG. 11 are positive and negative and have different values, the tension AD value indicates the same transition state. That is, during the pre-rotation (D state), the tangential force at the secondary transfer nip portion T2 increases. At the timing when the transfer material and the toner image enter the secondary transfer nip portion T2 (the start point of the E state), the tangential force rapidly decreases and gradually disappears. And it will be in a steady state, without tension AD value becoming substantially zero. The tangential force increases again in the post-rotation (F state).

  On the other hand, a state T in FIG. 11 shows a case where the conveyance speed of the intermediate transfer belt 11 by the secondary transfer drive motor 29 and the conveyance speed of the intermediate transfer belt 11 by the belt drive motor 28 are substantially equal. In this case, the tension AD value changes while maintaining substantially zero throughout the image forming operation. That is, the tangential force at the secondary transfer nip T2 does not increase during the pre-rotation (D state) or the post-rotation (F state).

  That is, consider a case where the conveyance speed of the intermediate transfer belt 11 by the secondary transfer drive motor 29 is changed with respect to the conveyance speed of the intermediate transfer belt 11 by the belt drive motor 28. Then, there is a condition in which the tension AD value is substantially equal during the pre-rotation and in a state where the transfer material and the toner image are present in the secondary transfer nip T2. The tension AD value when this becomes equal becomes the control target value.

  FIG. 12 is a relationship diagram between the secondary transfer driving motor and the tension AD value according to the fifth embodiment. Specifically, it is a graph in which the tension AD value immediately before the start of the E state and the tension AD value in the latter half of the E state are plotted according to the secondary transfer drive motor 29 rotation speed setting. Here, immediately before the start of the E state is immediately before the transfer material and the toner image enter the secondary transfer nip T2, and in the latter half of the E state, the toner image is interposed in the secondary transfer nip T2. This is when the steady state is reached.

  In FIG. 12, the increase / decrease rate with respect to the rotational speed of the secondary transfer drive motor 29 at which the intermediate transfer belt 11 and the secondary transfer roller 20 are set at a constant speed is displayed in%. As shown in FIG. 12, the tension AD value immediately before the transfer material and the toner image enter the secondary transfer nip T2 shows a change approximated by a multi-order equation. The tension AD value when the toner image enters the secondary transfer nip portion T2 and is in a substantially steady state shows a change approximated by a primary expression. This graph is based on experimentally obtained data.

  The two approximate equations intersect at one point. This intersection means that the tension AD value does not change even when the transfer material and the toner image are interposed in the secondary transfer nip T2 (E state) from the previous rotation (D state). As described above, the control target value is determined from the correlation between the detection result of the tension detection mechanism 50 and the speed of the secondary transfer roller 20 driven by the secondary transfer drive motor 29. That is, the above-described tension AD value at the intersection becomes the control target value.

  As described above, in the present embodiment, the tension AD value that is the intersection of the above two approximate equations is calculated and used as the control target value. As a result, the control target value is determined even when the tension state difference between the A part and the B part generated during the pre-rotation cannot be resolved in a state where the transfer material and the toner image are present in the secondary transfer nip T2. be able to.

  In the present embodiment, the tension AD value in which the tension difference between the A part and the B part is substantially zero has been described. However, when the two optical distance measuring sensors 55 are configured as in the second and third embodiments, the same effect can be obtained by detecting the tension AD value difference instead of the tension AD value. Is obtained.

[Sixth Embodiment]
In the present embodiment, another embodiment of the correction mode for determining the control target value will be described. The rest is the same as any one of the first to fifth embodiments described above, and only the difference will be described.

  In the present embodiment, only the toner image is interposed in the secondary transfer portion at the E portion in FIGS. 6, 9, and 11. The toner image is set to have a density, width, and length so that the difference in tension between the A portion and the B portion is sufficiently small while the toner image is interposed in the secondary transfer nip portion T2 (E state). The In this embodiment, as a toner image used in this correction mode, a halftone image of 50% black is formed with an LTR size in the width direction and 30 mm in the length direction.

  When the density of the toner image is low or the width is short, the lubricating effect by the toner becomes insufficient, and the looseness of the A part and the tension of the B part are not sufficiently eliminated during the secondary transfer. As a result, the tension detection difference detected during the pre-rotation (D state) and the state where the transfer material and the toner image are present in the secondary transfer nip T2 (E state) is not clear, and an appropriate control target value is obtained. I can't get it. Similarly, when the length of the toner image is short, the toner image passes through the secondary transfer nip portion T2 before the looseness of the A portion and the tension of the B portion are sufficiently eliminated during the secondary transfer. Will pass. As a result, the tension detection difference detected during the pre-rotation (D state) and the state where the transfer material and the toner image are present in the secondary transfer nip T2 (E state) is not clear, and an appropriate control target value is obtained. I can't get it.

  As described above, in this embodiment, only the toner image is sent to the secondary transfer nip portion T2, and the tension value at this time is detected and set as the control target value. As a result, the control target value can be determined without consuming the transfer material. This is more preferable for the user.

T1 ... primary transfer unit 2 ... photosensitive drum 11 ... intermediate transfer belt 12 ... belt drive roller 20 ... secondary transfer roller 50 ... tension detection mechanism 71 ... CPU
75 ... Secondary transfer drive motor controller 77 ... Intermediate transfer belt drive motor controller

Claims (9)

An image carrier;
An image forming unit for forming an image on the image carrier;
An image bearing belt;
A primary transfer portion for transferring a toner image on the image carrier to the image carrier belt;
A first driving member that doubles as a tension roller of the image bearing belt and drives the image bearing belt;
A second drive member in contact with the image bearing belt and having a conveying force;
A tension detection mechanism for detecting the tension state of the image bearing belt;
A control unit,
The control unit determines a driving speed of the second driving member by using, as a control target value, a detection result of the tension detection mechanism in a state where the tension state of the image bearing belt is substantially equal over the entire circumference. An image forming apparatus.   The image forming apparatus according to claim 1, wherein the second driving member is in contact with an outer peripheral surface of the image bearing belt. The tension detection mechanism is
A movable member that contacts the image bearing belt and moves in accordance with a tension state of the image bearing belt;
The image forming apparatus according to claim 1, further comprising a movable member detection unit configured to detect a position of the movable member. The tension detection mechanism is
4. The tension state of the image carrier belt on the upstream side and the downstream side of the second drive member in the moving direction of the image carrier belt is detected. 5. The image forming apparatus described. The tension detection mechanism is
A first movable member that contacts the image carrier belt upstream of the second drive member and moves in accordance with the tension state of the image carrier belt;
A second movable member that contacts the image carrier belt downstream of the second drive member and moves in accordance with the tension state of the image carrier belt;
A first movable member detector for detecting the position of the first movable member;
A second movable member detector for detecting the position of the second movable member;
5. The difference in the tension state of the image carrying belt upstream and downstream of the second drive member in the moving direction of the image carrying belt is detected. 6. The image forming apparatus described in the item. The tension detection mechanism is
A movable member that contacts the image carrier belt on each of the upstream side and the downstream side of the second drive member in the moving direction of the image carrier belt and moves integrally with the tension state of the image carrier belt;
A movable member detector for detecting the position of the movable member;
5. The difference in the tension state of the image carrying belt on the upstream side and the downstream side of the second drive member in the moving direction of the image carrying belt is detected. 6. The image forming apparatus described in 1. The control unit
When there is a speed difference between the first driving member and the second driving member, at least one of a transfer material or a toner image is interposed in a contact portion between the image carrying belt and the second driving member. The image forming apparatus according to claim 1, wherein a correction mode in which a detection result of the tension detection mechanism is a control target value is performed.   The correction mode for determining the control target value based on a correlation between a detection result of the tension detection mechanism and a speed of the second driving member, according to any one of claims 1 to 6. The image forming apparatus described in the item.   8. The correction mode, wherein the secondary transfer bias is set to be smaller than that during normal image formation in a state where at least one of a transfer material and a toner image is present in the secondary transfer nip portion. Item 9. The image forming apparatus according to Item 8.

Images