JP5477695B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile machine that employs an image forming method such as an electrophotographic method, an electrostatic recording method, an ionography method, and a magnetic recording method. More specifically, the present invention relates to an image forming apparatus that obtains a final image by transferring a toner image on a toner image carrying belt such as an intermediate transfer belt to a sheet-like member.

  In recent years, in a color image forming apparatus, a toner image on a photosensitive drum is primarily transferred onto an intermediate transfer belt, which is a toner image carrying belt, in a primary transfer portion, and four color toner images on the intermediate transfer belt are secondary-transferred. An intermediate transfer system that performs secondary transfer to a sheet-like member at the transfer nip is often employed. This intermediate transfer belt type image forming apparatus has the advantage that it can be used with various types of sheet-like members such as thin paper, thick paper, postcards and envelopes, and is highly versatile. However, when a single-sheet member having a thickness of a certain level or more enters the secondary transfer nip, the speed of the intermediate transfer belt that has been driven at a constant speed fluctuates for a short time, and the primary transfer is performed. There was a problem that the image was disturbed in the part.

  More specifically, as shown in FIG. 20, the leading end of the sheet-like member S enters the secondary transfer nip, so that the secondary transfer nip is expanded by the thickness of the sheet-like member S. When the secondary transfer nip is expanded by the thickness of the sheet-like member S, a load is suddenly applied to the driving of the secondary transfer counter roller 116, and the rotation of the secondary transfer counter roller 116 is temporarily delayed. As a result, the belt amount fed from the driving roller 114 around which the upstream end of the secondary transfer nip upstream stretch surface is wound is larger than the belt amount fed by the secondary transfer counter roller 116. As a result, the stretch surface on the upstream side of the secondary transfer nip becomes loose. On the other hand, the amount of belt sent out by the driven roller 115 by the driving force of the driving roller 114 is larger than the amount of belt sent by the secondary transfer counter roller 116. As a result, the stretched surface between the secondary transfer counter roller 116 and the driven roller 115 of the intermediate transfer belt 110 (hereinafter referred to as the secondary transfer nip downstream stretched surface) is pulled to the opposite side of the belt moving direction. The tension on the secondary transfer nip downstream tension surface increases. Then, the driven roller 115 is pulled and decelerated in the direction opposite to the belt moving direction, and an area between the driven roller 115 and the driving roller 114 of the intermediate transfer belt (hereinafter referred to as a primary transfer area) is opposite to the belt moving direction. And the speed of the intermediate transfer belt in the primary transfer nip is reduced. Therefore, when the image is transferred from the photosensitive drum 104 to the intermediate transfer belt 110 at the primary transfer portion while this deceleration occurs, the image is disturbed due to the speed fluctuation.

  Such a defect assumes the speed fluctuation waveform of the intermediate transfer belt that occurs at the time of entry before the sheet-like member enters the secondary transfer nip, and controls the drive roller drive so as to cancel the assumed speed fluctuation waveform. It is known to avoid by feedforward control.

  However, the speed fluctuation waveform of the intermediate transfer belt differs depending on changes in the waist of the sheet-like member, the secondary transfer pressure, the secondary transfer roller hardness, and the like. For this reason, in the case of the feedforward control, the assumed speed fluctuation waveform to be canceled differs from the actual speed fluctuation waveform, and there is a possibility that the belt speed fluctuation is deteriorated.

  Such a problem can also be avoided by suppressing the stretching surface on the downstream side of the secondary transfer nip of the intermediate transfer belt from being pulled to the side opposite to the belt moving direction. It has been known.

  In Patent Document 1, when the sheet-like member enters the secondary transfer nip, a tension is applied to the downstream side of the secondary transfer nip of the intermediate transfer belt that becomes tensioned by a spring from the outside so as to bend the belt inward. An image forming apparatus is described in which a tension roller for biasing is provided, and the tension roller adjusts the tension of the stretched surface on the downstream side of the secondary transfer nip of the intermediate transfer belt. Specifically, when the tension surface of the secondary transfer nip downstream tension surface of the intermediate transfer belt is pulled to the opposite side of the belt movement direction and the tension of the secondary transfer nip downstream tension surface increases, the tension roller is displaced outward. Thus, the tension on the stretched surface on the downstream side of the secondary transfer nip is reduced.

  However, in the image forming apparatus of Patent Document 1, when the sheet-like member enters the secondary transfer nip, the intermediate transfer belt is stretched by a predetermined tension on the stretched surface on the downstream side of the secondary transfer nip. When the sheet-like member enters the secondary transfer nip, a load is suddenly applied to the driving of the secondary transfer counter roller, the rotation temporarily slows down, and the tension on the stretched surface on the downstream side of the secondary transfer nip increases. After that, the tension roller is displaced and the tension on the stretching surface downstream of the secondary transfer nip is weakened. Therefore, immediately after the sheet-like member enters the secondary transfer nip, a force is applied to pull the downstream transfer nip downstream stretched surface opposite to the belt moving direction, and the tension is applied to the secondary transfer nip downstream stretch. It reaches the tension surface on the downstream side of the surface. As a result, it was not possible to sufficiently suppress disturbance in the image at the primary transfer portion or the like. Further, in the image forming apparatus described in Patent Document 1, in order to reduce the size of the apparatus, the intermediate transfer belt is stretched by being bent inward by 90 ° or more on the stretching surface on the downstream side of the secondary transfer nip. For this reason, local stress is generated at a position where the intermediate transfer belt is bent inward by 90 ° or more, and there is a problem that it is difficult to obtain durability of the intermediate transfer belt due to this local stress.

  The present applicant has proposed an image forming apparatus of Japanese Patent Application No. 2008-221817 in order to suppress the speed fluctuation of the primary transfer portion of the intermediate transfer belt. This image forming apparatus is provided with interlocking means that interlocks when the sheet member enters and exits between the intermediate transfer belt and the secondary transfer roller, and the interlocking means contacts the intermediate transfer belt in the vicinity of the secondary transfer roller. It has a contact member, and operates the contact member in a direction to change the tension of the intermediate transfer belt in conjunction with the entry and exit of the sheet member between the intermediate transfer belt and the secondary transfer roller. It is. Specifically, interlocking means that integrally includes a tension roller as an abutting member that abuts against the intermediate transfer belt in the vicinity of the secondary transfer roller is provided on a support that supports the secondary transfer roller. The interlocking means is displaced in a direction in which the tension roller weakens the tension on the stretch surface on the downstream side of the secondary transfer nip in conjunction with the movement of the secondary transfer roller being pushed down. When the sheet-like member starts to bite into the secondary transfer nip, the secondary transfer roller starts to be pushed down, so that an increase in tension when the sheet-like member enters the secondary transfer nip can be suppressed by the interlocking means.

  In this configuration, the amount that the tension roller can be displaced in order to suppress an increase in tension is limited by the positional relationship between the support of the secondary transfer roller and the tension roller. On the other hand, since the interlocking unit is provided with a tension roller that is in contact with the intermediate transfer belt in the vicinity of the secondary transfer roller and is integrally provided with the support, the degree of freedom in design is small with respect to the positional relationship. For this reason, there is a case where a sufficient effect of suppressing an increase in tension cannot be obtained. In addition, since the tension roller is integrally provided on the support that supports the secondary transfer roller, the transfer pressure of the secondary transfer roller may decrease due to the reaction force from the tension roller. It is necessary to design a large spring pressure for the transfer roller. For this reason, the deflection in the axial direction of the secondary transfer unit may be a problem.

  In addition, in Japanese Patent Application No. 2009-041361, the present applicant controls an intrusion detecting means for detecting that a sheet-like member has entered the secondary transfer nip upstream stretch surface and the tension of the transfer nip downstream stretch surface. And a tension control means for controlling the tension of the stretched surface on the downstream side of the transfer nip when the sheet-like member transfer nip enters, using the rush detection signal output from the rush detection means as a trigger, and the sheet-like member enters the transfer nip. An image forming apparatus in which tension control means is configured so as to perform control to weaken the tension before the start is proposed. In this image forming apparatus, it is detected that the sheet-like member has entered the stretch surface on the upstream side of the transfer nip, and using this as a trigger, the tension control means causes the stretch surface on the downstream side of the transfer nip when the sheet-like member enters the transfer nip. Is controlled to be less than the tension before the sheet-like member enters the transfer nip. As a result, when the sheet-like member enters the transfer nip, the transfer nip downstream stretch surface is loosened, and the transfer nip downstream stretch surface is pulled to the opposite side of the belt movement direction by the sheet-like member entry. However, the slackened belt simply returns to its original stretched state. Therefore, it is possible to suppress the speed fluctuation of the belt stretching surface downstream of the transfer nip downstream stretching surface when the sheet-like member enters. However, in this image forming apparatus, a rush detection means for detecting that a sheet-like member has rushed into the upstream surface of the transfer nip is required. The entry detection means includes a rotary encoder serving as rotation information acquisition means for the entrance roller provided so as to be in contact with the inner surface of the transfer nip upstream side stretched surface, a memory that stores in advance the linear velocity of the entrance roller before entry, and the rotary encoder Is obtained, and the linear velocity of the entrance roller is calculated, and the linear velocity of the entrance roller stored in the memory in advance is compared with the calculated linear velocity. And a microprocessor having a function of outputting to the drive control unit of the means. Providing the intrusion detecting means having such a configuration increases the cost. Further, erroneous detection may occur depending on the setting of the threshold value.

  In the above, an intermediate transfer type image forming apparatus in which the toner image carrying belt is an intermediate transfer belt has been described. However, when transferring a toner image on the intermediate transfer belt to a sheet-like member for image quality improvement, image transfer is performed. Even in a transfer simultaneous fixing type image forming apparatus that simultaneously performs image fixing and image fixing, the same problem may occur in the transfer section. Also in this case, when a sheet-like member having a thickness of a certain level or more enters the transfer fixing unit, the speed of the intermediate transfer belt that has been driven at a constant speed is reduced for a short time until the primary transfer unit etc. This can cause a problem that the image is disturbed.

  Even in the direct transfer type image forming apparatus in which the toner image carrying belt is a photosensitive belt and the toner image on the photosensitive belt is directly transferred to the sheet-like member at the transfer portion, the same problem occurs in the transfer portion. Can occur. In this case, when a sheet-like member having a thickness of a certain degree or more enters the transfer portion, the speed fluctuation of the photosensitive belt that has been driven at a constant speed until then occurs. Due to the speed fluctuation of the photosensitive belt, a deviation occurs between the exposure light irradiation timing for forming the latent image and the timing when the surface of the photosensitive belt passes the exposure position, and the positional deviation of the latent image formed on the photosensitive belt is shifted. A malfunction that occurs may occur.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a low-cost image forming apparatus that suppresses fluctuations in the speed of the toner image carrying belt caused by the entry of a sheet-like member into the transfer nip. That is.

In order to achieve the above object, a first aspect of the present invention provides a toner image carrying belt that is stretched by a plurality of stretching rollers and carries a toner image on its surface and moves endlessly, and among the plurality of stretching rollers. in the image forming apparatus having a transfer roller which one and facing each other across the toner image bearing belt, to form a transfer nip for transferring the toner image on the toner image bearing belt in a sheet-like member, the displacement of the transfer roller Transfer roller displacement information measuring means for measuring information and a moving direction of the toner image carrying belt among a plurality of stretching surfaces formed by the toner image carrying belt being stretched by the plurality of stretching rollers and tension control means upstream end to control the tension of the toner image carrying belt of the transfer nip downstream a stretched surface serving as the transfer nip, the transfer roller displacement information measurement hand A drive command means for driving the tension control means based on the displacement information measured provided by the tension control means, with so as to bend with respect to the transfer nip downstream stretched surface of the toner image bearing belt An urging roller for urging, a bearing portion for rotatably supporting the shaft of the urging roller, a displacement means for moving the bearing portion in a direction orthogonal to the stretching surface on the downstream side of the transfer nip, An urging roller displacement information measuring means for measuring the displacement information of the urging roller, wherein the drive command means is configured to generate a target value based on the displacement information of the transfer roller measured by the transfer roller displacement information measuring means. Te, by feeding back the displacement information of the biasing roller which is measured by the urging roller displacement information measuring means, is characterized in operating the displacement means of the tension control means
Also, the invention of claim 2, the image forming apparatus according to claim 1, the second biasing roller for urging so as to bend with respect to the transfer nip downstream stretched surface of the toner image bearing belt It is characterized by providing .
Also, the invention of claim 3, the image forming apparatus according to claim 1 or 2, wherein provided in the transfer nip vicinity, a heating means for heating the toner image bearing belt or sheet-like member, the transfer nip The toner image on the toner image carrying belt is transferred to a sheet-like member and fixed at the same time.

  In the present invention, the tension control means for controlling the tension of the toner image carrying belt downstream of the transfer nip is operated so as to control the tension of the toner image carrying belt in conjunction with the displacement of the transfer roller. Since the displacement of the transfer roller starts when the sheet-like member enters the transfer nip, the tension control means controls the tension in conjunction with the displacement of the transfer roller, thereby matching the timing of entry of the sheet-like member into the transfer nip. Thus, fluctuations in tension of the toner image carrying belt can be suppressed. This tension control means can be operated in conjunction with the displacement of the transfer roller, unlike a tension roller that is provided integrally with a conventional transfer roller or a transfer roller support and contacts the toner image carrying belt in the vicinity of the transfer roller. As long as it is a thing, the structure which interlock | cooperates electrically is not restricted to what is interlock | cooperated mechanically. For this reason, there is a degree of design freedom in the arrangement of the tension control means, and the tension control means can be arranged at a position where the fluctuation in the tension of the toner image carrying belt can be satisfactorily suppressed. Therefore, the fluctuation in the tension of the toner image carrying belt on the downstream side of the transfer nip due to the entry of the sheet-like member is satisfactorily suppressed, and the fluctuation in the speed of the primary transfer portion of the toner image carrying belt due to the fluctuation in tension can be suppressed. In addition, by operating the tension control means in conjunction with the displacement of the transfer roller accompanying the entry into the transfer nip, there is no need to provide a separate entry detection means. As described above, the rush detection means is costly because it is composed of the toner carrying belt or the tension roller speed information acquisition means, the memory, the microprocessor, and the like. On the other hand, since the displacement of the transfer roller can be detected with a simple configuration using only a displacement amount sensor, for example, speed fluctuation of the toner image carrying belt can be suppressed at low cost.

  According to the present invention, there is an excellent effect that it is possible to provide a low-cost image forming apparatus that suppresses fluctuations in the speed of the toner image carrying belt caused by the sheet-like member entering the transfer nip.

1 is a schematic configuration diagram of an image forming apparatus according to an embodiment. The figure which shows the speed fluctuation of a drive roller and a driven roller. FIG. 3 is an explanatory diagram schematically showing a transfer paper on which a toner image in which positional deviation has occurred is transferred. (A) The figure which showed transmission of the force when a sheet | seat entered the prenip part. (B) is an enlarged explanatory view of the vicinity of the secondary transfer nip when the paper enters the prenip portion, and (c) is a graph for explaining the speed fluctuation when the paper enters the prenip portion. (A) The figure which showed transmission of the force when a sheet | seat penetrates into this nip part. FIG. 6B is an enlarged explanatory view of the vicinity of the secondary transfer nip when the sheet enters the main nip portion. (C) is a graph for explaining the speed fluctuation when the paper enters the main nip portion. Explanatory drawing of the mechanism in which both a drive roller and an entrance roller decelerate by paper sheet nip entry. FIG. 6A is a diagram illustrating transmission of force when the pre-nip region of the intermediate transfer belt returns. (B) is an enlarged explanatory view of the vicinity of the secondary transfer nip when the pre-nip region of the intermediate transfer belt returns. (C) is a graph for explaining the speed fluctuation caused when the pre-nip region of the intermediate transfer belt returns. Data investigating the relationship between the speed fluctuation of the driven roller and the driving roller and the displacement of the tension roller and the secondary transfer roller. FIG. 3 is an explanatory diagram of tension roller displacement control according to the first embodiment. 1 is a schematic configuration diagram of an image forming apparatus according to Embodiment 1. FIG. FIG. 3 is a control block diagram of the image forming apparatus according to the first exemplary embodiment. It is a schematic block diagram of the tension | tensile_strength control means of Example 1, (a) is when a drive motor is OFF, (b) is when a drive motor is ON. FIG. 3 is a schematic configuration diagram of an image forming apparatus according to a second embodiment. FIG. 4 is a schematic configuration diagram of an image forming apparatus according to a third embodiment. FIG. 9 is a control block diagram in the image forming apparatus according to the third embodiment. FIG. 10 is a block diagram for performing feedback control based on displacement information of the tension roller according to the third embodiment. FIG. 6 is a schematic configuration diagram of an image forming apparatus according to a fourth embodiment. FIG. 10 is a schematic configuration diagram illustrating another example of the image forming apparatus according to the fifth exemplary embodiment. The figure which shows the result of a verification test. FIG. 4 is a schematic diagram illustrating a state in which a tension state of an intermediate transfer belt is changed when a sheet-like member enters a secondary transfer nip.

Hereinafter, an embodiment of an image forming apparatus to which the present invention is applied will be described.
FIG. 1 is a main part configuration diagram showing the image forming apparatus. In this figure, the image forming apparatus includes four process units 100Y, 100, M, and K for forming yellow (Y), magenta (M), cyan (C), and black (K) toner images. ing. In addition, a sheet conveyance path including a plurality of guide plates for conveying a recording sheet in the apparatus, a registration roller pair 9, a fixing device 15, an optical writing unit (not shown), a transfer unit 50, and the like are also provided.

  This image forming apparatus has a so-called tandem configuration in which four process units 100Y, 100C, 100M, and 100K are arranged along an endless movement direction with respect to an intermediate transfer belt 2 described later. The process units 100Y, 100C, 100M, and 100K for each color have a common support using the photoreceptors 1Y, 1C, 1M, and 1K, which are latent image carriers, and various devices disposed around the photoreceptors as one unit. It can be attached to and detached from the printer unit main body. The configuration is the same except that the colors of the toners used are different.

  Taking the Y process unit 100Y as an example, a charging device 103Y, a developing device 101Y, a cleaning device 102Y, and the like are provided around the photoreceptor 1Y as a toner image carrier. The photosensitive member 1Y of the process unit 100Y is driven to rotate counterclockwise in the drawing by a driving unit (not shown). The charging device 103Y uniformly charges the peripheral surface of the rotationally driven photoreceptor 1Y to the same polarity as the toner charging polarity. An optical writing unit (not shown) drives a laser diode based on the image information, and irradiates the rotating charged photoconductor 1Y while deflecting the laser light in the direction of the rotation axis. Perform the scanning process. Thereby, an electrostatic latent image based on the Y image information is formed on the photoreceptor 1Y. As the photoreceptor 1 </ b> Y, a drum-like member is used in which a photosensitive layer is formed by applying a photosensitive organic photosensitive material to a base tube made of aluminum or the like. However, an endless belt may be used.

  The developing device 101Y develops the electrostatic latent image on the photoreceptor 1Y using a two-component developer (hereinafter simply referred to as developer) containing a magnetic carrier (not shown) and non-magnetic Y toner. Instead of the two-component developer, a type that develops with a one-component developer not containing a magnetic carrier may be used.

  The Y toner image formed on the photoreceptor 1Y by development is transferred to the front surface of the intermediate transfer belt 2 at a Y primary transfer nip described later. The transfer residual toner adhering to the photoreceptor 1Y after the Y toner image is transferred in this way is removed from the surface of the photoreceptor 1Y by the drum cleaning device 102Y. Prior to this cleaning, the surface of the photoreceptor 1 </ b> Y is discharged by receiving light from a discharge lamp (not shown).

  Although the Y process unit 100Y has been described, M, C, and K toner images are similarly formed on the surfaces of the photoreceptors 1M, C, and K in the M, C, and K process units.

  A transfer unit 50 is disposed below the four process units 100Y, 100C, 100M, and 100K. The transfer unit 50 has an intermediate transfer belt 2 as a toner image carrying belt. The intermediate transfer belt 2 is stretched by stretching rollers such as a driving roller 3, a secondary transfer counter roller 5, a tension roller 6, a driven roller 7, and an entrance roller 4. The intermediate transfer belt 2 is moved endlessly in the clockwise direction in the figure by the rotational driving of the driving roller 3 while being in contact with the photoreceptors 1Y, 1C, 1M, and 1K. As a result, primary transfer nips for Y, C, M, and K where the photoreceptors 1Y, 1C, 1M, and 1K contact the intermediate transfer belt 2 are formed.

  In the vicinity of the primary transfer nips for Y, C, M, and K, the intermediate transfer belt 2 is transferred to the photoreceptors 1Y, C, M, and K by primary transfer rollers 104Y, 104, M, and K disposed inside the belt loop. It is pushing toward. A primary transfer bias is applied to these primary transfer rollers 104Y, 104C, 104M, and 104K by a power source (not shown). As a result, a primary transfer electric field for electrostatically moving the toner images on the photoreceptors 1Y, 1C, 1M, and 1K toward the intermediate transfer belt 2 is formed in the primary transfer nips for Y, C, M, and K. Yes.

  In the drawing, a toner image is sequentially applied to each of the primary transfer nips on the front surface of the intermediate transfer belt 2 that sequentially passes through the primary transfer nips for Y, C, M, and K with endless movement in the clockwise direction. Overlaid and primary transferred. By this primary transfer of superposition, a four-color superposed toner image (hereinafter referred to as a four-color toner image) is formed on the front surface of the intermediate transfer belt 2.

  A secondary transfer roller 8 serving as an abutting member is disposed below the intermediate transfer belt 2 in the drawing, and a portion of the intermediate transfer belt 2 that is wound around the secondary transfer counter roller 5 is contacted from the belt front surface. A secondary transfer nip is formed in contact therewith. Thus, a secondary transfer nip is formed in which the front surface of the intermediate transfer belt 2 and the secondary transfer roller 8 are in contact with each other. The shaft center of the secondary transfer roller 8 is located upstream of the shaft center of the secondary transfer counter roller 5 in the paper transport direction. The secondary transfer roller 8 may be driven to rotate counterclockwise in the drawing by a motor (not shown), or may be configured to rotate around the intermediate transfer belt 2.

  A secondary transfer bias having the same polarity as the toner is applied to the secondary transfer counter roller 5 in the belt loop by a power source (not shown). On the other hand, the secondary transfer roller 8 outside the belt loop is grounded. Thereby, a secondary transfer electric field is formed in the secondary transfer nip.

  A registration roller pair 9 is disposed on the right side of the secondary transfer nip in the drawing. In parallel with image formation, the paper feed roller 18 is rotated to feed out the paper 19 as a sheet-like member from the paper feed cassette 20, separated one by one by the separation roller 17, and fed into the paper conveyance path, and conveyed by the conveyance roller 16. Then, it stops against the registration roller 9. Alternatively, the paper on the manual feed portion is fed out, put into the paper conveyance path, and abutted against the registration roller 9 and stopped.

  The sheet 19 sandwiched between the registration rollers is sent to the secondary transfer nip at a timing that can be synchronized with the four-color toner image on the intermediate transfer belt 2. In the secondary transfer nip, the four-color toner images on the intermediate transfer belt 2 are collectively transferred to a sheet as a sheet-like member due to the influence of the secondary transfer electric field and the nip pressure, and become a full color image combined with the white color of the sheet.

  On the front surface of the intermediate transfer belt 2 that has passed through the secondary transfer nip, untransferred toner that has not been transferred to the paper at the secondary transfer nip adheres. This untransferred toner is cleaned by a belt cleaning device (not shown) that contacts the intermediate transfer belt 2.

  The sheet 19 that has passed through the secondary transfer nip is fed toward the fixing device 15 by the conveyance force between the intermediate transfer belt 2 and the conveyance belt 11. The fixing device 15 is configured by pressing the pressure roller 12 against a fixing belt 13 a stretched between a fixing roller 13 and a heating roller 14. The fixing belt 13a is heated by an IH coil (not shown) in the heating roller 14, and is heated to a temperature necessary for image fixing. On the other hand, the pressure roller 12 also incorporates a heater (not shown) and is used for preheating during standby. The unfixed image on the paper is fixed on the paper 19 by applying heat and pressure at the nip portion between the fixing belt 13 a and the pressure roller 12. The heater of the fixing device 15 does not have to use an IH coil, and may be a system constituted by a pair of heat rollers.

  The fixing device 15 applies heat and pressure to fix the transferred image, and then the paper is discharged to a paper discharge tray (not shown). Alternatively, the image is again guided to the transfer position by a double-side reversing mechanism (not shown), and an image is recorded and fixed on the back surface, and then discharged to a paper discharge tray.

  The secondary transfer roller 8 that is in contact with the intermediate transfer belt 2 to form a secondary transfer nip includes a metal cored bar and an elastic member such as rubber coated on the peripheral surface thereof. In the secondary transfer nip, the portion of the intermediate transfer belt 2 that is wound around the secondary transfer counter roller 5 bites into the elastic member on the surface of the secondary transfer roller 8. Thereby, a wide secondary transfer nip is formed.

  The rotation shaft of the secondary transfer roller 8 is rotatably received by a bearing (not shown) fixed to a swing arm 10 as a holding body. The swing arm 10 is supported so as to swing about the swing shaft 10a, and the rotation shaft of the secondary transfer roller 8 and the secondary transfer counter roller 5 are swung along with the swing arm 10 itself. Change the distance from the rotation axis.

  An urging coil spring 10 b as urging means is fixed to the swing arm 10. By this biasing coil spring 10b, a biasing force in the counterclockwise direction around the swinging shaft 10a is applied to the swinging arm 10 so that the secondary transfer roller 8 is pressed against the intermediate transfer belt 2. .

  A cam contact surface with which the eccentric cam 10c contacts is provided at the end of the swing arm 10 opposite to the swing shaft 10a. When this eccentric cam 10c is rotationally driven by a cam motor (not shown), the swing arm 10 is rotated little by little in the clockwise direction in the drawing so as to push down the swing arm 10 against the biasing force of the bias coil spring 10b. . Then, the distance between the axes of the secondary transfer roller 8 and the secondary transfer counter roller 5 gradually increases. Along with this, the size of the secondary transfer nip gradually decreases, and eventually the secondary transfer roller 8 is separated from the intermediate transfer belt 2.

  As described above, in this image forming apparatus, the secondary transfer roller 8 is moved against the urging force of the urging coil spring 10b, so that the distance between the rotation shaft of the secondary transfer counter roller 5 and the secondary transfer roller 8 is increased. Then, the distance between the axes is adjusted. In this configuration, the eccentric cam 10c, the cam motor, a control unit that drives the cam, and the like function as a distance adjusting unit.

Next, problems of the intermediate transfer belt 2 of the conventional image forming apparatus will be described.
When the sheet 19 enters the secondary transfer nip, a driving roller 3 and a driven roller 7 that stretch a region (hereinafter referred to as a primary transfer region) that contacts the photoreceptors 1Y, C, M, and K of the intermediate transfer belt 2 are Then, the speed fluctuation as shown in FIG. 2 occurs. Such a phenomenon is particularly noticeable when a thick paper is passed (in this case, a continuous weight of 220 [kg] paper, a basis weight of 256 [g / m ² ] paper or more is assumed).

  On the other hand, the photoreceptors 1Y, 1C, 1M, and 1K are driven to rotate at substantially the same speed even when entering the secondary transfer nip of the paper leading edge. A speed difference occurs between them. Due to this speed difference, a transfer position shift occurs in the primary transfer nip.

FIG. 3 is an explanatory diagram schematically showing a sheet 19 on which a toner image having a misalignment is transferred as a final image when a monochrome image is formed.
A toner image including a transfer position shift in the primary transfer nip that occurs when the leading edge of the sheet enters the secondary transfer nip is defined on the sheet 19 at a position L from the leading edge of the sheet, where L is the distance from the primary transfer nip to the secondary transfer nip. Transfer or transfer / fixing. For this reason, as shown in FIG. 3, the final image on the discharged paper 19 is a portion I _{2 that} is lighter in color than the portion I ₁ of the normal image or a portion I _{3 that} is darker than the normal image portion I _1. The image has local density unevenness.
Note that, when images of a plurality of colors are formed, the distances from the primary transfer nip to the secondary transfer nip are different for each color, so that images having different distances L from the leading edge of the paper to the density unevenness are output for each color. .
As described above, in order to prevent the transfer position shift in the primary transfer portion that occurs when the paper enters, it is necessary to suppress the speed fluctuations of the driving roller 3 and the driven roller 7 when the paper enters. The mechanism by which the speed fluctuations of the driving roller 3 and the driven roller 7 occur will be described below.

In the present image forming apparatus, as shown in FIG. 1, the axial center of the secondary transfer roller 8 is located upstream of the axial center of the secondary transfer counter roller 5 in the sheet conveying direction. Therefore, the nip where the outer peripheral surface of the intermediate transfer belt 2 contacts the secondary transfer roller 8 has the following two nip portions. That is, the inner peripheral surface of the intermediate transfer belt 2 is in a secondary transfer nip (hereinafter referred to as a main nip portion) that is in contact with the secondary transfer counter roller 5 and upstream of the secondary transfer nip in the sheet conveying direction. The inner peripheral surface of the intermediate transfer belt 2 has a nip portion that is not in contact with the secondary transfer counter roller 5 (hereinafter referred to as a pre-nip portion).
As described above, since the image forming apparatus according to the present embodiment has the main nip portion and the pre-nip portion, the speed fluctuations of the driving roller 3 and the driven roller 7 are roughly divided into the following three fluctuations. .

(1) Speed fluctuation due to the leading edge of the paper 19 entering the pre-nip portion FIG. 4 is an explanatory diagram of speed fluctuation that occurs when the paper 19 enters the pre-nip section.
When the leading edge of the sheet enters the pre-nip portion, a stretch region (hereinafter referred to as pre-nip region T3) between the secondary transfer counter roller 5 and the entrance roller 4 which is a stretch surface on the upstream side of the transfer nip of the intermediate transfer belt 2 is pushed inward. It is. When the pre-nip region of the intermediate transfer belt 2 is pushed inward, the portion on the upstream side in the intermediate transfer belt moving direction from the contact portion of the pre-nip region T3 with the front end of the sheet moves as shown in FIG. Pull in the direction. As a result, as shown in FIG. 4B, the inlet roller 4 receives a pulling force I2 in the same direction as the belt moving direction, and the inlet roller 4 receives a torque E2 in the same direction as the rotational direction. As a result, the entrance roller 4 is accelerated. Further, as indicated by an arrow F (2) in FIG. 4A, the force I2 pulling in the same direction as the belt moving direction is transmitted to the driving roller 3 via the intermediate transfer belt 2, and is rotated to the driving roller 3 in the rotational direction. As shown by the thick line in FIG. 4C, the driving roller 3 is accelerated. Finally, as shown by an arrow F (2) in FIG. 4A, a force I2 pulling in the same direction as the belt moving direction is transmitted to the driven roller 7 via the intermediate transfer belt 2. As a result, the driven roller 7 is accelerated as indicated by the bold line in FIG.

  Further, the downstream portion of the intermediate transfer belt 2 in the intermediate transfer belt moving direction from the contact portion of the pre-nip region T3 of the intermediate transfer belt 2 with the front end of the sheet is pulled in the direction opposite to the belt moving direction. As a result, as shown in FIG. 4B, the secondary transfer counter roller 5 is subjected to a pulling force I1 in the direction opposite to the belt moving direction, and the secondary transfer counter roller 5 is reverse to the rotation direction. Torque E1 is applied. Thereby, the secondary transfer counter roller 5 is decelerated. Further, the force I1 pulling in the direction opposite to the belt moving direction is transmitted to the tension roller 6 and the driven roller 7 via the intermediate transfer belt 2 as indicated by an arrow F (1) in FIG. Considering the transmission path, the force I2 pulling in the same direction as the belt moving direction is transmitted to the driven roller 7, and before the driven roller 7 is accelerated, the driven roller 7 is decelerated by the force I1 pulling in the direction opposite to the belt moving direction. Should do. However, as shown in FIG. 4C, the driven roller 7 is not decelerated by the force I1 pulling in the direction opposite to the belt moving direction. In this experiment, because the secondary transfer roller 8 was driven by another motor, the force I1 pulling in the direction opposite to the belt moving direction was extinguished by the rotational driving force of the secondary transfer roller 8. It is done.

As shown in FIG. 4B, the primary transfer region T1 of the intermediate transfer belt 2 is accelerated by the driving roller 3 being accelerated by the force I2 that is pulled in the belt moving direction. As a result, as shown in FIG. 3, a portion I ₂ having a lighter color than the portion I ₁ of the normal image is generated.

(2) Speed fluctuation due to the leading edge of the paper 19 entering the main nip part FIG. 5 is an explanatory diagram of the speed fluctuation that occurs when the leading edge of the paper enters the main nip part, which is the secondary transfer nip.
The speed fluctuation due to the leading edge of the paper 19 entering the main nip portion is caused by the secondary transfer counter roller 5 being pushed by the leading edge of the paper 19 pushing down the secondary transfer roller 8 when the leading edge of the paper 19 enters the main nip portion. It is a phenomenon that slows down.
Here, the mechanism by which the secondary transfer counter roller 5 decelerates will be described with reference to FIG.
As shown in FIG. 6, in order for the leading edge of the sheet 19 to enter the nip portion, the leading edge of the sheet 19 needs to push down the secondary transfer roller 8 by an amount corresponding to the thickness of the sheet 19. As described above, the secondary transfer roller 8 is urged in the direction of arrow A in the figure by the urging coil spring 10b. In order to push down the secondary transfer roller 8, the force (arrow B in the figure) that the leading edge of the paper 19 pushes down the secondary transfer roller 8 needs to exceed the urging force of the urging coil spring 10b. The force B that pushes down the leading edge of the sheet 19 is a conveying force that conveys the leading edge of the sheet to the right side in the figure, and the conveying force is the rotational driving force C of the secondary transfer counter roller 5. In other words, when the leading edge of the sheet 19 comes into contact with the secondary transfer roller 8, the secondary transfer counter roller 5 has not only torque for conveying the belt but also the leading edge of the sheet to push down the secondary transfer roller 8. Necessary torque is generated. As a result, as shown in FIG. 5B, the rotational load (arrow D1 in the figure) of the secondary transfer counter roller 5 increases, and the secondary transfer counter roller 5 decelerates. In this experiment, the secondary transfer roller 8 is driven to rotate. For this reason, the rotational driving force of the secondary transfer roller 8 also acts as a force at which the leading edge of the sheet 19 pushes down the secondary transfer roller 8, and the secondary transfer roller 8 only has a secondary transfer roller 8 that pushes down the secondary transfer roller 8. The torque applied to the transfer counter roller 5 is reduced.

  On the other hand, the entrance roller 4 is also decelerated similarly to the secondary transfer counter roller 5. This is because when the leading edge of the paper 19 enters the main nip portion, the paper 19 does not advance until the leading edge of the paper 19 pushes down the secondary transfer roller 8, and therefore the belt due to the frictional force between the intermediate transfer belt 2 and the paper. A traveling load J2 is generated. Due to this travel load, torque D2 in the direction opposite to the rotation direction acts on the entrance roller 4, and the entrance roller 4 similarly decelerates. The belt running load J2 is transmitted to the driving roller 3 via the intermediate transfer belt 2 as shown by an arrow F (3) in FIG. However, the belt traveling load transmitted to the driving roller 3 is canceled by the driving force of the motor that drives the driving roller 3 to rotate, and is not transmitted to the driven roller 7 upstream of the driving roller 3 in the belt moving direction.

When the secondary transfer counter roller 5 decelerates, a stretch region (hereinafter referred to as a tension control region T2) between the secondary transfer counter roller 5 and the driven roller 7 which is a stretch surface on the downstream side of the transfer nip of the intermediate transfer belt 2 is a belt moving direction. As shown by an arrow F (4) in FIG. 5A, a force J1 in the direction opposite to the belt moving direction is transmitted to the tension roller 6 and the driven roller 7. When the force J1 pulling in the direction opposite to the belt moving direction is transmitted to the driven roller 7, the driven roller 7 is decelerated as shown by the thick line in FIG. As the driven roller 7 decelerates, the primary transfer region T1 of the intermediate transfer belt 2 is pulled in the direction opposite to the belt moving direction, and torque in the direction opposite to the rotation direction of the driving roller 3 is applied. As a result, the drive roller 3 decelerates as indicated by the thick line in FIG. As described above, the driven roller 7 and the driving roller 3 are decelerated, so that the primary transfer region T1 of the intermediate transfer belt 2 is decelerated, and the portion I ₃ having a darker color than the normal image portion I ₁ as shown in FIG. Will occur.

(3) Speed fluctuation due to return of pre-nip region T3 of the intermediate transfer belt 2 after entering the front end main nip FIG. 7 is an explanatory diagram of fluctuation due to return of the pre-nip region T3 of the intermediate transfer belt 2 after entering the front end main nip. is there.
As shown in FIG. 7B, when the leading edge of the sheet pushes down the secondary transfer roller 8 and enters the main nip portion, the intermediate transfer belt 2 that has been pushed inward by the leading edge of the sheet 19 so far. The pre-nip region T3 is restored. As a result, a phenomenon opposite to the above (1) speed fluctuation due to entry of the pre-nip portion occurs. That is, a force G1 that pushes the belt in the same direction as the belt movement direction is generated on the downstream side in the belt movement direction from the position where the leading edge of the paper in the pre-nip region T3 is pushing the belt. On the other hand, a force G2 that pushes the belt in the direction opposite to the belt moving direction is generated on the upstream side of the belt moving direction from the position where the leading edge of the paper in the pre-nip region T3 presses the belt. The force G2 for pushing the belt in the direction opposite to the belt moving direction is transmitted to the entrance roller 4 and the drive roller 3 through the intermediate transfer belt 2 as shown by an arrow F (6) in FIG. As a force G2 that pushes the inlet roller 4 in the direction opposite to the belt moving direction is generated, a torque H1 in the direction opposite to the inlet roller rotation direction is generated in the inlet roller 4 as shown in FIG. Slows down. Similarly, torque in the direction opposite to the rotation direction of the drive roller is generated in the drive roller 3, and the speed of the drive roller 3 decreases as shown by the thick line in FIG.

  On the other hand, the force G1 for pushing the belt in the direction opposite to the belt moving direction is, as shown by an arrow F (5) in FIG. 7A, via the intermediate transfer belt 2, the secondary transfer counter roller 5, the tension roller 6, It is transmitted to the driven roller 7. When the force G1 for pushing the belt in the direction opposite to the belt moving direction is transmitted to the secondary transfer counter roller 5, as shown in FIG. 7B, the torque in the same direction as the rotation direction of the secondary transfer counter roller 5 is obtained. H2 is generated and the secondary transfer counter roller 5 is accelerated. Similarly, a torque in the same direction as the rotation direction of the driven roller 7 is generated in the driven roller 7, and the speed of the driven roller 7 increases as shown by a thick line in FIG. 7C.

  In FIG. 7C, the driving roller 3 and the driven roller 7 fluctuate even after the speed fluctuates due to the return of the intermediate transfer belt 2. This is because the torsional vibration is generated in the shaft of the driving roller 3 to which the driving force of the driving motor is transmitted due to the speed fluctuation of the driving roller 3 in the above (1) to (3), and the intermediate transfer belt 2 is returned by the torsional vibration. Even after the speed fluctuation due to the above, it is considered that the speed fluctuation occurred in the driving roller 3 and the driven roller 7.

FIG. 8 shows data obtained by examining the relationship between the speed fluctuation of the driven roller 7 and the driving roller 3 and the displacement of the tension roller 6 and the secondary transfer roller 8.
As shown in FIG. 8, it can be seen that the linear velocity of the driven roller 7 starts to drop, that is, the tension roller 6 starts to be suddenly displaced in the belt outer direction almost simultaneously with the start of the main nip entry variation of (2). This means that the tension roller 6 is pushed down to the outside of the belt due to an increase in belt tension between the secondary transfer counter roller 5 and the driven roller 7 of the intermediate transfer belt 2. If the tension roller 6 is not provided, there is no relaxation of tension due to the tension roller 6 being pushed down to the outside of the belt, so that the load torque generated when the front end of the paper enters the main roller nip propagates to the driven roller 7 with almost no loss. Thus, the deceleration of the driven roller 7 increases.
On the other hand, if the belt tension in the tension control region T2 of the intermediate transfer belt 2 is weak when the front end of the sheet is in the main nip portion, the force J1 is applied to the tension control region T2 of the intermediate transfer belt 2 in the direction opposite to the belt moving direction. Even if (see FIG. 5B) works, the force can be made difficult to propagate to the driven roller 7 and the driving roller 3. As a result, the speed reduction of the driven roller 7 and the driving roller 3 can be suppressed, and the speed reduction of the primary transfer region T1 of the intermediate transfer belt 2 can be suppressed. Therefore, it is possible to prevent the portions I ₃ darker than the portion I ₁ of the normal image as shown in FIG. 3 results.

  Further, it can be seen that the secondary transfer roller 8 starts to be displaced in the belt outer direction (downward in the drawing) almost simultaneously with the tension roller 6 starting to be displaced in the belt outer direction. Therefore, if the tension roller 6 is displaced more rapidly in the belt outer direction by external force in conjunction with the displacement of the secondary transfer roller 8, the belt tension in the tension control region T2 can be further weakened. The load torque that decelerates the secondary transfer counter roller 5 is difficult to propagate to the driven roller 3, and the speed reduction of the driven roller 7 and the driving roller 3 can be suppressed, and the speed reduction of the primary transfer region T <b> 1 of the intermediate transfer belt 2 is suppressed. be able to.

  Therefore, in the present invention, the tension roller 6 is displaced more suddenly outward by the external force in conjunction with the displacement of the secondary transfer roller 8 to weaken the belt tension in the tension control region T2 (hereinafter referred to as the belt). (Tension control). Hereafter, it demonstrates concretely using each Example of this invention.

[Example 1]
FIG. 9 is an explanatory diagram of tension roller displacement control according to the first embodiment. As described above, when the sheet enters, the secondary transfer roller 8 starts to move in the belt outer direction (downward in the drawing) almost simultaneously with the tension roller 6 starting to move in the belt outer direction. Therefore, if the displacement of the tension roller 6 is controlled in conjunction with the secondary transfer roller 8, the tension roller 6 can be displaced at the timing when the driven roller 3 starts to decelerate, and the tension control region T2 of the intermediate transfer belt 2 can be displaced. The belt tension can be weakened. As a result, the tension control region T2 can reduce the force that pulls the driven roller 7 in the direction opposite to the belt moving direction, and can suppress a decrease in the speed of the driven roller 7. As a result of the speed reduction of the driven roller 7 being suppressed, the force that the driven roller 7 pulls the driving roller 3 in the direction opposite to the belt moving direction is reduced, and the speed reduction of the driving roller 3 is suppressed. As a result, the speed reduction of the primary transfer region T1 of the intermediate transfer belt 2 is suppressed, and the density reduction can be suppressed.

  The displacement of the tension roller 6 is controlled by using a value obtained by multiplying the measured displacement L of the secondary transfer roller 6 by a predetermined coefficient a as a target value. The amount can be adjusted. Although the suppression effect is generally proportional to the displacement amount of the tension roller 6, the displacement amount of the secondary transfer roller 8 can be set even when a plurality of displacement amounts of the tension roller 6 are set to cope with a plurality of paper thicknesses. Can be handled only by setting one coefficient a in advance. However, since it is also assumed that it is affected by fluctuations in the external environment, a plurality of values may be held, and the values may be changed as appropriate.

FIG. 10 is a schematic configuration diagram of the image forming apparatus according to the first embodiment.
As shown in FIG. 10, a secondary transfer roller displacement sensor 24 is mounted as a secondary transfer roller displacement information measuring means for measuring the displacement of the secondary transfer roller 8. Further, the secondary transfer roller displacement sensor 24 is connected to the drive command unit 25 and the tension control means 28. Based on the displacement information of the secondary transfer roller 8 measured by the secondary transfer roller displacement sensor 24, the drive command unit 25 generates a target value (drive voltage value) for the displacement of the tension roller 6, and the tension control means 28 Control.

FIG. 11 is a control block diagram of the image forming apparatus according to the first embodiment.
The drive command unit 25 includes a memory 27 and a microprocessor 26. The microprocessor 26 performs various processes based on a program stored in the memory 27. A signal of displacement information measured by the secondary transfer roller displacement sensor 24 is input to the microprocessor 26 in the drive command unit 25. The microprocessor 26 calculates the target value of the tension roller displacement by performing an offset process based on the time when the secondary transfer roller is pressed before entering the sheet and a process of multiplying by a predetermined coefficient. A drive voltage value corresponding to the target value is sent to the drive driver 29 of the tension control means 28.

FIG. 12 is a schematic configuration diagram of the tension control means 28.
As shown in FIG. 12A, the shaft of the tension roller 6 is rotatably supported by the bearing portion 34. The bearing portion 34 is urged toward the intermediate transfer belt 2 by a tension spring 36. An eccentric cam 34 that is rotationally driven by the drive motor 30 is provided, and a lever 33 whose one end is rotatably supported by the apparatus main body is in contact with the eccentric cam 34. The other end side of the lever 33 is supported by a pin 35 provided in the bearing portion 34. When the drive motor 30 is turned off, the tension roller 36 urges the tension roller 6 toward the intermediate transfer belt 2 with a predetermined pressure to apply a predetermined tension to the intermediate transfer belt 2. When a predetermined drive voltage is applied from the drive driver 29 to the drive motor 30 and the drive of the motor 30 is turned on and the eccentric cam 34 rotates as shown in FIG. 12B, the lever 33 rotates and is provided in the bearing portion 34. Push the pin 35 downward in the figure. As a result, the bearing portion 34 is displaced outside the belt against the urging force of the tension spring 36, and the pressing force of the tension roller 6 to the intermediate transfer belt 2 is weakened. As described above, in the first embodiment, the displacement means for moving the bearing portion 34 in the direction orthogonal to the transfer nip downstream-side stretching surface includes the drive motor 30, the eccentric cam 32, the lever 33, and the like. Then, the displacement means is driven by the drive driver 29 to displace the bearing portion 34 of the tension roller 6 to control the tension by the tension roller 6. As a displacement means for displacing the bearing portion 34, in addition to the above-described configuration, an electromagnetic solenoid may be used to move the bearing portion 34 in a direction orthogonal to the transfer nip downstream-side stretch surface. .

  In this embodiment, as described in the background art, when a sheet-like member having a thickness of a certain degree or more enters the secondary transfer nip, the speed of the intermediate transfer belt that has been driven at a constant speed until then is entered. To solve the problem that the image is disturbed at the primary transfer portion. For this reason, when using a sheet-like member that can ignore the speed fluctuation of the intermediate transfer belt when entering the secondary transfer nip, such as plain paper, a separate switching means is provided, and the tension control means 28 is You may make it not operate.

[Example 2]
Next, Example 2 will be described.
FIG. 13 is a schematic configuration diagram of an image forming apparatus according to the second embodiment.
As shown in FIG. 13, in the second embodiment, a second tension is applied to the tension control region T2 separately from the tension roller 6 that urges the intermediate transfer belt 2 on the secondary transfer nip downstream tension surface in FIG. A roller 41 is provided. Then, the tension roller 6 is left as it is, and the second tension roller 41 is displaced more rapidly outward by the external force in conjunction with the displacement of the secondary transfer roller 8, and the belt tension in the tension control region T2 is reached. Take control.

  With such a configuration, even if the second tension roller 41 is displaced more than expected due to a malfunction or the like, the tension roller 6 can maintain a certain belt tension above a certain level. The secondary transfer roller displacement sensor 24, the drive command unit 25, the tension control unit, and the like have the same configuration as in the first embodiment.

[Example 3]
Next, Example 3 will be described.
FIG. 14 is a schematic configuration diagram of an image forming apparatus according to the third embodiment.
As shown in FIG. 14, in the third embodiment, in addition to the first embodiment, a tension roller displacement sensor 31 as an urging roller displacement information measuring means for measuring displacement information of the tension roller 6 is mounted. The displacement sensor 31 is connected to the drive command unit 25. The drive command unit 25 generates a target value (drive voltage value) for the displacement of the tension roller 6 based on the displacement information of the secondary transfer roller 8 measured by the secondary transfer roller displacement sensor 24. Based on the displacement information of the tension roller 8 measured by the tension roller displacement sensor 24 with respect to the target value, the tension roller displacement is feedback-controlled so as to follow the target value.

FIG. 15 is a control block diagram of the image forming apparatus according to the third embodiment.
As shown in FIG. 15, the displacement information signal of the secondary transfer roller 8 measured by the secondary transfer roller displacement sensor 24 is input to the microprocessor 26 in the drive command unit 25. The microprocessor 26 calculates the target value of the tension roller displacement by performing an offset process based on the time when the secondary transfer roller is pressed before entering the sheet and a process of multiplying by a predetermined coefficient. On the other hand, a displacement information signal of the tension roller 6 measured by the tension roller displacement sensor 31 is also input to the microprocessor 26 in the drive command unit 25. Similarly to the secondary transfer roller displacement, the microprocessor 26 performs an offset process for making the reference before the paper entry, compares the tension roller displacement with the target value, and eliminates the deviation from the target value. A driving voltage value is generated, and the driving voltage value is sent to the driving driver 29 of the tension control means 28.

  FIG. 16 is a block diagram for performing the feedback control based on the displacement information of the tension roller 6. An output signal of the tension roller displacement sensor 31, that is, information P (i−1) of the displacement amount of the tension roller 6 is given to a calculation unit (subtractor) 37. The calculation unit 37 calculates a difference e (i) between the target value Ref (i) of the tension roller displacement amount, which is a control target value, and the detected displacement amount P (i−1) of the tension roller. The difference e (i) is input to the control controller unit 38. The control controller unit 38 includes a low-pass filter 39 for removing high-frequency noise and a proportional element (gain Kp) 40. In the control controller unit 38, a control voltage u (i) used for driving the drive motor 30 is obtained. A drive signal is generated by the drive driver 29 based on the drive voltage u (i) and output to the drive motor 30. The driving force of the drive motor 30 thus controlled to drive rotates the eccentric cam 32, and the tension roller 6 moves according to a predetermined target displacement amount. Such a feedback loop control operation is repeated. Here, the control system is an example, and any other PID control system, modern control system, robust control system, or the like may be used.

  Thus, by providing the tension roller displacement sensor 31 and performing feedback control on the target value, the tension roller 6 can be displaced with high accuracy.

[Example 4]
Next, Example 4 will be described. In the fourth embodiment, the toner image on the intermediate transfer belt is transferred to the paper 19 at the secondary transfer nip, and at the same time, the fixing is performed.
FIG. 17 is a schematic configuration diagram of an image forming apparatus according to the fourth embodiment.
As shown in FIG. 17, a heating device 21 serving as a heating unit is provided near the secondary transfer nip and upstream of the secondary transfer nip in the sheet-like member conveyance direction. The heating device 21 heats the sheet 19 to a temperature sufficient to fix the toner, and the transfer and fixing are simultaneously performed in the secondary transfer nip.

  FIG. 18 is a modification of the fourth embodiment. In this modification, the intermediate transfer belt 2 is heated by the heating device 21, the toner on the intermediate transfer belt 2 is melted, and transfer and fixing are simultaneously performed in the secondary transfer nip.

  In the fourth embodiment shown in FIGS. 17 and 18, as in the first embodiment, the tension control unit 8 controls the tension in conjunction with the displacement of the secondary transfer roller 8 detected by the secondary transfer roller displacement sensor 24. The belt tension in the region T2 is controlled. As a result, the speed reduction of the primary transfer region T1 of the intermediate transfer belt 2 is suppressed, and the density reduction can be suppressed.

Next, a verification test conducted by the present inventors will be described.
In the verification test, first, a belt conveyance tester having substantially the same characteristics as the image forming apparatus in the intermediate transfer system shown in FIG. 1 was prototyped, and a simulation model for reproducing it was constructed. Next, using this simulation model, it is a test for verifying the degree of suppression effect when the displacement of the tension roller 6 is controlled in conjunction with the displacement of the secondary transfer roller 8.
FIG. 19 is a diagram illustrating a result of the verification test. FIG. 19A is a diagram showing the tension roller displacement when the tension roller displacement control is not performed and the tension roller displacement when the tension roller displacement control is performed. FIG. 19B is a diagram showing the driven roller linear speed fluctuation when the tension roller displacement control is not performed and the driven roller linear speed fluctuation when the tension roller displacement control is performed. Unlike the image forming apparatus shown in FIG. 1, the driven roller linear speed fluctuation in the speed fluctuation portion due to the intrusion of the pre-nip portion is suppressed because the secondary transfer roller is driven and the layout is different. There is no impact on verifying the effect.

In this simulation, it is assumed that the paper having a basis weight of 359 [g / m ² ] is passed, and the coefficient a shown in Example 1 is set to 2. That is, the tension roller displacement is controlled with a displacement amount twice the displacement amount L of the secondary transfer roller 8 as a target value. As the coefficient a is increased, the first linear velocity drop immediately after the entry can be suppressed, but on the other hand, the residual vibration (linear velocity drop after the second time) also increases. In this case, the determination may be made while comparing the suppression effect and the adverse effect.

  When the tension roller displacement control as shown in FIG. 19 (a) is performed in conjunction with the displacement of the secondary transfer roller 8, as shown in FIG. 19 (b), the driven roller linear velocity fluctuation is controlled by the displacement control. It can be seen that it is reduced by about 35 [%] compared to the case where it is not performed.

As described above, according to the image forming apparatus of the present embodiment, the intermediate transfer belt 2 is a toner image carrying belt that is stretched by a plurality of stretching rollers and carries a toner image on the surface and moves endlessly, and the plurality of stretching rollers. The secondary transfer opposing roller 5 that is one of the two is opposed to the toner image on the intermediate transfer belt and is transferred to the sheet 19 that is a sheet-like member. And a transfer roller 8. Of the plurality of stretching surfaces formed by the intermediate transfer belt 2 being stretched by a plurality of stretching rollers, the stretching surface whose downstream end in the moving direction of the intermediate transfer belt 2 is the main nip portion. Tension control means 28 for controlling the tension in the tension control region T2 which is the tension surface on the downstream side of the transfer nip. The tension control means 28 controls the tension in the tension control region T2 in conjunction with the displacement of the secondary transfer roller 8.
Since the displacement of the transfer roller starts when the sheet 19 enters the transfer nip, the tension control unit 28 is configured to control the tension in conjunction with the displacement of the transfer roller. Accordingly, it is possible to suppress a change in tension in the tension control region T2 of the intermediate transfer belt 2. The tension control means 28 only needs to operate in conjunction with the displacement of the secondary transfer roller 8 and does not have to be provided integrally with the secondary transfer roller 8 or the support body. It can be designed so that the tension control in the tension control region T2 is efficiently performed. Accordingly, a sufficient effect of suppressing a change in tension can be obtained, and the speed fluctuation of the primary transfer portion of the intermediate transfer belt 2 generated by the entry of the sheet 19 can be suppressed satisfactorily. Further, by operating the tension control means 28 in conjunction with the displacement of the secondary transfer roller 28 accompanying the entry into the secondary transfer nip, it is not necessary to provide an entry detection means separately, so that the cost can be reduced.

  The secondary transfer roller displacement sensor 24 is a transfer roller displacement information measuring unit that measures displacement information of the secondary transfer roller 8, and the tension control unit 28 is driven based on the displacement information measured by the secondary transfer roller displacement sensor 24. A driving command unit 25 is provided. The tension control means 28 controls the tension in the tension control region T2 based on the displacement information measured by the secondary transfer roller displacement sensor 24. Thereby, tension control based on the displacement information by the secondary transfer roller displacement sensor 24 can be performed, and the tension control in the tension control region T2 can be efficiently performed.

  Further, the tension control means 28 rotatably supports the tension roller 6 that is an urging roller that urges the intermediate transfer belt 2 to bend with respect to the stretching surface on the downstream side of the transfer nip, and the shaft of the tension roller 6. A bearing 34 and a drive motor 30, an eccentric cam 32, a lever 33, and the like constituting displacement means for moving the bearing in a direction orthogonal to the transfer nip downstream stretch surface are provided. Then, based on the target value generated based on the displacement information of the secondary transfer roller 8 measured by the secondary transfer roller displacement sensor 24, the driving command unit 25 operates the displacement means of the tension control means 28. With such a configuration, the shaft of the tension roller 6 is displaced according to the displacement information of the secondary transfer roller 8, and the contact pressure on the tension roller 6 with respect to the intermediate transfer belt 2 is appropriately set to a desired value. Therefore, the tension control in the tension control region T2 can be performed efficiently.

  Further, as described in the second embodiment, a second tension roller 41 is provided in the tension control region T2 separately from the tension roller 6 of FIG. Then, the tension roller 6 is left as it is, and the second tension roller 41 is displaced more rapidly outward by the external force in conjunction with the displacement of the secondary transfer roller 8, and the belt tension in the tension control region T2 is reached. Take control. With such a configuration, even if the second tension roller 41 is displaced more than expected due to a malfunction or the like, the tension roller 6 can maintain a certain belt tension above a certain level.

  Further, as described in the third embodiment, a tension roller displacement sensor 31 is provided as a biasing roller displacement information measuring unit that measures displacement information of the tension roller 6 constituting the tension control unit 28, and the transfer roller displacement information measuring unit is provided. The tension control means 28 is driven by feeding back the biasing roller displacement information measured by the biasing roller displacement information measuring means to the target value generated based on the displacement information of the transfer roller measured by the above. . By having such a configuration, the tension roller 6 can be displaced with high accuracy.

  Further, as described in the fourth embodiment, a heating device 21 that is provided in the vicinity of the secondary transfer nip and that heats the intermediate transfer belt 2 or the sheet 19 is provided, and the intermediate transfer belt 2 is disposed in the secondary transfer nip. Even the image forming apparatus configured to transfer the upper toner image onto the sheet 19 and fix it at the same time can obtain the above-described effects.

1Y, C, M, K: photoconductor 2: intermediate transfer belt 3: drive roller 4: entrance roller 5: secondary transfer counter roller 6: tension roller 7: driven roller 8: secondary transfer roller 9: registration roller 10: Swing arm 11: Conveying belt 15: Fixing device 19: Paper 21: Heating device 24: Secondary transfer roller displacement sensor 25: Drive command unit 26: Microprocessor 27: Memory 28: Drive control means 29: Drive driver 30: Drive motor
31: Tension roller displacement sensor 32: Eccentric cam 33: Lever 34: Bearing portion 35: Pin 36: Tension spring 37: Calculator (subtractor)
38: Control controller section 50: Transfer unit T1: Primary transfer area T2: Tension control area T3: Pre-nip area

JP-A-2005-338884

Claims (3)

A toner image carrying belt that is stretched by a plurality of stretching rollers and carries a toner image on its surface and moves endlessly, and is opposed to one of the plurality of stretching rollers across the toner image carrying belt, In an image forming apparatus having a transfer roller for forming a transfer nip for transferring a toner image on a toner image carrying belt to a sheet-like member,
Transfer roller displacement information measuring means for measuring displacement information of the transfer roller;
Of the plurality of stretching surfaces formed by the toner image carrying belt being stretched by the plurality of stretching rollers, the stretching end in which the upstream end portion in the moving direction of the toner image carrying belt becomes the transfer nip. Tension control means for controlling the tension of the toner image carrying belt on the downstream side of the transfer nip as a surface ;
Drive command means for driving the tension control means based on displacement information measured by the transfer roller displacement information measurement means ,
The tension control means includes an urging roller that urges the toner image carrying belt to bend with respect to the tensioning surface on the downstream side of the transfer nip, and a bearing portion that rotatably supports a shaft of the urging roller. , A displacement means for moving the bearing portion in a direction orthogonal to the transfer nip downstream-side stretching surface, and an urging roller displacement information measuring means for measuring displacement information of the urging roller,
The driving command means is configured to detect the displacement of the urging roller measured by the urging roller displacement information measuring means with respect to a target value generated based on the displacement information of the transfer roller measured by the transfer roller displacement information measuring means. An image forming apparatus , wherein information is fed back to operate a displacement means of the tension control means . The image forming apparatus 請 Motomeko 1, image forming, characterized in that a second biasing roller for urging so as to bend with respect to the transfer nip downstream stretched surface of the toner image bearing belt Equipment . The image forming apparatus 請 Motomeko 1 or 2, wherein provided in the transfer nip vicinity, a heating means for heating the toner image bearing belt or sheet-like member, on the toner image bearing belt in the transfer nip toner An image forming apparatus configured to transfer an image to a sheet-like member and fix the image at the same time.

Images