JP2003270891A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus for forming an image of high image quality by surely cancelling the expansion/contraction of a pixel while preventing the occurrence of a hollow-character phenomenum. <P>SOLUTION: The moving velocity of the image carrying surface of each photoreceptor drum 11 is made equal to that of a transfer paper 2. The ratio (W<SB>1</SB>/W<SB>2</SB>) of a transfer nip width W<SB>1</SB>at a 1st transfer position Pt1 between each photoreceptor drum 11 and an intermediate transfer belt 40 to a transfer nip width W<SB>2</SB>at a 2nd transfer position Pt2 between the intermediate transfer belt 40 and the transfer paper 2 is made equal to the ratio (Vd/Vb) of the moving velocity Vd of the image carrying surface of each photoreceptor drum 11 to the moving velocity Vb of the image transfer surface of the intermediate transfer belt 40. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a printer and a fax machine. Specifically, an image on an image carrier such as a photoconductor drum is superposed on a first transfer member such as an intermediate transfer belt and primary-transferred, and the image on the first transfer member is transferred to a second transfer medium such as a recording medium. The present invention relates to an image forming apparatus that performs secondary transfer at once to a transfer body.

[0002]

2. Description of the Related Art In recent years, the need for color copying has increased, and ink jet type image forming apparatuses have become the mainstream in low speed areas, but electrophotographic image forming apparatuses are becoming popular in the medium and high speed areas. In particular, as an image forming apparatus of a system suitable for speeding up, there is a tandem type color image forming apparatus in which a plurality of photosensitive drums as image bearing members are arranged side by side in the conveying direction of a transfer sheet as a transfer member. Also,
There is also an intermediate transfer belt type image forming apparatus that uses a belt-shaped intermediate transfer member as a first transfer member and transfers an image formed on the belt to a transfer paper as a second transfer member.

In the above tandem type color image forming apparatus, as disclosed in, for example, Japanese Unexamined Patent Publication No. 10-246995, four photosensitive drums provided with an optical scanning unit are provided in a conveying direction of a transfer sheet on a conveying belt. It is located in. Then, the light beam emitted from the optical scanning unit is scanned (main scanning) in the direction along the axis of the photosensitive drum, and the surface of the rotating photosensitive drum is exposed. As a result, an electrostatic latent image is formed on each photoconductor drum.
Different colors (cyan, magenta,
Toners of yellow and black are supplied from each developing device, and the electrostatic latent image is developed with the toners. Then, the transfer belt conveys the transfer paper to each photoconductor drum, and during the conveyance, the transfer charger sequentially transfers the toner images on the photoconductor drums onto the same transfer paper so that the toner images overlap each other on the same scanning line. It After that, the toner images transferred and superposed on the transfer paper are fixed and discharged onto the paper discharge tray. As described above, the tandem type color image forming apparatus forms images of different colors on the plurality of photoconductor drums in parallel. By this image formation by the parallel processing, a color image of four colors can be formed by passing the transfer paper from the respective transfer positions from the respective photoconductor drums once, which is a method suitable for high-speed color image formation. There is also known a tandem type color image forming apparatus of an intermediate transfer belt type in which an intermediate transfer belt is provided instead of the above-mentioned transport belt. In this apparatus, the four images are not superposed on the transfer paper, but the images are superposed and transferred on the intermediate transfer belt, and the superposed images on the intermediate transfer belt are collectively transferred on the transfer paper.

In the above-mentioned tandem type image forming apparatus of the intermediate transfer belt type, the toner images of different colors formed on the photosensitive drum are transferred so as to be superposed on the intermediate transfer belt as the first transfer member. Form a color image. Therefore, if the positions of the toner images of the respective colors transferred to the intermediate transfer belt deviate, color misregistration occurs on the color image. Regarding measures to prevent this color misregistration,
Several proposals have been made in the past (see, for example, Japanese Patent No. 2929671, Japanese Patent Application Laid-Open No. 63-11967, Japanese Patent Application Laid-Open No. 59-182139). There is also a description of color misregistration when transferring with a speed difference between the photosensitive drum and the intermediate transfer belt or transfer paper (for example, Kido, Iijima: "Slip transfer mechanism research"). , Fuji Xerox Technical Report, No. 13. Hereinafter, this document is referred to as "technical document."

In the tandem type image forming apparatus, as shown in FIG.
4 (a) and 4 (b), even if the photosensitive drum 11 has an eccentricity or a variation in diameter, if the rotational angular velocity of each photosensitive drum is constant and the speed of the intermediate transfer belt does not fluctuate. No color shift occurs in the color image on the intermediate transfer belt. That is, even if the pixel extends at the exposure position due to the eccentricity of the photoconductor drum 11 as shown in FIG. 14A (Ie1-> Ie2), as shown in FIG.
Since the pixel Ie shrinks at the transfer position of (Ie2-> Ie
3) As a result, the pixels on the intermediate transfer belt 21 have a predetermined length. However, as described in the above-mentioned technical documents and gazettes, when the gear that is the driving force transmitting element of the photosensitive drum drive system or the intermediate transfer belt drive system has an eccentricity, the motor that is the drive source has a constant speed. Even if the photoconductor drum rotates, the rotational angular velocity of the photosensitive drum and the moving velocity of the intermediate transfer belt fluctuate, which causes color misregistration. As a countermeasure against this, in Japanese Patent No. 2929671, the integral multiple of the fluctuation period of the gears and the like is made equal to the photosensitive drum rotation time from the exposure position to the transfer position on the photosensitive drum. Further, in Japanese Patent Laid-Open No. 63-11967, an integral multiple of the fluctuation period of the photosensitive drum drive system is
It is set to be equal to the time taken for the intermediate transfer belt or the transfer paper to pass between the adjacent photoconductor drums. Furthermore, JP-A-59-
In Japanese Patent No. 182139, an integral multiple of the rotation cycle of the belt drive roller is set to be equal to the time required for the intermediate transfer belt or the transfer paper to pass between adjacent photoconductor drums. As described above, the image forming apparatus described in the above publications takes measures to prevent color misregistration due to periodical speed fluctuations in the drive system.

Further, conventionally, in order to remove a phenomenon called a hollow character phenomenon, it is known that a certain speed difference is provided between the photosensitive drum and the intermediate transfer belt or the transfer paper at the transfer position ( For example, JP-A-10-39648 and JP-A-62-35137.
(See Japanese Patent Publication). This "hollow character phenomenon"
This is a phenomenon in which the central part of the pixel details is missing. For example,
It is assumed that a moving speed difference (relative speed) ΔVh (= Vd−Vb) is provided at the first transfer position in order to obtain this hollow character phenomenon. However, Vb is the moving speed of the intermediate transfer belt or the transfer paper, and Vd is the peripheral speed of the photosensitive drum with no diameter variation. In this way, the moving speed difference ΔVh is provided at the first transfer position, and the length of the exposure pixel per unit time when the rotational angular speed of the photosensitive drum is a constant speed ωo and the diameter of the photosensitive drum is Ro is set. When Ie = Roωo, the length I of the exposure pixel when the photosensitive drum diameter Ro + ΔRo increases by ΔRoωo per unit time as shown by the following equation.

## EQU1 ## I = (Ro + ΔRo) ωo = Ie + ΔRoωo

The speed of the intermediate transfer belt is Vb = R
oωo−ΔVh, and the movement speed difference Δ at the first transfer position.
V = ΔRoωo + ΔVh occurs. Therefore, δI =
(W ₁+ Iw) · ΔV / Vd
The pixel length is changed by δI.

## EQU2 ## δI = (W ₁ + I) · ΔV / Vd = {W ₁ + Ie + ΔRoωo} · (ΔRoωo + ΔVh) / {ωo (Ro + ΔRo)}

Therefore, the elongation ΔRoωo of the pixel per unit time at the time of exposure changes the length of the pixel at the time of transfer by δI shown by the following equation.

## EQU3 ## δI = {W ₁ + (Ro + ΔRo) ωo} · (ΔRoωo + ΔVh) / {ωo (Ro + ΔRo)} = W ₁ · (ΔRoωo + ΔVh) / {ωo (Ro + ΔRo)} + (ΔRoωo + ΔVh)

When the transfer nip width W ₁ = 0, ΔRoωo
The pixel shrinks by + ΔVh. That is, the error ΔVh corresponding to the moving speed difference is generated. When the rotational angular velocity of the photoconductor drum is constant, the argument that the pixel does not change even if the diameter of the photoconductor drum varies is that the transfer nip width W _{1 at} the first transfer position is zero and the movement velocity difference ΔVh is In the case of zero. However, when the moving speed difference ΔVh is constant, it means that the entire image is expanded (magnification error). It is clear that the same discussion can be made when the photosensitive drum has an eccentricity. As described above, due to the influence of the transfer nip width W ₁ and the moving speed difference ΔVh at the first transfer position, an error (contraction) Ce shown in the following expression is generated.

[Equation 4] Ce = W ₁ · (ΔRoωo + ΔVh) / {ωo (R
o + ΔRo)} + ΔVh

Further, the error (contraction) E when the speed variation δV of the intermediate transfer belt is added is given by the following equation.
That is, if there is a movement speed difference ΔVh for eliminating the hollow character phenomenon and a fluctuation δV of the movement speed of the intermediate belt, an error E as in the following equation will occur.

[Equation 5] E = W ₁ · {ΔRoωo + (ΔVh + δV)} /
{Ωo (Ro + ΔRo)} + (ΔVh + δV)

Therefore, conventionally, an image forming apparatus has been proposed which is configured to reduce the expansion and contraction of the toner image at that time even when a speed difference is applied at the transfer position in order to eliminate the above-mentioned hollow character phenomenon. (See JP 2001-265081 A). This image forming apparatus includes a photosensitive drum 11 and intermediate transfer drums 21 and 22.
And the first transfer position and the intermediate transfer drum 2 facing each other.
In each of the second transfer positions where the first and the second transfer drums 31 and the intermediate transfer drum 31 are opposed to each other, a transfer of a slip transfer method is adopted in which an image is transferred while causing a speed difference in the moving speeds of the surfaces which are opposed to each other. is doing. Then, the sign of the moving speed difference between the first transfer position and the second transfer position is reversed to cancel the expansion / contraction of the toner image. That is, this Japanese Patent Laid-Open No. 2001-2650
In the image forming apparatus described in Japanese Patent No. 81,
As shown in, the speed difference between each photosensitive drum 11 and the two intermediate transfer drums 21 and 22 as the first transfer body is ΔV _1, and the speed difference between the intermediate transfer drums 21 and 22 and the second transfer body is The speed of each component is set so that the speed difference between the intermediate transfer drum 31 and the intermediate transfer drum 31 is different from that of ΔV ₂ . This is intended to reduce the expansion and contraction of the toner image. In particular, it is said that the expansion and contraction of the toner image can be canceled by setting the speed of each component so that ΔV ₁ + ΔV ₂ = 0.

Further, Japanese Patent Laid-Open No. 2000-338745 also mentions a configuration in which the peripheral speed of the photosensitive drum and the moving speed of the transfer paper (recording medium) are equal, and the intermediate transfer member speeds are not equal. This has the same meaning as ΔV ₁ + ΔV ₂ = 0. That is, a speed difference is provided between the photoconductor drum and the intermediate transfer member, and the length of the pixel is restored at the second transfer position by the amount corresponding to the change in the length of the pixel.

[0013]

However, when the inventors of the present invention conducted experiments with an actual device, the above ΔV ₁
It has been found that it is difficult to realize an image forming apparatus capable of reliably canceling the expansion and contraction of the toner image even if the speed difference is set so as to satisfy the condition of + ΔV ₂ = 0. Then, as a result of the analysis by the present inventors,
It has been found that the expansion and contraction of the toner image cannot be reliably canceled in some cases due to factors not mentioned in Japanese Patent Laid-Open No. 265081 and Japanese Patent Laid-Open No. 2000-338745.

The present invention has been made in view of the above problems, and an object thereof is to reliably cancel the expansion and contraction of pixels while preventing the occurrence of a hollow character phenomenon.
An object of the present invention is to provide an image forming apparatus capable of obtaining a high quality image.

[0015]

In order to prevent the occurrence of the above-mentioned hollow character phenomenon, when the speed difference of the surface moving speed of the members which are in contact with each other at the transfer position is made different,
In order to prevent expansion and contraction of pixels at that time, the above ΔV
_An image forming apparatus has been proposed so as to satisfy the condition of ₁ + ΔV ₂ = 0 (for example, see Japanese Patent Laid-Open No. 2001-265081). However, in an actual device, the above ΔV ₁ + ΔV
_Even if the above-mentioned speed difference is set so as to satisfy the condition of ₂ = 0, it is difficult to realize an image forming apparatus capable of reliably canceling expansion and contraction of pixels. As a result of the analysis by the present inventors, it has been found that the expansion and contraction of pixels may not be reliably canceled due to the following factors not mentioned in the above-mentioned conventional Japanese Patent Laid-Open No. 2001-265081. For example, as the above-mentioned factor, the transfer nip width at each transfer position is not zero but has a certain width. Another factor is that the transfer process conditions other than the speed difference and the transfer nip (for example, the surface condition of the image bearing member or the transfer member) are different from each other at each transfer position. As yet another factor, when a plurality of image carriers are provided, the eccentricity of each image carrier and the transfer medium causes a difference in speed between the first transfer positions when the same pixel of the image is transferred. It was different. Therefore, in order to achieve the above object, the invention of claim 1 is one or more image carriers that are rotationally driven, and an image forming unit that forms a plurality of different images on the image carrier, A first transfer unit that transfers an image on the image carrier to a first transfer member that is driven so as to pass a first transfer position that faces the image carrier, and a first transfer unit that opposes the first transfer member. In the image forming apparatus, the second transfer member is driven so as to pass through the second transfer position, and the second transfer unit transfers the image on the first transfer member. Of the image transfer surface of the second transfer member is equal to the moving speed of the image transfer surface of the second transfer member, and the transfer nip width W ₁ between the image transfer member and the first transfer member at the first transfer position is , the ratio of the nip width W ₂ between the first transfer body and the second transfer body at the second transfer position (W / W
₂ ) is the moving speed Vd of the image bearing surface of the image bearing member and the first
Ratio (Vd / V
It is characterized in that it is equal to b). Here, examples of the above-mentioned "image carrier" include a photosensitive drum and a photosensitive belt. Further, examples of the above-mentioned “first transfer member” include an intermediate transfer belt and an intermediate transfer drum.
Further, examples of the above-mentioned "second transfer member" include an intermediate transfer belt and an intermediate transfer drum, and a recording medium such as a transfer paper. In the image forming apparatus of the first aspect, the moving speed of the image carrying surface of the image carrier is equal to the moving speed of the image carrying surface of the second transfer body. Under this condition, the ratio (W ₁ / W ₂ ) of the transfer nip width between the first transfer position and the second transfer position is
The ratio (Vd / Vb) of the moving speed Vd of the image bearing surface of the image carrier and the moving speed Vb of the image transferring surface of the first transfer body is set to be equal. Thus, as will be described later in detail, in order to eliminate the hollow character phenomenon, a constant speed difference is generated between the image transfer surface of the image carrier and the image transfer surface of the first transfer member at the first transfer position. Even when the (relative speed) is provided, the expansion and contraction of the pixels transferred onto the second transfer member can be accurately canceled regardless of the change in the transfer nip width. In order to achieve the above-mentioned object, the invention of claim 2 is one or a plurality of image carriers that are rotationally driven, an image forming unit that forms a plurality of different images on the image carrier, and A first transfer unit that transfers an image on the image carrier to a first transfer member that is driven so as to pass a first transfer position that faces the image carrier, and a first transfer unit that opposes the first transfer member. In the image forming apparatus, the second transfer member is driven so as to pass through the second transfer position, and the second transfer unit transfers the image on the first transfer member. The difference between the moving speed Vd of the image bearing surface of the first transfer member and the moving speed Vb of the image transfer surface of the first transfer member is ΔVh (= Vd−Vb).
Of the size of the pixel which expands and contracts under the influence of other transfer process conditions except the transfer nip width and the surface moving speed difference between the transfer position and the second transfer position of the pixel to the size of the pixel when there is no such influence. Where κ ₁ and κ ₂ are the influence coefficients defined by, respectively, the moving speed Vd of the image bearing surface of the image bearing member and the movement of the image transferring surface of the second transferring member passing through the second transferring position. Speed difference δ with speed V ₂ (= Vd−
V ₂ ) satisfies the relational expression of δ = (1−κ ₁ / κ ₂ ) · ΔVh. In the image forming apparatus of the second aspect, the speed difference δ (= Vd−V ₂ ) satisfies the predetermined relational expression. Thus, as will be described later in detail, in order to eliminate the hollow character phenomenon, a constant speed difference is generated between the image transfer surface of the image carrier and the image transfer surface of the first transfer member at the first transfer position. Even when the (relative speed) is provided, even when other transfer process conditions except the transfer nip width and the surface movement speed difference at the first transfer position and the second transfer position are different, It is possible to accurately cancel the expansion and contraction of the pixel transferred to. According to a third aspect of the present invention, in the image forming apparatus according to the second aspect, the transfer nip width W ₁ between the image carrier and the first transfer member at the first transfer position, and the second
The ratio (W ₁ / W ₂ ) of the transfer nip width W ₂ between the first transfer member and the second transfer member at the transfer position is It is characterized by being equal to the ratio (Vd / Vb) to the moving speed Vb of the image transfer surface of the first transfer member. According to a fourth aspect of the present invention, in the image forming apparatus according to the first, second or third aspect, on the moving path of the image transfer surface of the first transfer body from the first transfer position to the second transfer position. The time required for the image transfer surface of the first transfer member to pass between the transfer positions adjacent to each other is a natural number times the cycle of speed fluctuations occurring on the image transfer surface of the first transfer member. To do.
Here, the moving speed of the image carrying surface of the image carrier and the moving speed of the image carrying surface of the second transfer body may be equalized. According to a fifth aspect of the present invention, in the image forming apparatus according to the fourth aspect, the first transfer body has an endless image transfer surface, and the first transfer body rotates a plurality of times to form an image on the image carrier. The images are configured to be superimposed and transferred onto the first transfer member, and are adjacent to each other on the moving path of the image transfer surface of the first transfer member from the second transfer position to the first transfer position. The time required for the image transfer surface of the first transfer member to pass between the transfer positions is a natural number times the cycle of speed fluctuations occurring on the image transfer surface of the first transfer member. is there. The invention of claim 6 relates to claim 1,
The image forming apparatus of 3, 4 or 5 is provided with means for driving and controlling each image carrier so that the average moving speeds of the image carrying surfaces of the image carriers become equal to each other, and the image at the first transfer position is provided. The transfer nip width between the carrier and the first transfer member does not change, and the image forming means exposes the image bearing surface of each image carrier based on the image data to form a latent image. Then, the latent image on the image bearing member is developed to form an image, and the exposure timing or the exposure position of each image bearing member with respect to the image bearing surface is different from that of the eccentricity and diameter of each image bearing member. In addition, it is set according to at least one of the distances between the image carriers. According to a seventh aspect of the present invention, in the image forming apparatus according to the sixth aspect, the surface portion of the first transfer body which comes into contact with the image carrier has flexibility or elasticity, It is characterized in that it is provided with means for bringing the first transfer body into pressure contact with the image carrier with a constant pressure contact force. According to an eighth aspect of the invention, in the image forming apparatus according to the first, second, third, fourth, fifth, sixth or seventh aspect, the first transfer body and the second transfer body are roller-shaped transfer bodies, The rotational angular velocity of at least one of the image carrier, the first transfer member and the second transfer member is
It is characterized in that it is set according to the variation in the diameter. In order to achieve the above object, the invention of claim 9 is one or a plurality of image carriers that are rotationally driven, an image forming unit that forms a plurality of different images on the image carrier, and A first transfer unit that transfers the image on the image carrier to one or a plurality of first transfer members that are driven so as to pass through a first transfer position facing the image carrier; Second transfer means for transferring the image on the first transfer body to the second transfer body driven so as to pass through the second transfer position facing the second transfer body, and to the second transfer body. In an image forming apparatus including a third transfer unit that transfers an image on the second transfer member to a recording medium that is driven so as to pass through a third transfer position that is opposite to the second transfer member, It is a roller-shaped transfer member, and the diameter and rotation angular velocity of the image carrier are respectively the diameter and rotation of the second transfer member. Equal to the speed,
The angle (eccentricity phase) formed between the position where the same pixel is formed and the maximum eccentric angle position on the image carrying surface of the image carrier and the image transferring surface of the second transfer body are the same. It is characterized by. In the image forming apparatus according to claim 9, each of the diameter and the rotational angular velocity of the image carrier is the second
Is equal to the diameter and rotational angular velocity of the transfer body of. Moreover, the angle (eccentricity phase) formed between the position where the same pixel is formed and the maximum eccentric angle position on the image carrying surface of the image carrier and the image transferring surface of the second transfer body are the same. . Therefore, even if the image carrier and the second transfer member are eccentric, the speed difference between the image carrier and the first transfer member when an arbitrary pixel of the image is primarily transferred is , And the speed difference between the first transfer member and the second transfer member when the pixel is secondarily transferred becomes the same. Therefore, even if a speed difference is provided at the transfer position to suppress the occurrence of the hollow character phenomenon,
Whether or not the image carrier and the second transfer member are eccentric,
It is possible to accurately cancel the expansion and contraction of the pixels on the second transfer member. The invention of claim 10 is,
In the image forming apparatus of 3, 4, 5, 6, 7, 8 or 9, the second transfer member is attached to the recording medium driven so as to pass through the third transfer position facing the second transfer member. A third transfer unit that transfers the upper image, wherein the moving speed of the image bearing surface of the first transfer body is equal to the moving speed of the image transfer surface of the third transfer body, The ratio (W of the transfer nip width W ₂ between the first transfer member and the second transfer member and the transfer nip width W ₃ between the second transfer member and the recording medium at the third transfer position (W ₂ / W ₃ ) is equal to the ratio (Vb1 / Vb2) of the moving speed Vb1 of the image transfer surface of the first transfer member and the moving speed Vb2 of the image transfer surface of the second transfer member. Is. According to an eleventh aspect of the present invention, in the image forming apparatus according to the first, second, third, fourth, fifth, sixth, seventh, eighth or ninth aspect, the third transfer position facing the second transfer member is passed. A recording medium driven by the third transfer means for transferring the image on the second transfer body,
The difference between the moving speed Vb1 of the image bearing surface of the transfer body and the moving speed Vb2 of the image transfer surface of the second transfer body is ΔVh (=
Vb1-Vb2), and the size of the pixel that expands and contracts under the influence of other transfer process conditions except the transfer nip width and the surface movement speed difference at the second transfer position and the third transfer position. When the influence coefficients defined by the ratio to the pixel size when there is no pixel are κ ₂ and κ ₂ , respectively, the moving speed Vb1 of the image bearing surface of the first transfer member and the third
Speed difference between the moving speed _{V 3} of an image transfer surface of the recording medium passing through the transfer position of the δ (= _{Vb1-V 3)} is, δ = (1-κ
₂ / κ ₃ ) · ΔVh relational expression is satisfied. The invention of claim 12 is the invention of claim 10 or 11.
In the image forming apparatus, the image of the second transfer member is formed between the transfer positions adjacent to each other on the moving path of the image transfer surface of the second transfer member from the second transfer position to the third transfer position. It is characterized in that the passing time of the transfer surface is a natural number times the cycle of the speed fluctuation occurring on the image transfer surface of the second transfer member. Here, the first
The moving speed of the image carrying surface of the transfer body may be equal to the moving speed of the image transfer surface of the third transfer body.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. [Embodiment 1] First, the overall configuration and operation of a tandem type color image forming apparatus according to a first embodiment of the present invention will be described. FIG. 2 is a schematic diagram of the overall configuration of the color image forming apparatus of this embodiment. This color image forming apparatus includes cyan (C), magenta (M), yellow (Y),
Four sets of toner image forming units 1C, 1M, 1Y, 1BK for forming an image of each color of black (BK) (hereinafter, subscripts C, M, Y, and BK of the respective symbols are cyan, magenta, and
Indicates that it is a member for yellow and black. ) Are sequentially arranged from the upstream side in the moving direction (the direction of arrow A in the figure) of the upper stretched portion of the intermediate transfer belt 40 as the first transfer body. The toner image forming units 1C, 1M, 1Y,
1BK is a photoconductor drum 11C, 11M, 11Y as an image bearing member, which is driven to rotate in the direction of arrow B in the figure.
11BK, charging rollers 12C, 12M, 12Y, 12BK as charging means for charging the surface of each photoconductor drum,
Developing unit 1 as developing means for developing an electrostatic latent image formed on the surface of each photoconductor drum to form a toner image.
3C, 13M, 13Y, 13BK, photoconductor cleaning unit 14 for cleaning the surface of each photoconductor drum
It is equipped with C, 14M, 14Y, 14BK and the like. Each of the developing units 13C, 13M, 13Y, and 13BK develops the electrostatic latent image on the corresponding photoconductor drum with a cyan toner, a magenta toner, a yellow toner, and a black toner, which are toners of different colors, to develop the electrostatic latent image of each color. A toner image is formed. Further, the toner image forming units 1C, 1M, 1Y, 1BK are arranged in a predetermined manner such that the rotation axes of the respective photosensitive drums are parallel to each other and in the moving direction (direction A) of the upper tension portion of the intermediate transfer belt. It is set to be arranged at a pitch of.

In addition to the toner image forming portions 1C, 1M, 1Y, 1BK, the color image forming apparatus irradiates the uniformly charged surface of each photosensitive drum with laser light L corresponding to image information. An optical writing unit 3 as an exposing means for forming an electrostatic latent image, a sheet feeding cassette (not shown), a registration roller pair (not shown), an intermediate transfer unit, a fixing device (not shown), a sheet discharge tray (not shown), and the like are provided. In addition,
Image forming means for forming an image on each photoconductor drum is composed of the toner image forming sections 1C, 1M, 1Y, 1BK charging rollers, a developing unit and a photoconductor cleaning unit, and the optical writing unit 3. It

The optical writing unit 3 is provided with a laser light source, a polygon mirror, an f-θ lens, a reflection mirror and the like, which are not shown, and is rotated at each predetermined exposure position Pex based on image data. Drums 11C, 11
The surface of M, 11Y, 11BK is irradiated with the laser beam L while scanning in the main scanning direction.

The intermediate transfer unit includes an endless belt-shaped intermediate transfer belt 40 as a first transfer member onto which an image on each photosensitive drum is transferred. The intermediate transfer belt 40 includes a belt driving roller 41 as a driving rotating body that is driven while being in contact with the intermediate transfer belt 40, a transfer opposing roller 42, a driven roller 43, and the intermediate transfer belt 4
Tension roller 48 for applying a predetermined tension to 0
And is driven to rotate in the direction of arrow A in the figure at a predetermined timing. The intermediate transfer unit also includes pressure contact rollers 44, 45, 46 for pressing the intermediate transfer belt 40 against the surface of each photoconductor drum with a predetermined pressure contact force. Further, inside the intermediate transfer unit, a transfer charge is applied at the first transfer position Pt1 located on the opposite side of the exposure position Pex with the photosensitive drums sandwiched therebetween, and the toner on each photosensitive drum is provided. Image transfer belt 40
Transfer corona chargers 5C, 5M, 5Y, and 5BK are provided as first transfer means for transferring onto the top. Further, at the second transfer position Pt2 where the image on the intermediate transfer belt 40 is transferred onto the transfer paper 2 as the second transfer body, the intermediate transfer belt 40 is sandwiched and the transfer opposing roller 42 is opposed. As a second transfer means,
A next transfer roller 47 is provided.

The belt drive roller 41 is rotationally driven by the drive force of a drive motor 50 as a drive source being transmitted through a drive transmission system including drive transmission members such as gears 51 and 52.

In the color image forming apparatus having the above structure,
For example, in the cyan toner image forming unit 1C, the surface of the photosensitive drum 11C uniformly charged by the charging roller 12C is irradiated with the laser light L modulated and deflected by the optical writing unit 3 while being scanned. And the photoconductor drum 1
An electrostatic latent image is formed on the surface of 1C. Then, the electrostatic latent image on the photosensitive drum 11C is developed by the developing unit 13C to become a cyan toner image. Intermediate transfer belt 4
At the first transfer position Pt1 where 0 passes, the toner image on the image bearing surface (outer peripheral surface) of the photosensitive drum 11C is transferred to the image transfer surface (outer peripheral surface) of the intermediate transfer belt 40. The surface of the photoconductor drum 11C after the transfer of the toner image is cleaned by the photoconductor cleaning unit 14C and is neutralized by a neutralization unit (not shown) to prepare for the formation of the next electrostatic latent image. The above toner image forming process is performed for the other photoconductor drums 11M, 11Y, and 11BK.
This is executed in accordance with the movement of the intermediate transfer belt 40, and the toner image of each color on each photoconductor drum is transferred onto the intermediate transfer belt 40 in an overlapping manner. On the other hand, the transfer sheet 2 fed from a sheet feeding cassette (not shown) is conveyed by a conveying roller while being guided by a conveying guide (not shown), and is sent to a temporary stop position where a pair of registration rollers is provided. The transfer sheet 2 sent out at a predetermined timing by the registration roller pair is conveyed so as to pass a second transfer position Pt2 facing the intermediate transfer belt 40. The transfer paper 2 onto which the color image is transferred on the intermediate transfer belt 40 is discharged onto a paper discharge tray (not shown) after the toner image is fixed by a fixing device (not shown).

Next, in the color image forming apparatus having the above-described structure, the line width ( A configuration and the like for reducing expansion and contraction of pixels) and improving image quality will be described. Less than,
If you want to give a common explanation about the members corresponding to each color,
The subscripts Y, M, C, and BK of the reference numerals of the respective members are omitted as necessary.

(Measures Against Variations in Diameter of Photoreceptor Drums) First, countermeasures against variations in diameter of the photoreceptor drums will be described. As shown in FIG. 3, the photosensitive drum 11
The photosensitive drum drive unit (not shown) is controlled so as to change the rotational angular velocities (ω ₁ , ω ₂ ) of the photosensitive drum 11 according to the size of the diameter. As a result, it is attempted to reduce the fluctuation of the speed difference (relative speed) between the surface moving speed (peripheral speed) Vd of the photosensitive drum 11 and the surface moving speed Vb of the intermediate transfer belt 40 at the first transfer position Pt1. To do. Furthermore, in order to eliminate the occurrence of color misregistration of the image formed by each photoconductor drum and transferred to the intermediate transfer belt 40, the exposure timing (t1, t2) to each photoconductor drum 11 is set as shown in FIG. The diameter is changed according to the diameter of the photosensitive drum 11. Each of t1 and t2 in the figure is an elapsed time from the control reference time. For example, when the diameter of the photoconductor drum 11 is large, the rotational angular velocity of the photoconductor drum is slowed as described above, so that the exposed portion is at the first transfer position Pt.
It takes a long time to move to 1. Therefore, the exposure data is sent to the optical writing unit 3 earlier, and the exposure timing is advanced.

When the diameter of the photoconductor drum 11 varies by ΔRo, the rotational angular velocity of the photoconductor drum 11 is controlled so that the speed difference (relative speed) at the first transfer position Pt1 is constant ΔVh. , The following advantages will come out. The rotational angular velocity ω for maintaining the peripheral velocity when there is no error in the diameter of the photosensitive drum 11 is (Ro + ΔRo) ω = Ro
From ωo, it is expressed by the following equation.

(6) ω = {Ro / (Ro + ΔRo)} ωo

The length Ie of the exposure pixel on the photosensitive drum 11 per unit time is Ie = (Ro + ΔRo) ω = Roωo
Therefore, the exposed pixel does not extend. Then, when the speed difference of the moving speeds at the first transfer position Pt1 is ΔV = ΔVh, the change (expansion amount or contraction amount) δI of the line width of the image is expressed by the following equation. Here, W ₁ in the equation is the transfer nip width, and Iw is the line width of the image on the photosensitive drum.

[Formula 7] δI = (W ₁ + Iw) · ΔVh / Vd = (W ₁ + Roωo) · ΔVh / (Roωo) = W ₁ · ΔVh / (Roωo) + ΔVh

As can be seen from this equation, even if the diameter of the photosensitive drum 11 varies from one photosensitive drum to another, the expansion and contraction of pixels can be prevented from being affected.

(Countermeasure 1 for Reducing Pixel Expansion / Contraction) Further, in the image forming apparatus of this embodiment, the expansion / contraction of pixels generated in the image on the transfer paper due to the periodical speed fluctuation of the intermediate transfer belt 40 can be prevented. is doing. The periodic speed fluctuation of the intermediate transfer belt 40 causes the belt drive roller 41,
It is caused by eccentricity of gears, timing belts, pulleys, driven rollers 43, etc. forming a drive transmission system from the drive motor, tooth accumulated pitch error, and the like. This intermediate transfer belt 40
Regarding the color misregistration that occurs due to the periodical speed fluctuation of the intermediate transfer belt 40, the time required for the intermediate transfer belt 40 to pass through the interval between the adjacent photoconductor drums, that is, the interval between the adjacent first transfer positions Pt1 is the speed of the intermediate transfer belt 40. If it is a natural number multiple of the fluctuation period, color shift will not occur by the conventional technique. However, due to the periodical speed fluctuation of the intermediate transfer belt 40, between the surface moving speed (peripheral speed) of the photosensitive drum 11 and the moving speed of the intermediate transfer belt 40 at the first transfer position of each photosensitive drum. There may be a speed difference that fluctuates periodically. When such a speed difference occurs, the expansion and contraction of pixels may occur in the image on the transfer paper 2 transferred from the intermediate transfer belt 40 as described above.

In order to reduce the expansion and contraction of the pixels, in the image forming apparatus of this embodiment, each photosensitive drum 11C,
The transfer paper 2 at the second transfer position Pt2 has an average surface moving speed (average peripheral speed) of 11M, 11Y, and 11BK, respectively.
It is configured to be equal to the moving speed of. Further, the passage time for the intermediate transfer belt 40 to pass the distance from each first transfer position Pt1 to the second transfer position Pt2 is set to be a natural number times the cycle of the speed fluctuation occurring in the intermediate transfer belt 40. is doing. Further, each second transfer position Pt2
When the passing time of the intermediate transfer belt 40 for the distance from the first transfer position Pt1 to the first transfer position Pt1 is set to be a natural number times the cycle of the speed fluctuation occurring in the intermediate transfer belt 40,
Furthermore, the expansion and contraction of pixels is reduced.

FIG. 1 is an explanatory diagram for explaining the passage time (Tbd0, Tbd1, Tbd2, Tdp1, Tdp2) for the intermediate transfer belt 40 to pass between the transfer positions in the image forming apparatus of this embodiment. In FIG. 1, Tbd0,
Tbd1 and Tbd2 indicate the passage time for passing between the adjacent first transfer positions Pt1. Further, Tdp1 is moved by a distance between the first transfer position Pt1 and the second transfer position Pt2 facing the black photoconductor drum 11BK located on the most downstream side in the moving direction of the intermediate transfer belt. It shows the passage time of passing through. Further, the intermediate transfer belt 40 moves Tdp2 from the second transfer position Pt2 to a first transfer position Pt1 facing the cyan photosensitive drum 11C located on the most upstream side in the moving direction of the intermediate transfer belt. It shows the passing time. The conditions for reducing the expansion and contraction of the pixel using these symbols are as follows.

[Equation 8] Vda = Vp Tdp1 = M1 × Tr Tdp2 = M2 × Tr Tbd0 = Tbd1 = Tbd2 = M3 × Tr

Here, Vda is the average surface moving speed (average peripheral speed) of the photosensitive drum, and Vp is the second transfer position P.
It is the moving speed of the transfer paper 2 at t2. Further, Tr is a cycle of speed fluctuation of the intermediate transfer belt 40. M1-M
3 is a natural number.

When the equation (8) is satisfied, the passage time (T
bd0, Tbd1, Tbd2, Tdp1, Tdp2) is a natural number times the speed fluctuation of the intermediate transfer belt 40. Tdp2 =
When the condition of M2 × Tr is added, each frequency component of the periodic fluctuation of the intermediate transfer belt becomes more sinusoidal, and the expansion and contraction of pixels can be further reduced. Even if the condition of Vda = Vp does not hold, if the following equation (Equation 9) holds,
If the pixel shrinks at the first transfer position Pt1, the pixel becomes stretched at the second transfer position Pt2, so it is clear that the expansion and shrinkage of the pixel can be reduced even in this case.

[Equation 9] Tdp1 = M1 × Tr Tdp2 = M2 × Tr Tbd0 = Tbd1 = Tbd2 = M3 × Tr

In the actual construction of the image forming apparatus, the diameter of the belt drive roller 41, the diameter of the gear of the drive transmission system, the timing belt are satisfied so that the conditions shown in the equations (8) and (9) are satisfied. Select the length, the diameter of the pulley, etc.

As described above, in this embodiment, the transit time required for the intermediate transfer belt 40 to pass the distance from the first transfer position Pt1 to the second transfer position Pt2 is the moving speed generated in the intermediate transfer belt 40. It is a natural number times the cycle of fluctuations. Therefore, at the first transfer position Pt1, any pixel of the image on the photoconductor drum 11 is transferred to the intermediate transfer belt 40.
The transfer speed of the intermediate transfer belt 40 at the time of the primary transfer to the transfer paper 2 is the same as that at the time of the primary transfer to the transfer paper 2 at the second transfer position Pt2.
Moreover, the surface moving speed of the photosensitive drum 11 is equal to the moving speed of the transfer paper 2. Therefore, the speed difference between the photoconductor drum 11 and the intermediate transfer belt 40 when any pixel is primarily transferred, and the intermediate transfer belt 40 and the transfer paper 2 when the pixel is secondarily transferred. Will be the same as the speed difference between. Therefore, even when the moving speed of the intermediate transfer belt 40 periodically changes, for example, when the pixel extends at the first transfer position Pt1 due to the change, the pixel contracts at the second transfer position Pt2 by the extension. The expansion and contraction of the pixels transferred onto the transfer paper 2 can be reduced.

In the above embodiment, the case where the full-color image is formed on the intermediate transfer belt 40 by passing the intermediate transfer belt 40 once through the four first transfer positions Pt1 has been described. On the other hand, the intermediate transfer belt 4
The image transfer area at the same position on 0 is the first transfer position Pt1.
There may be a case where a full-color image is formed on the intermediate transfer belt 40 by passing a plurality of times. In the latter case, in the equations of the above equations 8 or 9,
It is preferable that the transit time for the intermediate transfer belt 40 to pass the distance from the second transfer position Pt2 to the first transfer position Pt1 be a natural number times the cycle of the speed fluctuation occurring in the intermediate transfer belt 40. . In this case, the image transfer area on the intermediate transfer belt 40 on which the image is transferred passes directly through the second transfer position Pt2 and reaches any one of the first transfer positions Pt1 so that an arbitrary pixel of another image is transferred. Intermediate transfer belt 4
Primary transfer to 0. The moving speed of the intermediate transfer belt 40 during the primary transfer is also the intermediate transfer when the pixel on the intermediate transfer belt 40 is secondarily transferred onto the transfer paper 2 at the second transfer position Pt2 according to the above additional condition. It becomes the same as the moving speed of the belt 40. Therefore, expansion and contraction of the pixels transferred onto the transfer paper 2 can be reduced.

In the above embodiment, the tandem type color image forming apparatus in which four photoconductor drums are arranged side by side has been described. It can also be applied to a monochrome image forming apparatus. In this case, the passage time for the intermediate transfer belt to pass the distance from the first transfer position to the second transfer position is set to be a natural number times the cycle of the speed fluctuation occurring in the intermediate transfer belt 40. Particularly in the case of a multiple transfer type color image forming apparatus, a color image is formed on the intermediate transfer belt by the image transfer area on the intermediate transfer belt passing the first transfer position a plurality of times. Therefore, in this case, the transit time for the intermediate transfer belt to pass the distance from the second transfer position where the transfer is performed to the transfer sheet to the first transfer position is the natural period of the speed fluctuation cycle that occurs in the intermediate transfer belt. Make it several times.

FIG. 5 is an explanatory diagram for explaining the transit time (Tdp1, Tdp2) for the intermediate transfer belt 40 to pass between the transfer positions in the image forming apparatus having only one photosensitive drum. In FIG. 5, Tdp1 indicates that the intermediate transfer belt 40 moves a distance from the first transfer position Pt1 facing the photoconductor drum 11 to the second transfer position Pt2 along the moving path of the intermediate transfer belt 40. The passing time to pass is shown. Further, Tdp2 is the intermediate transfer belt 40.
From the second transfer position Pt2 along the moving path of
The transfer time of the intermediate transfer belt 40 moving and passing the distance to the transfer position Pt1 is shown. Using these symbols, the conditions for reducing the expansion and contraction of the pixel when there is one photoconductor drum are expressed by the following equation.

[Equation 10] Vda = Vp Tdp1 = M × Tr Tdp2 = L × Tr

Here, Vda is the average surface moving speed (average peripheral speed) of the photosensitive drum, and Vp is the second transfer position P.
It is the moving speed of the transfer paper 2 at t2. Further, Tr is a cycle of speed fluctuation of the intermediate transfer belt 40. M and L
Is a natural number. Under the above-described conditions, even if the intermediate transfer belt undergoes periodic speed fluctuations due to eccentricity of the belt drive roller, the same pixel can be transferred from the photosensitive drum to the intermediate transfer belt during the primary transfer. The speed of the intermediate transfer belt and the speed of the intermediate transfer belt during the secondary transfer onto the transfer paper become equal. Tdp even when Vda ≠ Vp
If 1 = M × Tr and Tdp2 = L × Tr are established, the expansion / contraction of pixels is reduced.

(Pixel Expansion / Reduction Mitigation Measure 3) Next, another pixel expansion / contraction mitigation measure in the image forming apparatus having the above configuration will be described. The total shrinkage amount E2 at the second transfer position Pt2 can be obtained as follows. First,
The contraction δI ₂ of the pixel Ie = Roωo on the transfer paper 2 which is the second transfer body is expressed by the following equation.

(11) δI ₂ = (W ₂ + Ie) · ΔV ₂ / Vt ₁

However, W ₂ is the transfer nip width at the second transfer position, and ΔV ₂ is the moving speed difference (relative speed, ΔV ₂ = Vt ₁ −V ₂ = Vb−V) at the second transfer position. ₂ ). Further, Vt ₁ is the linear velocity (= Vb) of the intermediate transfer belt which is the first transfer member, and V ₂ is the moving velocity of the transfer paper 2. Here, ΔV ₂ + ΔVh = δ (or Vd =
Since the relationship of V ₂ + δ) is established, δI ₂ is given by the following equation.

[Equation 12] δI_Two= (W_Two+ Ie) / ΔV_Two/ Vt₁       = W_Two・ ΔV_Two/ Vb + Ie · ΔV_Two/ Vb       = W_Two・ (Δ-ΔVh-δU) / (ωoRo-ΔVh-δU)         + [Roωo] ・ (δ-ΔVh-δU) / (ωoRo-ΔVh-δU)

However, since the time for forming the same pixel is different, δU is the belt speed variation amount at the second transfer position, and δV is the velocity variation amount at the first transfer position. The total shrinkage amount for E2 is as follows. Pixels I _w1 = Ie−E per unit time are formed on the second transfer member, which causes pixel shrinkage at the second transfer portion, and the total shrinkage amount E2 is obtained. That is, the last formed pixel I _w2 per unit time is Ie in the second transfer member pixel I _w1 and Ie−δI ₂ = I in the second transfer portion.
From the relationship of e · (1-δ · I ₂ / Ie), (1-δI ₂ /
Since it is multiplied by Ie), naturally E is also (1-δI ₂ / Ie). Therefore, it becomes as follows.

I _w2 = Ie−δI ₂ −E · (1−δI ₂ / Ie) E2 = δI ₂ + E · (1−δI ₂ / Ie)

Here, considering δI ₂ / Ie << 1, E2 = E + δI ₂ holds. After that, the total shrinkage amount E
For 2, use this formula.

[Equation 14] E2 = E + δI_Two     = W₁・ {ΔRoωo + (ΔVh + δV)} / {ωo (Ro + ΔRo)} + (Δ Vh + δV)       + W_Two・ (Δ-ΔVh-δU) / (ωoRo-ΔVh-δU)       + [Roωo] ・ (δ-ΔVh-δU) / (ωoRo-ΔVh-δU)     = W₁・ {ΔRoωo + (ΔVh + δV)} / {ωo (Ro + ΔRo)}       + (ΔVh + δV)       + W_Two・ (Δ-ΔVh-δU) / (ωoRo-ΔVh-δU)       + (Δ-ΔVh-δU) (However, ωoRo >> ΔVh + δU)

Here, W ₁ and W ₂ in the equation are the transfer nip widths at the first transfer position and the second transfer position, respectively. Further, Ro and ΔRo are the photoconductor drum 1 respectively.
No. 1 diameter and its variation. Further, ωo is the rotational angular velocity of the photosensitive drum 11. ΔVh is a speed difference (= Vd−Vb) between the peripheral speed Vd of the photosensitive drum 11 and the moving speed Vb of the intermediate transfer belt 40 at the first transfer position Pt1, which is provided to prevent the occurrence of the hollow character phenomenon.
Is. δ is a speed difference (= Vd−Vp) between the peripheral speed Vd of the photosensitive drum and the moving speed Vp of the transfer paper 2. δV and δU are the speed fluctuation amount of the moving speed of the intermediate transfer belt 40 at the first transfer position and the speed fluctuation amount of the moving speed of the intermediate transfer belt 40 at the second transfer position, respectively.

From the relation expressed by the equation (14), the influence of the variation of the diameter of the photosensitive drum is removed, the peripheral speed Vd of the photosensitive drum is made constant (= ωoRo), and the relation of the equation (8) or (9) is applied. When the body drum, the intermediate transfer belt, and the second transfer position are arranged, the relationship of δV = δU is established, and therefore the following equation is established.

[Equation 15] E2 = W ₁ · (ΔVh + δV) / (ωoRo)
+ W ₂ · (δ-ΔVh-δV) / (ωoRo-ΔVh-δ
V) + δ

If δ = 0, the condition for E2 = 0 is as follows.

[Equation 16] E2 = W ₁ · (ΔVh + δV) / (ωoRo)
-W ₂ · (ΔVh + δV) / (ωoRo-ΔVh-δV)
= 0 The transfer nip width W ₂ at the second transfer position Pt2 is summarized as follows.

[Expression 17] W ₂ = (ωoRo-ΔVh-δV) · W ₁ /
(ΩoRo)

Since δV fluctuates here, if δV = 0, the following equation is obtained.

[Equation 18] W ₂ = {1-ΔVh / (ωoRo)} · W ₁

When the peripheral speed of the photosensitive drum when there is no error in the diameter and eccentricity of the photosensitive drum is Vdo, the following equation is obtained.

## EQU19 ## W ₂ / W ₁ = Vb / Vdo (or W ₁ /
W ₂ = Vdo / Vb)

Therefore, the total shrinkage amount E at the second transfer position
In order to set 2 to zero, the transfer nip widths W ₁ and W ₂ and the transfer paper speed Vp may be set so as to satisfy the following equation.

Δ = Vd−Vp = 0 W ₂ / W ₁ = Vb / Vdo (or W ₁ / W ₂ = Vdo
/ Vb)

By setting the transfer nip width and the transfer paper speed in this manner, it is possible to reduce the expansion and contraction of pixels due to the periodical speed fluctuation of the intermediate transfer belt, as compared with the prior art.

(Pixel Expansion / Contraction Reduction Countermeasure 3) Next, still another pixel expansion / contraction reduction countermeasure in the image forming apparatus having the above configuration will be described. The discussion so far has been described on the assumption that the expansion / contraction of pixels due to the difference in moving speed (relative speed) between the first transfer position Pt1 and the second transfer position Pt2 depends on the transfer nip width. However, actually, the first transfer position Pt1 and the second transfer position Pt2 have different transfer process conditions other than the transfer nip width. Therefore, the amount of expansion and contraction of the pixel is influenced by the transfer process conditions other than the transfer nip width, and may differ even with the same transfer nip width. For example, when a lubricant is applied to the surface of the intermediate transfer belt 40, the amount of expansion and contraction of pixels changes. Focusing on this point, the influence coefficients κ ₁ and κ ₂ are defined as the degree of influence of the transfer process condition other than the transfer nip width on the expansion / contraction amount of the pixel. The influence coefficients κ ₁ and κ ₂ are respectively the first transfer position Pt1 and the second transfer position Pt.
2 is defined as the ratio of the size of the pixel that expands and contracts under the influence of other transfer process conditions except the transfer nip width and the speed difference in 2 to the size of the pixel when there is no such influence.
For example, when a lubricant such as zinc stearate is applied to improve the cleaning property of toner or the like on the intermediate transfer belt 40, the expansion and contraction of pixels with respect to the speed difference at the transfer position becomes small. That is, the influence coefficients κ ₁ and κ ₂ are smaller than 1. This influence coefficient is determined by experimentally keeping the speed of the intermediate transfer belt constant, changing the rotational angular velocity of the photoconductor drum with a small eccentricity, and exposing the basic pixel on the photoconductor drum. Is measured by measuring. At this time, the transfer nip width is changed by changing the pressure contact force of the transfer roller. Hereinafter, the influence of the transfer process condition on the expansion / contraction amount of the pixel will be described using the influence coefficients κ ₁ and κ ₂ .

The expansion / contraction amount δ _1κ of the pixel at the first transfer position Pt1 between the photosensitive drum 11 and the intermediate transfer belt 40 is expressed by the following equation.

[Equation 21] δ_1κ= Κ₁・ {W₁+ (Ro + ΔRo) ωo} ・ (ΔRoωo + ΔVh + δV) / { ωo (Ro + ΔRo)}       = Κ₁・ W₁・ (ΔRoωo + ΔVh + δV) / {ωo (Ro + ΔRo)} + κ ₁ ・ (ΔRoωo ＋ ΔVh + δV)

Due to the influence of the transfer nip width W ₁ and the speed difference ΔVh, an error (contraction) E _κ expressed by the following equation is generated at the first transfer position Pt1.

[Equation 22] E _κ = κ ₁ · W ₁ · (ΔRoωo + ΔVh + δ
V) / {ωo (Ro + ΔRo)} + κ ₁ · (ΔVh + δV)
＋ (κ ₁ -1) ・ ΔRoωo

The case where κ ₁ = 1 is the content discussed in the measures 1 and 2 for reducing pixel expansion and contraction. κ
_{When 1 1} ≠ 1, the expansion / contraction correction function of the pixel at the time of exposure due to the variation in the diameter of the photoconductor drum, which is shown in the third term at the end of the equation (22), does not work. The same applies to the eccentricity of the photosensitive drum 11. That is, κ ₁ =
Although it was possible to cancel when 1, the expansion and contraction of pixels occurred when κ ₁ ≠ 1. Therefore, κ ₁ ≠
When 1, the condition must be set such that ΔRoωo = 0. This condition is "speed difference ΔVh to prevent hollow character phenomenon" and "intermediate transfer belt speed fluctuation δV".
When is excluded, it is equivalent to "the speed difference must be zero because the photosensitive drum has an eccentricity and a diameter variation."

Let us look at the canceling effect on the expansion and contraction of pixels at the first transfer position and the second transfer position due to the speed difference (relative speed) between the photosensitive drum 11 and the intermediate transfer belt 40. The following discussion is about the variation in the diameter of the photosensitive drum, but if the photosensitive drum is eccentric, it can be considered that the eccentricity is added. Incidentally, as a countermeasure against the eccentricity of the photosensitive drum, it is possible to deal with it by a method described later.

The shrinkage amount δI ₂ of the pixel on the transfer paper 2 is expressed by the following equation. However, since the time for forming the same pixel is different, δV is the variation amount of the moving speed of the intermediate transfer belt 40 at the first transfer position Pt1, and δU is the second amount.
The speed fluctuation amount of the intermediate transfer belt 40 at the transfer position Pt2 of

[Expression 23] δI ₂ = κ ₂ · (W ₂ + Ie) · ΔV ₂ / Vb = κ ₂ · W ₂ · ΔV ₂ / Vb + κ ₂ · Ie · ΔV ₂ / Vb = κ ₂ · W ₂ · (δ-ΔVh -δU) / (ωoRo-ΔVh- δU) + κ 2 · Roωo · (δ-ΔVh-δU) / (ωoRo-ΔVh-δU) ≒ κ 2 · W 2 · (δ-ΔVh-δU) / (ωoRo-ΔVh -ΔU) + κ ₂ · (δ-ΔVh-δU)

Therefore, the total shrinkage amount E2 at the second transfer position Pt2 is given by the following equation.

[Equation 24] E2 = E _κ + δI ₂ = κ ₁ · W ₁ · (ΔRoωo + ΔVh + δV) / {ωo (Ro + ΔRo)} + κ ₁ · (ΔVh + δV) + (κ ₁ −1) ΔRoωo + κ ₂ · W ₂・ (Δ-ΔVh-δU) / (ωoRo-ΔVh-δU) + κ ₂・ (δ-ΔVh-δU)

From the relationship expressed by the equation (24), the influence of the variation in the diameter of the photosensitive drum is removed, the peripheral speed of the photosensitive drum is set to a constant ωoRo, and the photosensitive drum, the intermediate transfer belt and the second transfer position are set in the above relationship. , Then δV = δU
Since the relation of is established, the following equation is established.

[Equation 25] E2 = κ ₁ · W ₁ · (ΔVh + δV) / {ωo
Ro} + κ ₁ · (ΔVh + δV) + κ ₂ · W ₂ · (δ-Δ
Vh-δV) / (ωoRo-ΔVh-δV) + κ ₂ · (δ
-ΔVh-δV)

The condition that E2 = 0 holds is as follows.

[Expression 26] κ ₁ · (ΔVh + δV) + κ ₂ · (δ−ΔVh−δV) = 0 (α) κ ₁ · W ₁ · (ΔVh + δV) / {ωoRo} + κ ₂ · W ₂ · (δ-ΔVh-δV) / (ωoRo-ΔVh-δV) = 0 W ₁ / {ωoRo} -W ₂ / (ωoRo-ΔVh-δV) = 0 W ₁ / {ωoRo} = W ₂ / (ωoRo -ΔVh-δV) (β)

In the above formula (α), the intermediate transfer belt 4
Ignoring the variation δV of the moving speed of 0, the following equation is obtained.

[Expression 27] κ ₁ · ΔVh = κ ₂ · (ΔVh−δ) δ = (1−κ ₁ / κ ₂ ) · ΔVh κ ₂ · δ = Δκ · ΔVh However, Δκ = κ ₂ −κ ₁ ... (Αo)

In the expression (β), the variation of the moving speed of the intermediate transfer belt 40 causes an error, but if δV = 0 is set, the following expression is obtained.

[Equation 28] W ₂ = W ₁ · {(1-ΔVh / (ωoRo)}

When the peripheral speed of the photosensitive drum 11 when there is no error in the diameter and eccentricity of the photosensitive drum 11 is Vdo,
It becomes like the following formula.

[Formula 29] W ₂ / W ₁ = Vb / Vdo (βo)

Transfer nip width W at the second transfer position Pt2
Choose ₂ to follow the above formula. In addition, the above equation (α
If the moving speed of the transfer paper 2 and the transfer nip width W ₂ at the second transfer position Pt2 are set so as to satisfy the relationship of o) and the expression (βo), the image quality with less expansion and contraction of pixels can be obtained.

(Countermeasures for Reducing Expansion / Contraction of Pixels at First Transfer Position) Next, a description will be given of measures for reducing expansion / contraction of pixels due to eccentricity of the photosensitive drum due to exposure data correction or the like. At the same time, the influence of variations in the diameter of the photosensitive drum will be described. As for the eccentricity error of the photoconductor drum, the exposure data is corrected by detecting the eccentricity angle and the amplitude. This principle is disclosed in JP-A-200
It is described in Japanese Patent Publication 1-333761. The publication also mentions variations in the diameter of the photosensitive drum.

Modeling is performed as shown in FIG. In FIG. 6, when ε is the eccentric amount and θ is the angle of the eccentric position from the x-axis, the moving speed of the contact point T between the belt and the drum is as follows in coordinate display.

[Equation 30] (-Εsinθ · ω, εcosθ · ω), ω = dθ / dt (1)

Direction S of rotation around the drum rotation center O
The velocity Vs of is as follows.

[Equation 31] Vs = Vcosα−εsinθ · ω · cosα + εcosθ · ω · sinα (2)

However, V is the moving speed of the intermediate transfer belt,
α is an angle formed by a line orthogonal to a line r connecting the contact point T from the drum rotation center O at the position of the contact point T and the surface of the intermediate transfer belt. Therefore, the rotational angular velocity ω of the photosensitive drum 11 is given by the following equation.

[Equation 32] ω = Vs / r   = (Vcosα-εsinθ ・ ω ・ cosα + εcosθ ・ ω ・ sinα) / r (3 )

Here, the following equation is obtained from the cosine formula. However, R is the radius of the photosensitive drum 11.

[Equation 33] r ² = R ² + ε ² −2Rεcos (π / 2−θ) = R ² + ε ² −2Rε sin θ (4)

Further, the following equation is obtained from the sine theorem.

[Equation 34] ε / sin α = r / sin (π / 2−θ) = r / cos θ (5) sin α = ε cos θ / r, cos α = (R−ε sin θ) / r (6)

By substituting the equations (4) and (6) into the above equation (3), the following equation is obtained.

Equation 35] ω = {V · R- (V + ωR) εsinθ + ωε 2} / (R 2 + ε 2 -2Rεsinθ) ω (R 2 + ε 2 -2Rεsinθ) = V · R- (V + ωR) εsinθ + ωε 2 V = Rω ··・ (7)

Even if the photosensitive drum 11 is eccentric, the first transfer position between the photosensitive drum 11 and the intermediate transfer belt 40 is set as shown in FIG. ω and the moving speed V of the intermediate transfer belt 40 are V =
If the relationship of Rω is satisfied, a slip-free condition can be obtained. Then, the moving speed Vv of the intermediate transfer belt 40
Is Vv = V + ΔVh and V = Rω, the slip velocity (relative velocity) at the first transfer position (vertical position of the photosensitive drum circular cross section) is constant ΔVh. In principle, this configuration is applied to the photosensitive drum 11 and the intermediate transfer belt 40 at the transfer position.
The speed difference (relative speed) is constant because it satisfies the same condition as the suspension. The speed difference between the moving speed of the intermediate transfer belt 40 at the first transfer position and the peripheral speed of the photosensitive drum 11, which is likely to occur due to the eccentricity of the photosensitive drum 11, is corrected by moving the transfer position. Because they are doing it.

(Setting of Exposure Data Generation Timing and Exposure Position) Next, a description will be given of the exposure data generation method and the like when there are variations in the diameter of the photosensitive drum and eccentricity. Figure 7
At, the angle Θt of the first transfer position Pt1 is obtained from the exposure position Pex. In FIG. 7, the transfer position is determined by a dotted-line triangle OGE (notable “transfer triangle” related to transfer) determined at the moment of exposure. That is, the drum center of gravity (center of the drum circular cross section) G was exposed at the position of the rotation angle θ (angle GOx) (referred to as “eccentric position” here) (on the photosensitive drum shown by the dotted line in FIG. 7). After the image is rotated by the rotation angle Θt, the image is shifted from the ideal transfer position (x = 0) (x =
-S). The rotation angle Θt from the exposure of the photosensitive drum 11 to the transfer is expressed by the following equation.

Θt = π−β (8) where β is the angle GEO. Further, when the following relational expression (9) between the angles β and θ is used, the rotation angle Θt becomes
It becomes like 0).

(37) sinβ = (ε / R) · cosθ (9) Θt = π−sin ⁻¹ {(ε / R) cosθ} (10)

When the contact portion between the intermediate transfer belt 40 and the photoconductor drum 10 has a maximum value (vertex) in the belt side direction of the photoconductor circular cross section, the rotation from stable exposure to transfer is performed. The angle Θt is obtained. Based on this information, high-quality image data in which image distortion and color misregistration are corrected can be generated.

For the generation of image data, the main scanning image generation timing is adjusted so that the pixels that should be there are always transferred to the ideal transfer position. When the diameter of the photosensitive drum is ideal, the transfer is performed after moving by .pi.Ro. However, when the photosensitive drum 11 is eccentric and the diameter of the drum varies, the image is transferred after the drum rotation angle Θt and the transfer position deviates from the ideal transfer position T by -s. The transfer image on the intermediate transfer belt 40 moves at a speed V after transfer. Then, the exposure data is transferred by −s from the ideal transfer position T after θt / ω = τ. That is, the transfer is performed after the intermediate transfer belt 40 has moved the distance of Vτ. When the radius of the ideal photoconductor drum 11 is Ro and the drum rotation angular velocity at that time is ωo, the following equation holds.

[Equation 38] V = Roωo

If the photoconductor drum is ideal, then π / ωo =
It should be transcribed after τo time. Therefore, on the intermediate transfer belt 40, the image that should be at x = Vτo is x =
Can be Vτ. That is, an ideal image can be formed by generating image data corresponding to x = Vτ on the exposure side. After all, it is sufficient to generate the data before d = V · (τo−τ).

V = Rω = Roωo (11) Θt = π-sin ^-1 {(ε / R) cosθ} (12) d = V · (π / ωo-Θt / ω) = R · [πω / ωo−π + sin ⁻¹ {(ε / R) cos θ}] (13) d = π · (Ro-R) + Rsin ⁻¹ {(ε / R) cos θ} (14) )

If the photosensitive drum 11 has no eccentricity, d
This means that the exposure data may be generated by shifting the value by π (Ro-R). When the diameter of the photosensitive drum varies toward the larger side, data after d minutes may be generated. At this time, the peripheral speed of the photosensitive drum 11 is constant V
Therefore, the main scanning pitch is constant. That is, it is sufficient to shift the data by d according to the above equation (14). The exposure data is sent (advanced) ahead of time or delayed depending on the diameter or eccentricity of the photosensitive drum and the angle θ.

When the exposure position is different from that shown in FIG. 7, the exposure data delayed or advanced with respect to FIG. 7 is generated by the rotation time corresponding to the rotation angle up to the exposure position Pex (E) shown in FIG. do it.

Therefore, when a speed difference (relative speed) is provided between the photosensitive drum 11 and the intermediate transfer belt 40 at the first transfer position as a measure against the hollow character phenomenon, the side of the intermediate transfer belt is set. If it is deviated from the reference speed, the exposure data generation timing may be changed by d corresponding to the above equation (14). Further, in order to correct the variation in the photosensitive drum mounting position (interval), the exposure image generation timing for a time corresponding to each drum interval belt passing time error from the ideal photosensitive drum interval belt passing time can be changed. Good.

Instead of shifting the exposure data by d according to the above equation (14), the exposure position of the exposure beam in the sub-scanning direction may be changed to a position corresponding to shifting by d. Good. In the case of the optical writing unit 3 of the type in which the laser beam is scanned on the photosensitive drum 11 by the polygon mirror, the movement of the exposure beam in the sub-scanning direction is more than the main scanning width immediately before the exposure on the photosensitive drum 11. An oscillating mirror having a length mirror may be provided and the oscillating mirror may be driven for correction. Further, in the case of the optical writing unit 3 using the LED (light emitting diode) array, a mechanism for changing the exposure position by the LED array may be provided or the exposure timing of the main scanning may be changed.

(Stabilization of the speed difference at the first transfer position) Next, the speed difference (relative speed) between the peripheral speed of the photosensitive drum 11 and the moving speed of the intermediate transfer belt 40 at the first transfer position. A configuration for stabilizing the will be described. First
At the transfer position, the center of the contact portion (transfer nip portion) where the photosensitive drum 11 and the intermediate transfer belt 40 are in contact with each other is located at the maximum value (vertex) in the intermediate transfer belt side direction of the circular cross section of the photosensitive drum. It is preferable that it is configured as follows. With this configuration, the speed difference (relative speed) between the peripheral speed of the photosensitive drum and the intermediate transfer belt speed at the contact portion (transfer nip portion) between the photosensitive drum 11 and the intermediate transfer belt 40 becomes substantially constant. , Or both can move almost integrally without slipping. The central portion of the transfer nip portion surely moves without slipping integrally.

The intermediate transfer belt 40 is preferably formed by a belt having a single layer or a plurality of layers, the surface of which has flexibility or elasticity. For example, an elastic rubber layer typified by conductive silicone rubber is provided on a belt base made of polyimide or the like. Further, the surface thereof may be provided with a surface layer for improving toner releasability or cleaning property. If such a configuration is adopted, the rigidity of the intermediate transfer belt 40 in the moving direction is increased, and the thickness direction of the intermediate transfer belt is flexible or elastic.

When such an intermediate transfer belt 40 is used, the maximum value (vertex) is the center position of the transfer nip width W ₁ in FIGS. 8A and 8B. Therefore, if the rotational angular velocity of each photoconductor drum 11 is controlled to be constant corresponding to the photoconductor drum diameter, even if each photoconductor drum 11 has eccentricity and diameter variation, the photoconductor drums at the first transfer position The speed difference (relative speed) between the peripheral speed and the intermediate transfer belt speed can be made substantially constant, or the surfaces of both can be made to slide almost integrally. With respect to the eccentricity of the photosensitive drum 11, the speed difference (relative speed) near the center of the transfer nip width W ₁ becomes constant, or both surfaces at that position slide integrally.

If the transfer nip width W ₁ changes due to eccentricity, the expansion and contraction of pixels may change. Therefore, as shown in FIGS. 9A and 9B, in place of the transfer corona charger 5, a primary transfer roller 401 as a first transfer unit is provided with high rigidity on the back surface side of the intermediate transfer belt 40. It may be provided so as to be in pressure contact with the belt base member.
By pressing the primary transfer roller 401, the flexible or elastic surface portion of the intermediate transfer belt 40 can be pressed against the surface of the photosensitive drum with a constant pressing force. By thus pressing the intermediate transfer belt 40 with a constant pressing force, even if the photosensitive drum 11 having a diameter variation or an eccentricity rotates, the intermediate transfer belt 40 has a certain flexibility or elasticity. The surface portion is the photosensitive drum 11.
The amount of bite into is almost constant. Therefore, the transfer nip width W ₁ can be kept substantially constant. Here, the amount of deformation and the amount of bending of the surface portion of the intermediate transfer belt 40 are set so that the amount of the surface portion of the intermediate transfer belt 40 that bites into the photosensitive drum 11 by the pressure contact of the primary transfer roller 401 becomes substantially constant. The condition that it does is necessary. That is, the flexibility or elasticity of the surface portion of the intermediate transfer belt 40 and the tension and rigidity of the intermediate transfer belt 40 are required so that this condition can be maintained. In the configuration example of FIGS. 9A and 9B, the pressure transfer mechanism as a means for pressing the intermediate transfer belt 40 to the photosensitive drum 11 with a constant pressure contact force at the first transfer position uses the above-mentioned primary transfer. Roller 401 and fixed frame 40
2, the swing arm member 403, and the spring 404 are used, but the configuration is not limited to this. In the configuration example of FIGS. 9A and 9B, one end of the swing arm member 403 is fixed to the fixed frame 4 on the apparatus main body side.
02 is attached to the shaft 402a. The other end of the swing arm member 403 is attached to the rotary shaft 401 a of the primary transfer roller 401. The swing arm member 403 is swingable around a shaft 402a on a fixed frame 402 on one end side thereof, and a spring 404 attached in the middle of the swing arm member 403 in the figure. It is biased to rotate counterclockwise. The primary transfer roller 401 is rotatable around the other end of the swing arm member 403.

On the other hand, in the structure of the conventional apparatus (see FIG. 10), when the photosensitive drum 11 is eccentric and the surface of the intermediate transfer belt has almost no flexibility or elasticity,
The first transfer position Pt1 does not reach the maximum value (vertex) of the circular cross section of the photosensitive drum in the direction of the intermediate transfer belt. In this conventional apparatus, the rotational driving force is transmitted to the photoconductor drum 11 while the intermediate transfer belt 40 is pressed against the photoconductor drum 11 by the pressure contact force of the primary transfer roller 401. Then, the first transfer position Pt1 is a position close to a position that is vertically downward from the rotation center O of the photoconductor drum 11,
That is, it is closer to the y-axis than the device of the present embodiment. Therefore, it is clear that the speed difference at the first transfer position varies due to the eccentricity of the photosensitive drum 11.

The configuration shown in FIG. 9 of the present embodiment is
Second for transferring an image from the intermediate transfer belt 40 to the transfer paper 2
It can also be applied to the transfer position of. In the case of applying to the second transfer position, in FIG. 9, the above-mentioned secondary transfer roller 47 whose shaft is fixed is positioned at the position of the photosensitive drum 11, and the above-mentioned secondary transfer roller 47 is positioned at the position of the primary transfer roller 401. Transfer opposite roller 42
Are located (see FIG. 2). And 2
The transfer paper 2 is configured to pass between the next transfer roller 47 and the intermediate transfer belt 40. Further, when a roller-shaped rotating body such as an intermediate transfer drum is provided as the second transfer body of the second transfer means instead of the transfer paper 2, the shaft is fixed to the position of the photosensitive drum 11 in FIG. It suffices that the rotating body is positioned.

As described above, in the image forming apparatus of this embodiment, the relative speed difference at the time of pixel formation can be made smaller than that of the conventional apparatus. On the other hand, in the conventional apparatus, the point where the straight line passing through the rotation center O of the photosensitive drum is orthogonal to the intermediate transfer belt is the center of pixel formation. Therefore, the center of the transfer nip width does not coincide with the center of pixel formation, so that a speed difference (relative speed) between the photosensitive drum and the intermediate transfer belt occurs at the center of pixel formation.

(Drive Control of Photoconductor Drum and Intermediate Transfer Belt) Next, drive control of the photoconductor drum and the intermediate transfer belt in the image forming apparatus of the present embodiment will be described. In the present embodiment, a signal corresponding to a pixel pitch in the sub-scanning direction (hereinafter referred to as “sub-scanning pitch”) in synchronization with the movement of the intermediate transfer belt 40, for example, N times the sub-scanning pitch or 1 / N (N is a natural number). The signal generating means (encoder for detecting the rotation angle) for generating the timing signal is provided. Further, the intermediate transfer belt 40 is provided with a detection unit that detects the exposure start position. Then, each photosensitive drum 11
Each of them is provided with a detecting means for generating a pulse for one rotation to detect the reference position of the rotation angle, and a rotation angle detecting encoder. Further, a drive motor 50 as a drive source for moving the intermediate transfer belt is provided, and the drive motor 50 is rotationally controlled based on the signal output from the signal generating means corresponding to the sub-scanning pitch. A drive source including a drive motor is attached to each photoconductor drum 11,
At the first transfer position where the photosensitive drum 11 and the intermediate transfer belt 40 face each other, the speed difference (relative speed) between the average peripheral speed of the photosensitive drum and the moving speed of the intermediate transfer belt is substantially constant, or both of them are substantially constant. Control to move stably without slipping together.

Further, in the present embodiment, the photosensitive drum 11
Is rotated at a constant rotational angular velocity, and the intermediate transfer belt 4
0 at a constant speed, and at the first transfer position Pt1 where the intermediate transfer belt 40 and the photoconductor drum 11 face each other as described above, the maximum value (vertex) of the photoconductor circular section in the belt side direction. Is located at the center of the transfer nip portion. However, the rotational angular velocities of the respective photoconductor drums 11 are changed in accordance with variations in the diameter of the photoconductor drums so that the intermediate transfer belt speed and the peripheral speed of the photoconductor drums may be moved at a constant speed or integrally. The rotation of the photosensitive drum 11 is controlled. For this rotation control, the interval between the output pulses output from the detection unit that generates one pulse for each rotation of each photosensitive drum 11 and detects the reference position of the rotation angle is the same as that of the intermediate transfer belt 40. The rotation is controlled so that the value has a value corresponding to the speed difference or the speed of movement as a unit. Alternatively, rotation control may be performed such that the pulse interval of the encoder output for detecting the rotation angle of the photoconductor drum 11 becomes a value at which the intermediate transfer belt 40 and the intermediate transfer belt 40 move at a certain speed difference or integrally.

(Method of Detecting Eccentricity and Diameter of Photosensitive Drum) Next, a method of detecting eccentricity ε and drum radius R of the photosensitive drum 11 will be described. <Self-measurement method> Drum radius R of the photosensitive drum 11
Is obtained by moving the intermediate transfer belt 40 by a length L = 2πRo corresponding to the circumference of the ideal drum, and detecting the rotation angle θi of the rotation angle detection encoder directly connected to the photosensitive drum 11 at that time. , You can ask. That is,
It can be calculated by the following formula.

(40) R = L / θi

Alternatively, when there is no rotary encoder and only the reference position of rotation can be detected, the moving distance Lb of the intermediate transfer belt 40 when the photosensitive drum 11 makes one rotation may be obtained. That is, it can be obtained by the following equation.

(41) R = Lb / (2π)

A detector for detecting the movement or absolute position of the intermediate transfer belt 40, for example, a mark on the intermediate transfer belt 40 at a fixed interval is attached to the end of the intermediate transfer belt 40 where the transfer paper 2 does not pass, and a reference on the intermediate transfer belt 40 is set. It is also possible to use a linear encoder which is marked with a position and detects those marks to recognize the absolute position. With this detector, a rotation that can detect the reference position of rotation (detects by generating one pulse per revolution) without providing a rotation angle detection encoder that can detect the absolute position on each photosensitive drum 11. With the angle / reference position detector, the rotation angle position of each photoconductor drum can be predicted. In other words, the photosensitive drum 11 is rotated, and one rotation period of the rotation angle / reference position detector drum is measured by the linear encoder. From this measurement result, the angle per pulse of the linear encoder output can be known. Therefore, the drum diameter R of the photosensitive drum 11 can also be measured. Then, if the rotational angular velocity of each photoconductor drum 11 is controlled so as to be a constant rotational angular velocity according to the disc diameter, it is possible to eliminate the speed difference or slippage at the transfer position.

The position of the eccentricity ε of the photosensitive drum 11 can be detected by
For example, it is possible if the peripheral surface displacement based on the eccentricity of the photosensitive drum 11 is detected. The detector used for this detection can be configured using, for example, a light emitting element, a light receiving element, and an optical system. Here, the light emitting element emits a light beam to a displacement detection position on the peripheral surface of the photosensitive drum. Further, the light receiving element receives the light beam reflected by the photosensitive drum, and for example, a two-divided photodiode element can be used. The optical system is an optical system that changes the light detected on the light receiving element by changing the peripheral surface of the photosensitive drum due to eccentricity. An optical system using an error detection method or the like can be used. In this detector, when the distance between the detection positions changes, a photocurrent corresponding to the change amount flows in the light receiving element. By detecting this, the amount of eccentricity of the photosensitive drum 11 can be detected. Further, the eccentric position (θ, ε) from the x-axis can be found by detecting the peak position of the output signal change when the photosensitive drum 11 rotates and detecting the rotation angle information at that time. .

In the image forming apparatus of this embodiment, it suffices to be able to detect where the eccentric position (θ, ε) is at the rotation angle of the photosensitive drum. That is, since the rotation angle of the photoconductor drum 11 is detected by another means, it suffices to know where the detected eccentric position is in the rotation angle of the photoconductor drum 11 and how much the amplitude ε is.

<Factory Measurement Method> At the factory, the diameter R and the eccentric position ε of the photosensitive drum 11, and the angle from the home position of the rotary encoder that is interlocked with the rotation of the photosensitive drum 11 at the eccentric position ε. Measure θo. The information of the angle θo is recorded in a memory (flash memory or the like) in the image forming apparatus and can be used for deriving the value of “d” described above.

The operation sequence of the above measurement will be described with reference to FIGS. 11 and 12. Although not shown in FIG. 11, the photosensitive drums 11C, 11M, 11Y,
11BK includes a detector that detects a reference angular position (home position) of a rotation angle, a rotation angle encoder, and a detector (eccentricity detector) that detects displacement of the photosensitive drum surface to detect eccentricity. Is installed. Although not shown, a motor for driving the intermediate transfer belt 40 is also installed. Further, a polygon mirror, which is driven to rotate at a constant speed by a polygon motor and deflects a light beam emitted from a laser diode, performs main scanning on the photosensitive drum, and an exposure (writing) position on the photosensitive drum is fixed. .

First, when the power is turned on, the intermediate transfer belt 40 is driven. When the intermediate transfer belt 40 and the photoconductor drum 11 move integrally without slipping at a low speed without slipping, the photoconductor drum 11 is entangled and moves. Then, one rotation of the photosensitive drum 11 is detected by the output of the detector that detects the reference position of the rotation angle, and the number of output pulses of the belt movement detection linear encoder 412 at this time (in some cases, the phase of the pulse interval is also measured). To improve the accuracy) to measure the diameter of the photosensitive drum 11.
Further, the output of the eccentricity position detector is detected, and the output of the detector for detecting the reference position of the rotation angle for one rotation of the photosensitive drum 11 and the eccentricity by the rotation angle encoder provided in the photosensitive drum 11. Measure the position. The eccentricity amplitude can be detected by detecting the AC amplitude of the output waveform of the eccentricity position detector. The above detection is performed for all the photosensitive drums. Based on the above detection data, the correction value d = π (R0−
R) + Rsin ⁻¹ {(ε / R) cos θ} is calculated for each photoconductor drum for one rotation (θ = 0 to 2π) and stored in a memory in a controller (not shown) as a reference table.
Next, the reference mark 411 is detected by the front end position detector 413 at the end of the intermediate transfer belt 40, and it is assumed that each photoconductor drum 11 is in the ideal position and has the ideal shape. Main scanning data for transferring a test mark onto the photosensitive drum 11 is recorded. Here, a case where the timing phase of the main scanning of the polygon mirror due to disturbance or the like does not match the timing phase of the sub scanning generated by the movement of the intermediate transfer belt 40 will be considered. The main scan exposure image generation start timing is
The timing mark 410 for the belt movement detection linear encoder on the intermediate transfer belt 40 is performed based on the pulse signal detected by the belt movement detection linear encoder detector 412. It does not always coincide with the main scanning timing by the polygon mirror. Therefore, when the test mark recording generation timing comes and the main scanning timing of the polygon mirror does not come, the test mark is recorded at the delayed main scanning timing of the polygon motor. And FIG.
2 (a), test marks M (C), M (M), M (Y), M (B) recorded on the intermediate transfer belt 40.
K) and the reference mark 411: M (ref), after detecting an error, d caused by eccentricity and variation in diameter,
By correcting the error amount here, it is possible to correct the mounting error of the photoconductor drum 11. In this way, the correction data for the mounting position of the photosensitive drum 11 and the like and the correction data “d” due to the eccentricity and the diameter variation of the photosensitive drum 11 are obtained. If the exposure position on the photosensitive drum is changed by using this data or the timing of the generation data of this exposure image is changed as described above, an image without distortion and without distortion can be generated. Reference position error detector 41
Reference numeral 4 is composed of four sets of mark detection units each including the light emitting element LD shown in FIG. 12B, the light emitting section including the objective lens OL, the slit SL, and the light receiving section including the light receiving element PD. The four sets of detection units are arranged in a direction orthogonal to the traveling direction of the intermediate transfer belt and detect the four marks, respectively.

The photosensitive drum 11 is generally replaced even after the image forming apparatus is sold. Therefore, the diameter and the eccentricity are automatically measured in the apparatus, or are measured in advance in the factory, and, for example, a bar code label showing a value corresponding to the measurement result is created, and the bar code label is prepared. Attach it to the designated place. The image forming apparatus may read the bar code with a reader provided in the apparatus. Since the image forming apparatus detects the reference position of the rotation angle of the photoconductor drum, the bar code label position detector is provided, or a mark indicating the reference position is provided on the photoconductor drum to detect the reference position in the image forming apparatus. May be provided. However, when the measurement is performed in the factory, a step of measuring the drum diameter and the eccentricity and a step of applying a barcode label are included in the factory before the photosensitive drum 11 is shipped. Further, the image forming apparatus must be provided with a bar code reader.

The principle of measuring the diameter of the photosensitive drum 11 in the image forming apparatus is that the driving force of the photosensitive drum driving system is stopped and the intermediate transfer belt 4 is driven by the driving force of the belt driving system.
0 is driven to measure how much the photosensitive drum 11 rotates with respect to the specified belt movement distance. That is,
The photoconductor drum 11 is rotating. Then, in order to improve the measurement accuracy, it is preferable to select a value for the photosensitive drum 11 to make one rotation as the prescribed belt movement distance. The belt moving distance is detected by directly connecting it to a belt driving roller of a belt driving system with a rotary encoder, or by attaching a timing mark at the end of the intermediate transfer belt 40 and a linear encoder for reading the timing mark. And
The rotation angle of the photosensitive drum 11 is detected by directly connecting to the shaft of each photosensitive drum and attaching a rotation angle encoder. The rotation angle encoder directly connected to each photosensitive drum axis
It can be used to control the rotation of each photosensitive drum 11 with high accuracy. Further, since the rotary encoder directly connected to the belt drive roller or the linear encoder can be used for controlling the intermediate transfer belt 40 with high accuracy at a constant speed, the cost does not increase.

As another photosensitive drum drive, there is a case where it is driven via a gear and motor control is performed by an encoder attached to the motor shaft. In this case, the photosensitive drum drive shaft is equipped with a detector that generates one pulse per rotation. Then, it is measured by the number of pulses of the linear encoder or rotary encoder output of the belt drive system when the photosensitive drum 11 makes one rotation.

In the present embodiment, an intermediate transfer drum may be provided as a second transfer member to which the image is transferred from the intermediate transfer belt 40, and the image may be transferred from the intermediate transfer drum to the transfer paper. Good. In the case of this configuration, the above-described influence coefficients κ ₁ and κ ₂ at each transfer position can be made substantially equal.

As described above, in the first embodiment, the ratio (W ₁ / W ₂ ) of the transfer nip widths at the first transfer position and the second transfer position is the circumference of the photosensitive drum as the image carrier. It is configured to be equal to the ratio (Vd / Vb) of the speed Vd and the moving speed Vb of the intermediate transfer belt 40 as the first transfer body. Here, the peripheral speed of the photosensitive drum 11 is the second
The moving speed of the transfer paper 2 as the transfer body is set to be equal. In this case, as described above, in order to eliminate the hollow character phenomenon, a constant speed difference (relative speed) between the photosensitive drum 11 and the intermediate transfer belt 40 at the first transfer position Pt1.
Even when the sheet is provided, expansion and contraction of pixels transferred on the transfer paper 2 can be reduced. Further, in the first embodiment, the speed difference δ (= Vd−V ₂ ) is δ =
It can also be configured so as to satisfy the relational expression of (1-κ ₁ / κ ₂ ) · ΔVh. In this case, as described above, in order to eliminate the hollow character phenomenon, the first transfer position Pt1
Even when a constant speed difference (relative speed) is provided between the photoconductor drum 11 as the image carrier and the intermediate transfer belt 40 as the first transfer member, the transfer paper as the second transfer member. It is possible to reduce the expansion and contraction of the pixel transferred onto the second layer. In particular, even when other transfer process conditions except the transfer nip widths W ₁ and W ₂ and the surface moving speed difference at the first transfer position and the second transfer position are different, the pixels transferred onto the transfer paper 2 are Expansion and contraction can be reduced. In the first embodiment, the photosensitive drums are driven and controlled so that the average peripheral speeds of the photosensitive drums 11 become equal to each other, and the photosensitive drums 11 and the intermediate transfer belt at the first transfer position Pt1 are controlled. It is configured so that the transfer nip width W ₁ with respect to 40 does not change. As a result, even if there is eccentricity and diameter variation of the photosensitive drum 11, the change in the transfer nip width W ₁ at the first transfer position Pt1 and the first
The speed difference (relative speed) at the transfer position Pt1 is reduced. Therefore, the expansion and contraction of the pixel due to the eccentricity of the photosensitive drum 11 and the variation in the diameter are reduced. Moreover, the exposure timing or exposure position on the peripheral surface of each photoconductor drum 11 is
It is set according to at least one of the eccentricity and the variation in diameter of each photoconductor drum 11, and the inter-drum distance. By setting in this way, the displacement of the pixel due to the variation of the center position of the transfer nip is reduced. Further, if there is a variation in the distance between the photoconductor drums, the pixel data can be set by changing the timing of the image data to be exposed on the photoconductor drums 11 or changing the exposure position by an amount corresponding to the variation. The position shift of is reduced. Further, in the first embodiment, the photosensitive drum 1 as the image carrier of the intermediate transfer belt 40 as the first transfer body.
The surface portion in contact with 1 has elasticity, and the first transfer position Pt
In step 1, the intermediate transfer belt 40 is pressed against the photosensitive drum 11 with a constant pressing force. As a result, the pressure contact force acting between the intermediate transfer belt 40 and the photoconductor drum 11 can be kept substantially the same even if the diameter of the photoconductor drum is varied or eccentricity occurs. In addition, the intermediate transfer belt 4
The surface portion of the photosensitive drum side of 0 is provided with a certain spring property (elasticity) in the direction orthogonal to the running direction of the intermediate transfer belt. Therefore, the shapes of the photosensitive drums 11 submerged in the surface of the intermediate transfer belt 40 are substantially the same, and the transfer nip width can be kept constant, so that color misregistration and expansion / contraction of the image can be further reduced. In addition, in the first embodiment,
The transit time for the intermediate transfer belt 40 as the first transfer member to pass between the adjacent transfer positions on the moving system path from the first transfer position Pt1 to the second transfer position Pt2 is generated in the intermediate transfer belt 40. It is a natural number times the cycle of movement speed fluctuations. Therefore, the first transfer position Pt
1 when any pixel of the image on the photosensitive drum 11 as an image carrier is primarily transferred to the intermediate transfer belt 40,
At the second transfer position Pt2, the moving speed of the intermediate transfer belt 40 becomes the same when the pixel on the intermediate transfer belt 40 is secondarily transferred to the transfer paper 2 as a recording medium. Moreover,
The surface moving speed of the photosensitive drum 11 is equal to the moving speed of the transfer paper 2. Therefore, the speed difference between the photoconductor drum 11 and the intermediate transfer belt 40 when any pixel of the image is primarily transferred, and the intermediate transfer belt 40 and the transfer paper 2 when the pixel is secondarily transferred. The speed difference between and will be the same. Therefore, even if the moving speed of the intermediate transfer belt 40 periodically fluctuates, for example, the first transfer position P
When the pixel is expanded at t1, the pixel is contracted at the second transfer position Pt2 by the expanded amount. Therefore, the expansion or contraction of the pixel on the transfer paper 2 due to the periodical speed fluctuation of the intermediate transfer belt 40 is reduced. You can Further, in the first embodiment, the same position on the intermediate transfer belt 40 as the first transfer member passes over the first transfer position Pt1 a plurality of times, so that each photosensitive drum 11 as the image bearing member is transferred. It is also possible to configure so that the image of is transferred onto the intermediate transfer belt 40 in an overlapping manner. In the case of this configuration, the intermediate transfer belt 40 moves from the second transfer position Pt2 to the first transfer position Pt1.
The passing time for the intermediate transfer belt 40 to pass between the transfer positions adjacent to each other on the moving system path is set to a natural number times the cycle of the speed fluctuation occurring in the intermediate transfer belt 40. As a result, the intermediate transfer belt 40 is moved to the first transfer position Pt1.
After the toner image is primarily transferred onto the intermediate transfer belt 40
Even when the toner reaches the first transfer position Pt1 again after one round, and another toner image is primarily transferred, the speed difference between the first transfer position and the second transfer position becomes the same. Therefore, the expansion and contraction of the pixels on the transfer paper 2 due to the periodical speed fluctuation of the intermediate transfer belt 40 can be reduced.
Further, in the first embodiment, the image carrier 4
For example, the toner image on one photoconductor drum is separately transferred to two intermediate transfer bodies (belts and drums) as the first transfer body, and the toner image on each intermediate transfer body is transferred to the second transfer. Intermediate transfer body (belt or drum)
It is also possible to form a color image by secondarily transferring to The color image on the intermediate transfer body (belt or drum) as the second transfer body is tertiary-transferred onto the transfer paper 2 as the third transfer body. In the case of this configuration, the first
The transfer member moving speed and the transfer paper moving speed are equalized. Then, the passing time for the image transfer surface of the second transfer member to pass the distance from the second transfer position to the third transfer position on the moving path of the second transfer member is set to the second transfer member. It should be a natural number times the period of the speed fluctuations that occur.
As a result, the expansion and contraction of the pixels on the transfer paper 2 due to the periodical speed fluctuation of the second transfer member (belt or drum) can be reduced.

[Second Embodiment] Next, a second embodiment of the present invention will be described. In the image forming apparatus of this embodiment, an intermediate transfer drum is used instead of the intermediate transfer belt,
Moreover, the image on the photosensitive drum 11 is transferred to the transfer paper 2 through two intermediate transfer steps.

FIG. 13 is a schematic block diagram of an image forming apparatus according to this embodiment. In this image forming apparatus, four photosensitive drums 11C, 11M, 11Y, 11BK are arranged in parallel at equal intervals, and two photosensitive drums 11C, 11M, 11Y, and 11BK are provided as two first transfer bodies so as to face each two of them. Intermediate transfer drums 21 and 22 are provided. One intermediate transfer drum 2
In the first transfer positions Pt11 and Pt12, the magenta toner image on the photoconductor drum 11M and the yellow toner image on the photoconductor drum 11Y are superposed and transferred to the first transfer position Pt11. Further, the other intermediate transfer drum 22 has the photosensitive drum 11C at the first transfer positions Pt13 and Pt14.
The upper cyan toner image and the black toner image on the photosensitive drum 11BK are superimposed and transferred. And
An intermediate transfer drum (transfer electric field applying rotary member) 31 as a second transfer member is provided so as to face the two intermediate transfer drums 21 and 22. Secondly, the toner images on the intermediate transfer drums 21 and 22 are superposed and transferred onto the intermediate transfer drum 31 at the transfer positions Pt21 and Pt22. Third facing the intermediate transfer drum 31
At the transfer position Pt3, the color image transferred to the intermediate transfer drum 31 is collectively transferred to the transfer paper 2.

The basic structure of the image forming apparatus of this embodiment is the same as the structure described in the example of Japanese Unexamined Patent Publication No. 2001-265081, but in this publication, four photosensitive drums 11C and 11M are used. , 11Y, 11BK and the three intermediate transfer drums 21, 22, 31 do not mention the diameter variation and eccentricity. Regarding the intermediate transfer drums 21 and 22 which are the first transfer members, even if they have eccentricity and diameter variation, as long as the intermediate transfer drums 21 and 22 rotate at a constant angular velocity, the first embodiment ΔV described in
= ΔU is established. However, since each intermediate transfer drum is configured by providing a low resistance elastic rubber layer typified by conductive silicone rubber on a metal drum, the transfer nip width varies at each transfer position. That is, "first transfer position between the photoconductor drum 11 and the intermediate transfer drums 21 and 22" or "intermediate transfer drums 21 and 22 and the intermediate transfer drums and the intermediate transfer drums 21 and 22 due to the diameter variation and the eccentricity of the photosensitive drums and the intermediate transfer drums. Second between transfer drum 31
The transfer nip width fluctuates depending on the transfer position. This fluctuation in the transfer nip width causes expansion and contraction of pixels.

(Countermeasures for Reducing Influence of Drum Diameter Variation) Therefore, in this embodiment, the diameter variation of each photoconductor drum and the intermediate transfer drum 31 which is the third transfer body is corrected, and the expression (αo) described above is used. The rotational angular velocities of the photoconductor drums 11 and the intermediate transfer drum 31 are controlled so as to obtain the average peripheral velocity that satisfies the relationship. By this control, expansion and contraction of the formed pixels transferred / formed on the intermediate transfer drum 31 can be reduced. However, although the expansion and contraction of the pixel occurs due to the variation of the transfer nip width at each transfer position, if the average nip width W ₂ is set so as to satisfy the relationship of the above (βo) formula, the pixel formation is further reduced. Expansion and contraction can be reduced.

For the above control, it is necessary that the photosensitive drum and the intermediate transfer drum whose diameters are desired to be corrected can be independently driven and rotated. Further, the diameters of the respective photosensitive drums and the intermediate transfer drum may be measured in the factory, and the values may be stored in the flash memory in the image forming apparatus.
However, since the photosensitive drums are exchanged in the market, it is necessary to print the measured data with a barcode, attach the measured data to the photosensitive drum, and have a means that can be read by a barcode reader in the image forming apparatus. . Alternatively, encoders are attached to the rotary shafts of the photosensitive drum and the intermediate transfer drum whose diameters are to be corrected, and the encoders attached to the respective intermediate transfer drums when the intermediate transfer drum 31 as the second transfer member is rotated once, for example. It may be performed by counting the number of pulses. At this time, each intermediate transfer drum needs to be suspended.

(Countermeasures for reducing the influence of eccentricity of the drum) In this embodiment, in order to reduce the influence of the eccentricity, the diameter and the rotational angular velocity of the intermediate transfer drum 31 as the second transfer member are set to the photosensitive drum. The eccentricity phase of each photoconductor drum and the eccentricity phase of the intermediate transfer drum 31 are made equal to each other by making the diameter and rotational angular velocity of 11 equal. That is, the eccentricity phases (angles formed with the maximum eccentricity angle position) at the positions where the same pixel is formed are matched. By doing so, even if the intermediate transfer drums 21 and 22 as the first transfer bodies are eccentric, the photosensitive drum 11
With the eccentricity of the intermediate transfer drum 31, it is possible to reduce fluctuations in the speed difference (relative speed) when forming the same pixel. Therefore, the expansion and contraction of the formed pixels at the transfer positions Pt11 to Pt14, Pt21, and Pt22 can be reduced. The eccentricity of each drum is measured in the factory. Then, each drum may be attached in accordance with the eccentric phase instruction mark in the image forming apparatus. Eccentricity measurement data is attached to the photoconductor drums that are exchanged on the market, and it may be attached according to the eccentricity phase marking mark at the time of exchange. However, in the configuration of the image forming apparatus according to the present embodiment, it is not possible to simultaneously reduce the influence of the diameter variation of the photosensitive drum and the eccentricity. The data of the eccentricity phase or the diameter variation of the photoconductor drum 11 and the intermediate transfer drum 31 are measured in a production process in advance and assembled so that the eccentricity phase is matched, or the rotation angular velocity of the photoconductor drum 11 and the intermediate transfer drum 31 Change the setting.

(Countermeasure 1 for Reducing Pixel Expansion / Contraction) In the image forming apparatus of this embodiment, two photosensitive drums are in contact with each of the intermediate transfer drums 21 and 22 as the first transfer bodies. . Therefore, also in this embodiment, as in the case of the first embodiment, the average peripheral speed of the photosensitive drum 11 is made equal to the average peripheral speed of the intermediate transfer drum 31 as the second transfer member. ing. Then, the passing time required for the peripheral surfaces (image transfer surfaces) of the intermediate transfer drums 21 and 22 as the first transfer bodies to pass the distance from the first transfer position Pt1 to the second transfer position Pt2 is the intermediate transfer drum. It is set to be a natural number times the cycle of speed fluctuations occurring on the peripheral surfaces of 21 and 22. For example, the passage time for the peripheral surface of the intermediate transfer drum 21 to pass the distance from each of the first transfer positions Pt11 and Pt12 to the second transfer position Pt21 is the cycle of the speed fluctuation occurring on the peripheral surface of the intermediate transfer drum 21. It is a natural multiple of. Further, the passage time for the peripheral surface of the intermediate transfer drum 22 to pass the distance from each of the first transfer positions Pt13 and Pt14 to the second transfer position Pt22 is the cycle of the speed fluctuation occurring on the peripheral surface of the intermediate transfer drum 22. It is a natural multiple of. By configuring to satisfy such a condition, the intermediate transfer drum 21,
It is possible to reduce the expansion and contraction of pixels due to the periodical speed fluctuations that occur on the peripheral surface of 22. Further, by satisfying the above condition regarding the passage time, the passage time for the intermediate transfer drums 21 and 22 to pass between the first transfer positions facing the photoconductor drum 11 is also set to the intermediate transfer drum 21, It is a natural number times the cycle of speed fluctuations occurring on the circumferential surface of 22. Thereby, the intermediate transfer drums 21, 2
It is also possible to reduce the color shift in the image transferred onto the second unit. The periodical speed fluctuations generated on the peripheral surfaces of the intermediate transfer drums 21 and 22 are caused by the eccentricity of the gear, which is the transmission mechanism, the thickness error of the timing belt, the eccentricity of the pulley, and the like. It may also occur due to fluctuations transmitted from the photoconductor drum 11 or the intermediate transfer drum 31 as the second transfer member.

Further, in the image forming apparatus of the present embodiment, the intermediate transfer drum 31 as the second transfer member has the intermediate transfer drums 21 and 22 at the second transfer positions Pt21 and Pt22.
And is in contact with the transfer paper 2 as a recording medium at the third transfer position Pt3. Therefore, in this embodiment,
The moving speed of the transfer paper 2 at the transfer position Pt3 is set to be equal to the average peripheral speed of the intermediate transfer drums 21 and 22. At this time, the pixel formed on the transfer paper 2 is different from the exposure pixel length, so the pixel is corrected by changing the generation speed and timing of the exposure data. The peripheral surface (image transfer surface) of the intermediate transfer drum 31 as the second transfer member is the second
Transfer positions Pt21, Pt22 to the third transfer position Pt3
To the intermediate transfer drum 31
It is set to be a natural number times the cycle of speed fluctuations that occur on the peripheral surface of the. By configuring to satisfy such a condition, it is possible to reduce expansion and contraction of pixels due to periodical speed fluctuations that occur on the peripheral surface of the intermediate transfer drum 31.
Further, by satisfying the condition regarding the passage time, the passage time for the intermediate transfer drum 31 to pass between the second transfer positions Pt21 and Pt22 at positions facing the intermediate transfer drums 21 and 22 is also intermediate. It is a natural number times the cycle of speed fluctuations that occur on the peripheral surface of the transfer drum 31.
As a result, it is possible to reduce color misregistration in the images transferred onto the intermediate transfer drums 21 and 22. The periodic fluctuation of the peripheral speed of the intermediate transfer drum 31 is caused by the eccentricity of the gear, which is the transmission mechanism, the thickness error of the timing belt, the eccentricity of the pulley, and the like. Also, when it occurs due to fluctuations transmitted from the adjacent intermediate transfer drums 21 and 22, fluctuations transmitted from the transfer paper side at the third transfer position, fluctuations transmitted from the photosensitive drum, or the like. There is also. Further, as a measure for reducing the pixel expansion / contraction caused by the periodical speed fluctuation in the intermediate transfer drum 31, an image on the intermediate transfer belt is transferred to an intermediate transfer drum as a second transfer member in the intermediate transfer belt method. The same can be applied to the case where the transfer is performed from the transfer sheet to the transfer sheet (recording medium) as the third transfer member.

In the actual apparatus configuration, the configuration of the drive system such as the intermediate transfer drum is set so as to satisfy the above-mentioned condition regarding the passage time. For example, one drive source (drive motor) is used to transmit a transmission mechanism (gear, timing belt, press
Drive a photosensitive drum, a plurality of intermediate transfer drums or a transfer roller (electric field applying rotary member) as a driven part, and a cyclic speed is applied to each driven part by a load change to a transmission mechanism or a drive motor. Fluctuations may occur. In this case, the diameter or the transfer position of the transmission mechanism, the plurality of intermediate transfer drums or the transfer rollers (transfer electric field applying rotary member) may be set so as to satisfy the above-mentioned condition regarding the passage time.

(Pixel Expansion / Contraction Reduction Measure 2) Next, another pixel expansion / contraction reduction measure in the image forming apparatus of the present embodiment will be described. In the image forming apparatus shown in FIG.
In order to prevent the above-mentioned hollow character phenomenon, members contacting each other may have a certain speed difference in all transfer processes. In this case, the second transfer positions Pt21, Pt
When the peripheral speed difference or the transfer nip width at 22 is shifted from the condition that the expansion and contraction of pixels on the intermediate transfer drum 31 can be canceled, the passing speed Vp of the transfer paper 2 at the third transfer position Pt3 or the transfer nip at each transfer position Width W
Is set according to each influence coefficient, the peripheral speed of the photosensitive drum 11, the peripheral speeds of the intermediate transfer drums 21 and 22, and the peripheral speed of the intermediate transfer drum 31, as described above, the expansion and contraction of pixels can be canceled. You can To set the passing speed Vp and the transfer nip width W of the transfer paper 2, the analysis of the expansion and contraction of the pixels discussed in the first embodiment is performed as "second transfer positions Pt21, Pt22" in the present embodiment. It can be obtained by expanding to the third transfer position Pt3 ". Similarly, in this method, in the intermediate transfer belt method, an image on the intermediate transfer belt is transferred to an intermediate transfer drum as a second transfer body and then transferred to a transfer paper (recording medium) as a third transfer body. The same applies to the case.

As described above, in the second embodiment, the ratio (W ₁ / W ₂ ) of the transfer nip width between the first transfer position and the second transfer position is the photosensitive drum 11 as the image carrier.
Peripheral speed Vd and intermediate transfer drum 2 as a first transfer member
It is configured to be equal to the ratio (Vd / Vb) of the peripheral speed Vb of 1 and 22. Here, the peripheral speed of the photosensitive drum 11 is made equal to the moving speed of the intermediate transfer drum 31 as the second transfer member. In this case, as described above, in order to eliminate the hollow character phenomenon, the first transfer position Pt11
To Pt14, the photosensitive drum 11 and the intermediate transfer drum 21,
Even when a certain speed difference (relative speed) is provided between the pixel 22 and 22, it is possible to reduce expansion / contraction of pixels transferred onto the intermediate transfer drum 31 as the second transfer member. Further, in the second embodiment, the speed difference δ (= Vd
It is also possible to configure −V ₂ ) so as to satisfy the relational expression of δ = (1−κ ₁ / κ ₂ ) · ΔVh. In this case, as described above, in order to eliminate the hollow character phenomenon, the photosensitive drum 11 as the image carrier and the intermediate transfer drums 21 and 22 as the first transfer bodies are provided at the first transfer positions Pt11 to Pt14. Even when a constant speed difference (relative speed) is provided between them, expansion and contraction of pixels transferred onto the intermediate transfer drum 31 can be reduced. In particular, the transfer nip widths W ₁ , W at the first transfer position and the second transfer position
₂ and the transfer process conditions other than the difference in surface moving speed are different, the intermediate transfer drum 3 as the second transfer member
It is possible to reduce expansion and contraction of the pixels transferred onto the first pixel. Further, in the second embodiment, the diameter and the rotational angular velocity of each photoconductor drum 11 as the image bearing member are equal to the diameter and the rotational angular velocity of the intermediate transfer drum 31 as the second transfer member. The position (the eccentricity phase) formed between the position where the same pixel is formed on the drum 11 and the intermediate transfer drum 31 and the maximum eccentric angle position may be the same. In this case, the photosensitive drum 1
1 and the intermediate transfer drum 31 have eccentricity, the photosensitive drum 1 when any pixel of the image is primarily transferred
1 and the speed difference between the intermediate transfer drums 21 and 22 as the first transfer body, and the speed difference between the intermediate transfer drums 21 and 22 and the intermediate transfer drum 31 when the pixel is secondarily transferred. And will be close. Therefore, regardless of whether or not the photosensitive drum 11 and the intermediate transfer drum 31 are eccentric, even when a speed difference is provided at the transfer position in order to suppress the occurrence of the hollow character phenomenon, the expansion and contraction of the pixels on the intermediate transfer drum 31 is reduced. Can be reduced and a high quality image can be obtained. In the second embodiment, the respective photosensitive drums are drive-controlled so that the average peripheral speeds of the respective photosensitive drums 11 become equal to each other. Moreover, it has a structure as nip width _{W 1} does not change between the photosensitive drum 11 and the intermediate transfer drum 21 at the first transfer position Pt11～Pt14. As a result, the speed difference (relative speed) between the first transfer positions Pt11 to Pt14 becomes small even if the diameter of the photosensitive drum 11 varies. Therefore, the expansion and contraction of pixels due to the variation in the diameter of the photosensitive drum 11 is reduced. Further, the exposure timing or the exposure position with respect to the peripheral surface of each photoconductor drum 11 is set according to at least one of the variation in the diameter of each photoconductor drum 11 and the inter-drum distance.
As a result, the displacement of the pixels is reduced. Also, the second
In this embodiment, the image transfer surfaces of the intermediate transfer drums 21 and 22 as the first transfer members are located at the first transfer positions Pt.
The passing time for passing between the transfer positions adjacent to each other on the moving path from 11 to Pt14 to the second transfer positions Pt21 and Pt22 is respectively the intermediate transfer drums 21 and 22.
It is a natural number times the period of the fluctuation of the peripheral speed. Therefore, at the first transfer position Pt1, an arbitrary pixel of the image on the photoconductor drum 11 as an image carrier is transferred to the intermediate transfer drum 2.
When the first transfer is made to the first and the second transfer positions 22 and the second transfer position Pt.
At 21, Pt22, the peripheral speeds of the intermediate transfer drums 21 and 22 are the same when the pixels on the intermediate transfer drums 21 and 22 are secondarily transferred to the intermediate transfer drum 31 as the second transfer member. Moreover, the peripheral speed of the photosensitive drum 11 is equal to the peripheral speed of the intermediate transfer drum 31. Therefore, the speed difference between the photosensitive drum 11 and the intermediate transfer drums 21 and 22 when any pixel of the image is primarily transferred, and the intermediate transfer drums 21 and 22 when the pixel is secondarily transferred. And the speed difference between the transfer paper 2 are the same. Therefore, the intermediate transfer drum 2
Even if the peripheral velocities of Nos. 1 and 22 vary periodically, for example, if the variation causes the pixels to extend at the first transfer positions Pt11 to Pt14, the second transfer position Pt21,
At Pt22, the pixel shrinks, so the intermediate transfer drum 2
It is possible to reduce the expansion and contraction of the pixels on the intermediate transfer drum 31 due to the periodical peripheral speed fluctuations of 1 and 22. Further, in the second embodiment, the image transfer surface of the intermediate transfer drum 31 as the second transfer member is at the second transfer position P.
The passage time for passing between mutually adjacent transfer positions on the movement route from t21, Pt22 to the third transfer position Pt3 is each a natural number times the cycle of the peripheral speed fluctuation occurring in the intermediate transfer drum 31. Moreover, the peripheral speed of each of the intermediate transfer drums 21 and 22 as the first transfer member is set to the transfer paper 2
Equal to the moving speed of. Therefore, the expansion and contraction of the pixels on the transfer paper 2 due to the periodical fluctuation of the peripheral speed of the intermediate transfer drum 31 can be reduced.

[0111]

According to the first to twelfth aspects of the present invention, while preventing the occurrence of the hollow character phenomenon, the expansion and contraction of pixels are accurately canceled even if the transfer nip width fluctuates. There is an excellent effect that the image of can be obtained.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a main part of an image forming apparatus according to a first embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of the image forming apparatus.

FIG. 3 is an explanatory diagram of rotational angular velocity control of a photosensitive drum in the image forming apparatus.

FIG. 4 is an explanatory diagram of exposure timing control for a photosensitive drum in the image forming apparatus.

FIG. 5 is an explanatory diagram of a main part of an image forming apparatus according to a modified example.

FIG. 6 is a model diagram of a photosensitive drum in the image forming apparatus.

FIG. 7 is a model diagram of a photosensitive drum for explaining exposure data generation timing and exposure position setting.

FIGS. 8A and 8B are explanatory diagrams showing a change in transfer nip width when an eccentric photosensitive drum rotates.

9A and 9B are explanatory views of a pressure contact mechanism of the intermediate transfer belt.

FIG. 10 is a model diagram of the photosensitive drum and the like at the first transfer position in the conventional apparatus.

FIG. 11 is an explanatory diagram of a system for measuring the eccentricity ε and the drum radius R of the photosensitive drum.

FIG. 12A is an explanatory diagram showing a positional relationship between a test mark and a reference mark recorded on the intermediate transfer belt.
(B) is a schematic block diagram of a reference position error detector used for mark detection.

FIG. 13 is an explanatory diagram of a main part of an image forming apparatus according to a second embodiment of the present invention.

14A and 14B are explanatory views of the principle of preventing color misregistration in the conventional apparatus.

FIG. 15 is an explanatory diagram of a main part of a conventional device using an intermediate transfer drum.

[Explanation of symbols]

1 toner image forming unit 2 transfer paper (recording medium, second transfer member, third transfer member
Body 3 optical writing unit 5 transfer corona charger 11 photoconductor drum (image carrier) 12 charging roller 13 developing unit 14 photoconductor cleaning units 21, 22 intermediate transfer drum (first transfer body) 31 intermediate transfer drum ( Second transfer body) 40 Intermediate transfer belt (first transfer body) 41 Belt drive roller 42 Transfer opposite roller 43 Transfer roller 44, 45, 46 Pressure contact roller 47 Secondary transfer roller 48 Tension roller 50 Drive motor 51, 52 Gear 401 Primary transfer roller 402 Fixed frame 403 Swing arm member 404 Spring 410 Timing mark 411 Reference mark 412 Belt movement detection linear encoder detector 413 Tip position detector 414 Reference position error detector

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) G03G 15/16 F term (reference) 2H027 DA16 DA17 DA20 DE02 DE07 DE10 EB06 EC06 ED02 ED04 ED24 EE01 EE03 EE04 2H030 AA01 AB02 AD16 BB02 BB16 BB42 BB56 BB63 2H076 AB05 AB12 AB67 AB68 2H200 GA12 GA23 GA29 GA30 GA34 GA47 HA02 HA28 HB12 JA02 JC02 JC03 JC07 JC09 JC12 JC19 JC20 LA18 LA19 LA27 PLB

Claims (12)

[Claims] 1. An image carrier which is rotationally driven, an image forming unit which forms a plurality of different images on the image carrier, and a first transfer position which faces the image carrier. A first transfer member that transfers an image on the image carrier to a first transfer member that is driven to pass, and a first transfer member that drives to pass a second transfer position facing the first transfer member. In the image forming apparatus including the second transfer member configured to transfer the image on the first transfer member to the second transfer member, the moving speed of the image bearing surface of the image carrier is the second transfer unit. The transfer nip width W ₁ between the image carrier and the first transfer member at the first transfer position and the first transfer position at the second transfer position are equal to the moving speed of the image transfer surface of the transfer member.
The ratio (W ₁ / W ₂ ) of the transfer nip width W ₂ between the first transfer member and the transfer member of the second transfer member is equal to the moving speed Vd of the image bearing surface of the image carrier and the image of the first transfer member. An image forming apparatus having a ratio (Vd / Vb) to a moving speed Vb of a transfer surface. 2. One or a plurality of image bearing members that are driven to rotate, an image forming unit that forms a plurality of different images on the image bearing members, and a first transfer position facing the image bearing members. A first transfer member that transfers an image on the image carrier to a first transfer member that is driven to pass, and a first transfer member that drives to pass a second transfer position facing the first transfer member. In the image forming apparatus including the second transfer member configured to transfer the image on the first transfer member to the second transfer member, the moving speed Vd of the image bearing surface of the image carrier and the first The speed difference from the moving speed Vb of the image transfer surface of the transfer body is ΔVh (= V
d-Vb), the size of the pixel that expands and contracts under the influence of other transfer process conditions except the transfer nip width and the surface moving speed difference at the first transfer position and the second transfer position. When the influence coefficients defined by the ratio to the size of the pixel when there is no pixel are κ ₁ and κ ₂ , respectively, the moving speed Vd of the image bearing surface of the image bearing member and the second transfer position passing through the second transfer position. Moving speed V ₂ of the image transfer surface of the transfer body
The image forming apparatus is characterized in that the speed difference δ (= Vd−V ₂ ) with respect to satisfies the relational expression δ = (1−κ ₁ / κ ₂ ) · ΔVh. 3. The image forming apparatus according to claim 2, wherein a transfer nip width W1 between the image carrier and the first transfer member at the first transfer position, and a transfer nip width W1 at the second transfer position. The ratio (W ₁ / W ₂ ) of the transfer nip width W ₂ between the first transfer member and the second transfer member is the moving speed Vd of the image bearing surface of the image carrier and the first transfer member. The image forming apparatus has a ratio (Vd / Vb) to the moving speed Vb of the image transfer surface. 4. The image forming apparatus according to claim 1, 2 or 3, wherein the first transfer members are adjacent to each other on a moving path of an image transfer surface of the first transfer member from the first transfer position to the second transfer position. An image characterized in that the time taken for the image transfer surface of the first transfer member to pass between matching transfer positions is a natural multiple of the cycle of speed fluctuations occurring on the image transfer surface of the first transfer member. Forming equipment. 5. The image forming apparatus according to claim 4, wherein the first transfer body has an endless image transfer surface, and the first transfer body rotates a plurality of times to form an image on the image carrier. Transfer positions adjacent to each other on the moving path of the image transfer surface of the first transfer member from the second transfer position to the first transfer position, the transfer positions being configured to be superimposed and transferred onto the first transfer member. An image forming apparatus characterized in that a passage time for the image transfer surface of the first transfer member to pass through is a natural number times the cycle of speed fluctuations occurring on the image transfer surface of the first transfer member. 6. The image forming apparatus according to claim 1, 2, 3, 4 or 5, comprising means for driving and controlling each image carrier so that the average moving speeds of the image carrying surfaces of the image carriers become equal to each other. The image forming means is configured so that the transfer nip width between the image carrier and the first transfer body at the first transfer position does not change, and the image forming unit of each image carrier is based on image data. The image bearing surface is exposed to form a latent image, and the latent image on the image bearing member is developed to form an image. The exposure timing or exposure position of each image bearing member with respect to the image bearing surface is An image forming apparatus, wherein the image forming apparatus is set according to at least one of eccentricity and diameter variation of each image carrier and a distance between the image carriers. 7. The image forming apparatus according to claim 6, wherein a surface portion of the first transfer member which comes into contact with the image carrier has flexibility or elasticity, and the first transfer member has a first surface at a first transfer position. An image forming apparatus comprising means for bringing a transfer body into pressure contact with the image carrier with a constant pressure contact force. 8. The image forming apparatus according to claim 1, 2, 3, 4, 5, 6 or 7, wherein the first transfer member and the second transfer member are roller-shaped transfer members, An image forming apparatus, wherein at least one rotational angular velocity of the carrier, the first transfer member, and the second transfer member is set according to the variation in the diameter. 9. One or a plurality of image bearing members that are driven to rotate, an image forming unit that forms a plurality of different images on the image bearing members, and a first transfer position facing the image bearing members. First transfer means for transferring an image on the image carrier to one or a plurality of first transfer bodies driven so as to pass therethrough;
Second transfer means for transferring an image on the first transfer body to a second transfer body driven so as to pass a second transfer position facing the first transfer body, and a second transfer means. The recording medium driven to pass the third transfer position facing the body,
In an image forming apparatus including a third transfer unit that transfers an image on a second transfer member, the second transfer member is a roller-shaped transfer member, and a diameter and a rotational angular velocity of the image carrier are respectively Is equal to the diameter and rotational angular velocity of the second transfer member, and the maximum eccentric angle and the position where the same pixel is formed on the image carrying surface of the image carrier and on the image transfer surface of the second transfer member, respectively. An image forming apparatus characterized in that an angle formed by a position (eccentric phase) is matched. 10. Claims 1, 2, 3, 4, 5, 6, 7, 8
Or in the image forming apparatus of 9, Passes through a third transfer position facing the second transfer body
The image on the second transfer body on the recording medium that is driven like
A third transfer means for transferring, The moving speed of the image bearing surface of the first transfer member is equal to that of the third transfer member.
Equal to the moving speed of the image transfer surface of The first transfer member and the second transfer member at the second transfer position
Transfer nip width W between _TwoAnd at the third transfer position
Transfer nip width W between the second transfer body and the recording medium _ThreeRatio with
(W_Two/ W_Three) Is the moving speed of the image bearing surface of the first transfer member.
Degree Vb1 and moving speed Vb2 of the image transfer surface of the second transfer member
And a ratio (Vb1 / Vb2) equal to
Image forming device. 11. Claims 1, 2, 3, 4, 5, 6, 7, 8
Alternatively, in the image forming apparatus of No. 9, third transfer means for transferring the image on the second transfer member to a recording medium driven so as to pass through a third transfer position facing the second transfer member. And a difference in speed between the moving speed Vb1 of the image transfer surface of the first transfer body and the moving speed Vb2 of the image transfer surface of the second transfer body is ΔVh.
(= Vb1−Vb2), the size of the pixel that expands and contracts under the influence of other transfer process conditions except the transfer nip width and the surface moving speed difference at the second transfer position and the third transfer position, When the influence coefficients defined by the ratio to the pixel size when there is no influence are κ ₂ and κ ₂ , respectively, the moving speed Vb1 of the image bearing surface of the first transfer member and the third transfer position are passed. Moving speed V ₃ of the image transfer surface of the recording medium
The image forming apparatus is characterized in that a speed difference δ (= Vb1−V ₃ ) with respect to satisfies the relational expression δ = (1−κ ₂ / κ ₃ ) · ΔVh. 12. The image forming apparatus according to claim 10 or 11, wherein adjacent transfer operations are performed on the moving path of the image transfer surface of the second transfer member from the second transfer position to the third transfer position. An image forming apparatus characterized in that a transit time for the image transfer surface of the second transfer member to pass between positions is a natural number times the cycle of speed fluctuations occurring on the image transfer surface of the second transfer member. .