JP2016170247A - Image forming apparatus - Google Patents

Abstract

Kind Code: A1 A misalignment between toner images of a plurality of colors transferred to a transfer belt from a plurality of photosensitive drums that rotate as the transfer belt rotates is corrected. A plurality of photosensitive drums, a plurality of image forming means for forming toner images on the photosensitive drums, an endless transfer belt to which the toner images formed on the photosensitive drums are transferred, and A transfer device having a belt driving means 905 for rotating a transfer belt, a drum driving means 805 for applying torque to a photosensitive drum to be rotated following the rotation of the transfer belt, and color misregistration of a plurality of colors formed on the transfer belt. Based on the color shift correction amount calculated by detecting means 100 for detecting the amount of shift between the detection patterns, and by controlling the drum driving means to form the color shift detection pattern again when the amount of shift is greater than or equal to a predetermined value. and a controller 801 for correcting misalignment between the toner images of the plurality of colors using the image forming apparatus, and correcting the misalignment between the toner images of the plurality of colors based on the amount of misalignment when the amount of misalignment is less than a predetermined value. Device. [Selection drawing] Fig. 14

Description

  The present invention relates to an image forming apparatus in which a plurality of photosensitive drums rotate according to the rotation of a transfer belt.

  The electrophotographic image forming apparatus forms an image on a recording medium using an electrophotographic image forming process. Examples of the electrophotographic image forming apparatus include an electrophotographic copying machine (for example, a digital copying machine), an electrophotographic printer (for example, a color laser beam printer), an MFP (multifunction machine), a facsimile machine, and a printing machine.

  In an electrophotographic image forming apparatus (hereinafter referred to as an image forming apparatus), a photosensitive drum and a transfer belt that carry a toner image are required to be driven so that the surface speed is constant. The reason for this is that, first, when the exposure for drawing the electrostatic latent image on the photosensitive drum is time-synchronized exposure, if the surface speed of the photosensitive drum fluctuates, the irradiation position of the light beam deviates from the position where it should be irradiated. It is because it ends up. Second, when there is an AC speed difference between the surface speeds of the photosensitive drum and the transfer belt, the position where the toner image formed on the photosensitive drum is transferred onto the transfer belt varies from the position where it should originally be transferred. Resulting in. As a result, image defects such as color misregistration (positional misalignment between colors) and banding (periodic misregistration) occur in the image formed on the recording paper.

  For this reason, the motor as a drive source for the photosensitive drum and the transfer belt is secured with high accuracy and constant speed by speed feedback control using various speed detection sensors. Note that brushless DC motors are often used as motors because they are inexpensive, quiet, and highly efficient. In speed feedback control using a brushless DC motor, a method is used in which the motor is controlled so that the rotational speed of the shaft of the photosensitive drum is maintained at a constant speed based on a signal from a rotary encoder disposed on the shaft of the photosensitive drum. May be. However, in the above-described speed feedback control, the rotational speed of the photosensitive drum shaft is detected, but the surface speed of the photosensitive drum is not detected. Therefore, the eccentricity of the photosensitive drum rotational axis, the photosensitive drum diameter error, etc. Therefore, it is difficult to make the surface speed of the photosensitive drum constant. Similarly, it is difficult to control the surface speed of the transfer belt to be constant due to the eccentricity of the rotation shaft of the drive roller that rotates the transfer belt, the error in the diameter of the drive roller, the unevenness in the thickness of the transfer belt, and the like.

  Further, as a cause of image defects, mutual interference due to friction between the photosensitive drum and the transfer belt in the primary transfer portion can be raised. This is a problem that the speed fluctuation generated in one of the photosensitive drum and the transfer belt affects the other speed. In addition, when the recording paper that rushes into the secondary transfer section that transfers the toner image on the transfer belt to the recording paper is a thick paper, a sudden load fluctuation occurs on the transfer belt, so that the rotational speed of the transfer belt is high. Variation occurs. The high-frequency speed fluctuation of the transfer belt causes a positional shift of the toner image in the primary transfer portion that transfers the toner image from the photosensitive drum to the transfer belt. As described above, there are various causes of image defects, and it is very difficult to solve all of them.

  Therefore, as described in Patent Document 1, there is a technique in which an image cylinder (equivalent to a photosensitive drum) is driven by friction with an image transfer cylinder (equivalent to a transfer belt). This technique has the following advantages. First, since the image on the photosensitive drum becomes the image on the transfer belt, if the image is formed on the basis of the position on the photosensitive drum, the influence of the rotation unevenness of the photosensitive drum is eliminated. Second, even if the speed of the transfer belt changes due to a shock or the like when the recording paper enters the secondary transfer section, the consistency between the image on the photosensitive drum and the image on the transfer belt is ensured. No image defects will occur. However, in order to obtain the first advantage, it is important to form an image based on the rotational position of the photosensitive drum. Therefore, as disclosed in Patent Document 2, there is one that performs exposure control in synchronization with the rotational movement amount of the photosensitive drum.

  By the way, the above-described color misregistration occurs not only in the speed difference between the photosensitive drum and the transfer belt but also in the case where the exposure timing for forming the electrostatic latent image on the photosensitive drum is not accurately controlled. Therefore, the following control is performed as a measure against color misregistration. A color misregistration detection pattern for detecting color misregistration is formed on the transfer belt, light is emitted from the light emitting element to the color misregistration detection pattern, and the reflected light is detected by the light receiving element. The detection timing of the color misregistration detection pattern is obtained based on the detection waveform output from the light receiving element, thereby calculating the misregistration amount between the colors and correcting the exposure timing. Such color misregistration correction is generally used in image forming apparatuses (Patent Document 3).

JP 2002-333752 A JP-A-8-99437 JP 2012-3234 A

  In order to synchronize the rotation of the photosensitive drum and the rotation of the transfer belt, there are the following methods. Encoders are provided on the shaft of the photosensitive drum and the drive shaft of the transfer belt to detect the respective rotation speeds. One or both of the rotational speed of the photosensitive drum and the rotational speed of the transfer belt is controlled so that the detected rotational speed is the same. However, the control to make the surface speeds of the photosensitive drum and the transfer belt the same from the rotational speeds of the photosensitive drum shaft and the transfer belt drive shaft is based on the assumption that the diameters of the drive shafts of the photosensitive drum and the transfer belt are constant. . For example, when the surface of the photosensitive drum is shaved due to long-term use and the diameter of the photosensitive drum changes, the surface speed of the photosensitive drum changes, and a speed difference occurs between the surface speed of the photosensitive drum and the surface speed of the transfer belt.

  On the other hand, as described above, there are two main factors that cause color misregistration: a speed difference between the photosensitive drum and the transfer belt, and a deviation in exposure timing from a predetermined timing. However, the above-described color misregistration correction is intended to correct the color misregistration due to the deviation of the exposure timing from the predetermined timing, so the exposure timing is corrected based on the detected color misregistration amount. That is, when a difference occurs between the surface speeds of the photosensitive drum and the transfer belt, the exposure timing cannot be accurately corrected even if the above-described color misregistration correction is executed. This is because it is impossible to identify how much of the detected color misregistration amount is due to the speed difference between the photosensitive drum and the transfer belt, and how much is due to the exposure timing misalignment.

  Accordingly, the present invention provides an image forming apparatus capable of correcting a deviation between a plurality of color toner images transferred from a plurality of photosensitive drums rotating according to the rotation of the transfer belt to the transfer belt.

An image forming apparatus according to an embodiment of the present invention includes:
A plurality of photosensitive drums;
A plurality of image forming means for forming toner images of different colors on the plurality of photosensitive drums;
An endless transfer belt that is stretched by a plurality of rollers and onto which the toner images formed on the plurality of photosensitive drums are transferred, and at least one of the plurality of rollers is rotated to rotate the transfer. A belt driving means for rotating the belt, and a transfer device for transferring the toner image transferred onto the transfer belt to a recording medium;
Detecting means for detecting a shift amount between a plurality of color misregistration detection patterns formed on the transfer belt in order to correct a shift between the toner images of a plurality of colors;
The plurality of photosensitive drums are in contact with the transfer belt, and each of the plurality of photosensitive drums is rotated by the rotation of the transfer belt by the belt driving unit,
Drum driving means for applying torque to the plurality of photosensitive drums to be driven to rotate by rotation of the transfer belt;
Control means,
The control means includes
Controlling the plurality of image forming means to form a plurality of color misregistration detection patterns on the transfer belt;
Calculating a deviation amount between the color misregistration detection patterns detected by the detection means;
When the amount of deviation between the color misregistration detection patterns detected by the detection means is equal to or greater than a predetermined value, torque is applied to the plurality of photosensitive drums so that the plurality of photosensitive drums are applied to the transfer belt. The drum driving unit is controlled to rotate following the rotation, and then the color misregistration detection pattern of the plurality of colors is formed again, and the color misregistration correction amount is based on the detection result of the color misregistration detection pattern of the plurality of colors. And correcting misregistration between the toner images of the plurality of colors based on the color misregistration correction amount,
When the shift amount between the color misregistration detection patterns detected by the detection unit is less than a predetermined value, the shift between the toner images of the plurality of colors is based on the shift amount less than the predetermined value. Correct.

  According to the present invention, it is possible to correct a deviation between a plurality of color toner images transferred to a transfer belt from a plurality of photosensitive drums rotating according to the rotation of the transfer belt.

1 is a cross-sectional view of an image forming apparatus 1 according to an embodiment. Explanatory drawing of a color shift detection sensor. Explanatory drawing of an optical scanning device. Explanatory drawing of a transfer part friction torque and load torque. The figure which shows the time change of the load torque which acts on the drum shaft of a photosensitive drum. FIG. 3 is an explanatory diagram of a driving configuration of a photosensitive drum. FIG. 3 is an explanatory diagram of a driving configuration of a transfer belt. The flowchart which shows the assist torque derivation | leading-out sequence of a controller. The block diagram of a control system. Explanatory drawing of the analog signal output from a color shift detection sensor. The figure which shows a color shift detection pattern and a binary signal. The figure which shows a color shift detection pattern and a binary signal. The figure which shows the change of the distance of the same color pattern adjacent for a driven detection. 6 is a flowchart showing a color misregistration detection and follow-up detection sequence. 6 is a flowchart showing an image forming sequence.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described with reference to the accompanying drawings.

(Image forming device)
FIG. 1 is a cross-sectional view of an image forming apparatus 1 according to this embodiment. The image forming apparatus 1 includes a plurality of photosensitive drums 2 as photosensitive members (image carriers) that can rotate in the rotation direction R1, and an endless transfer belt (intermediate transfer belt) as an intermediate transfer member that can rotate in the rotation direction R2. ) 8. The image forming apparatus 1 includes a plurality of image forming units (image forming units) 9a and 9b that form toner images of different colors (yellow, magenta, cyan, and black) on the plurality of photosensitive drums 2a, 2b, 2c, and 2d. , 9c, 9d. The image forming units 9a to 9d include photosensitive drums 2a to 2d, chargers 3a, 3b, 3c, and 3d, optical scanning devices 5a, 5b, 5c, and 5d, developing devices 7a, 7b, 7c, and 7d, a primary transfer roller 6a, 6b, 6c, 6d and cleaning devices 4a, 4b, 4c, 4d.

  The chargers 3a to 3d uniformly charge the surfaces of the photosensitive drums 2a to 2d. The optical scanning devices 5a to 5d using a semiconductor laser as a light source emit light beams (laser beams) modulated by image information of yellow, magenta, cyan and black onto the uniformly charged surfaces of the photosensitive drums 2a to 2d. Then, electrostatic latent images are formed on the surfaces of the photosensitive drums 2a to 2d. The electrostatic latent images on the photosensitive drums 2a to 2d are developed by the developing devices 7a, 7b, 7c, and 7d to become toner images of respective colors. The toner images on the photosensitive drums 2a to 2d are sequentially transferred and superimposed on the transfer belt 8 by the primary transfer rollers 6a to 6d. The transfer belt 8 is stretched by a plurality of rollers. Specifically, the transfer belt 8 is stretched over a support roller 10, a drive roller 11, and a secondary transfer counter roller 21, and is supported so as to be rotatable in the rotation direction R2. The toner images of the respective colors superimposed on the transfer belt 8 are conveyed to the secondary transfer unit 22.

  On the other hand, a recording medium (hereinafter referred to as a sheet) S is fed from the paper feed cassette 17 or the manual feed tray 13 to the conveyance path. At that time, the sheet conveying unit such as the pickup roller 18, the feeding roller 19, the vertical pass roller 20, and the registration roller 16 for feeding the sheet S from the sheet feeding cassette 17 to the conveying path realizes a high-speed and stable conveying operation. Therefore, each is driven by an independent stepping motor. Similarly, the sheet conveying units such as the pickup roller 14 and the feeding roller 15 for feeding the sheet S from the manual feed tray 13 to the conveying path are also driven by independent stepping motors. The lateral registration position of the sheet S is corrected by the electrostatic conveyance means 30. The registration roller 16 conveys the sheet S to the secondary transfer unit 22 in synchronization with the toner image on the transfer belt 8.

  The toner images of the respective colors superimposed on the transfer belt 8 are collectively transferred onto the sheet S by the secondary transfer unit 22. The cleaning devices 4a to 4d have a cleaning blade 40 (FIG. 4) that contacts the surfaces of the photosensitive drums 2a to 2d. The cleaning blade 40 removes toner remaining on the surfaces of the photosensitive drums 2a to 2d after the primary transfer. The cleaning device 12 has a cleaning blade that contacts the surface of the transfer belt 8. The cleaning blade removes toner remaining on the surface of the transfer belt 8 after the secondary transfer. The sheet S on which the four-color toner images are collectively transferred by the secondary transfer unit 22 is conveyed to the fixing device 23. The fixing device 23 thermally fixes an unfixed toner image on the sheet S, and an image is formed on the sheet S. The sheet S on which the image is formed is discharged to the paper discharge tray 25 via the paper discharge roller 24.

  In double-sided printing, the sheet S that has passed through the fixing device 23 is guided from the paper discharge roller 24 to the double-sided reverse path 27 and then reversely conveyed in the reverse direction and conveyed to the double-sided path 28. The sheet S that has passed the double-sided path 28 passes through the vertical path roller 20 again and is conveyed to the secondary transfer unit 22 in the same manner as described above. The toner images of the respective colors are collectively transferred from the transfer belt 8 to the back surface of the sheet S conveyed to the secondary transfer unit 22. The sheet S having the toner image transferred on the back surface is discharged to the paper discharge tray 25 via the fixing device 26 and the paper discharge roller 24.

  The subscripts a, b, c and d of the reference symbols represent yellow, magenta, cyan and black, respectively. In the following description, the subscripts a, b, c, and d may be omitted.

(Color shift detection sensor)
The image forming apparatus 1 includes a color misregistration detection sensor (detection unit) 100. The color misregistration detection sensor 100 is disposed between the photosensitive drum 2 d located on the right side of the transfer belt 8 and the support roller 10 of the transfer belt 8. The color misregistration detection sensor 100 is arranged between the color misregistration detection patterns 111 transferred from the photosensitive drums 2a to 2d to the transfer belt 8 in order to correct a misregistration (color misregistration) between the toner images of a plurality of colors. The amount of deviation is detected. The color misregistration detection pattern 111 is formed as a toner image on the photosensitive drums 2a to 2d of the image forming units 9a to 9d for the respective colors, and is transferred from the photosensitive drums 2a to 2d to the transfer belt 8.

  FIG. 2 is an explanatory diagram of the color misregistration detection sensor 100. As illustrated in FIG. 2, the color misregistration detection sensor 100 includes a light emitting unit 101 and a light receiving unit 102. The light emitting unit 101 illuminates the transfer belt 8 at the position facing the light emitting unit 101 or the color misregistration detection pattern 111 formed on the transfer belt 8 with the light IL. The regular reflection light RL from the transfer belt 8 or the color misregistration detection pattern 111 formed on the transfer belt 8 is collected by the lens 103 and enters the light receiving unit 102. The light receiving unit 102 outputs an analog signal 121 having a potential that changes according to the amount of light incident on the light receiving unit 102.

(Optical scanning device)
The configuration of the optical scanning device 5 will be described in detail with reference to FIG. FIG. 3 is an explanatory diagram of the optical scanning device 5. A plurality of light beams emitted from a light source (semiconductor laser) 303 are incident on a rotating polygon mirror 301 that is rotated by a motor 302. The light beam reflected by the rotating polygon mirror 301 passes through the fθ lens 304, is reflected by the reflecting mirror 306, and scans on the rotating photosensitive drum 2 in the main scanning direction MS. The light beam reflected by the rotating polygon mirror 301 is incident on a photodetector (Beam Detector, hereinafter referred to as BD) 305 as a light receiving means. The BD 305 outputs a write timing synchronization signal in the main scanning direction. The image formation control unit 1109 provided in the main body of the image forming apparatus 1 outputs the image data 307 to the laser driver 308 provided in the optical scanning device 5. The laser driver 308 controls the light emission of the light source 303 by outputting a light emission control signal 309 based on the image data 307 to the light source 303.

(Driven drive)
In this embodiment, the photosensitive drum 2 rotates according to the rotation of the transfer belt 8 (hereinafter referred to as driven driving). The driven drive is a friction force (hereinafter referred to as a transfer portion friction torque _TF ) due to contact between the surface of the photosensitive drum 2 and the surface of the transfer belt 8 in the primary transfer portion between the primary transfer roller 6 and the photosensitive drum 2. This is realized by the rotational torque of the motor that drives the photosensitive drum 2.

FIG. 4 is an explanatory diagram of the transfer portion friction torque _TF and the load torque _TL . The load torque _TL is a torque that acts on the drum shaft 809 of the photosensitive drum 2 when the photosensitive drum 2 rotates at a predetermined process speed. The transfer portion friction torque _TF is obtained by converting the friction force generated in the primary transfer portion PT into torque acting on the drum shaft 809 of the photosensitive drum 2. The photosensitive drum 2 is always subjected to a load torque _TL opposite to the rotational direction R1 due to the frictional force of the cleaning blade 40 of the cleaning device 4 and the frictional force of a bearing portion (not shown) of the drum shaft 809. Since the load torque T _L is much larger than the maximum value T _FMAX of the transfer portion friction torque T _F (that is, T _L >> T _FMAX ), only the transfer portion friction torque T _F causes the photosensitive drum 2. Cannot be driven. Hereinafter, the maximum value T _FMAX of the transfer portion friction torque _TF is referred to as the maximum transfer portion friction torque.

FIG. 5 is a diagram showing the change over time of the load torque _TL acting on the drum shaft 809 of the photosensitive drum 2. FIG. 5A shows the change over time of the load torque _TL acting on the drum shaft 809 in the image forming process, and the maximum transfer portion friction torque T _FMAX (−T _FMAX ). The vertical axis indicates the torque acting on the drum shaft 809, and the torque acting on the rotation direction R1 of the photosensitive drum 2 is positive. Therefore, the load torque _TL acting in the direction opposite to the rotation direction R1 of the photosensitive drum 2 is located in the minus region in FIG. Since the load torque _TL changes transiently at the timing of applying the charging high voltage and the timing at which the transfer residual toner enters the cleaning blade 40, the load torque _TL is not always constant as shown in FIG. Absent.

However, it is known that such a transient change component (hereinafter referred to as torque fluctuation component) ΔT _L is sufficiently smaller than the magnitude of the load torque T _L that is steadily generated. Therefore, rotational torque of the same amount as the direct current component of the load torque _TL is applied to the photosensitive drum 2 by the rotational torque generating means (in this embodiment, a brushless DC motor 805 described later). The acting load torque _TL is canceled. DC components of the load torque T _L is, for example, a stationary component represented by the average value of the load torque T _L. The rotational torque applied to the photosensitive drum 2 by the rotational torque generating means is hereinafter referred to as assist torque T _AS .

FIG. 5B is a diagram showing a change over time of the torque fluctuation component ΔT _L of the load torque _TL when the assist torque T _AS is applied. By applying the assist torque T _AS to the drum shaft 809, an alternating component of the load torque T _L , that is, a torque fluctuation component ΔT _L (= T _AS −T _L ) remains. As shown in FIG. 5B, if the alternating torque fluctuation component ΔT _L is equal to or less than the maximum transfer portion friction torque T _FMAX (−T _FMAX ), the photosensitive drum 2 is driven by the transfer belt 8. It is possible.

ここで、Ｔ_ＦＭＡＸ：最大転写部摩擦トルク、J：等価ドラムイナーシャ、ω：角速度、dω/dt：角加速度、Ｔ_Ｌ：負荷トルク、Ｔ_ＡＳ：アシストトルク、ΔＴ_Ｌ：トルク変動成分である。 However, if the photosensitive drum 2 does not follow the speed fluctuation of the transfer belt 8, the followability to the AC speed fluctuation cannot be secured. That is, the driven drive is realized by satisfying the following equation of motion during the image forming process.

Here, T _FMAX : Maximum transfer portion friction torque, J: Equivalent drum inertia, ω: Angular velocity, dω / dt: Angular acceleration, T _L : Load torque, T _AS : Assist torque, ΔT _L : Torque fluctuation component.

Equation (1) and (2), by the action of the assist torque T _AS of the same size as the DC component of the load torque T _L in the direction opposite to the load torque T _L, the maximum transfer unit friction torque T _This shows that the amount of torque that _FMAX must act on is reduced. At the same time, the total value of the acceleration torque and the torque fluctuation component [Delta] T _L of the photosensitive drum 2 is always be less than or equal to the maximum transfer unit friction torque T _FMAX indicates that it is driven condition. Here, the acceleration torque is indicated by the multiplication of the equivalent inertia (hereinafter referred to as equivalent drum inertia) J on the drum shaft 809 and the angular acceleration (dω / dt) of the photosensitive drum 2. The angular acceleration of the photosensitive drum 2 is a value determined from the surface speed fluctuation component at the primary transfer portion PT of the transfer belt 8. The equivalent drum inertia J represents the total rotating load as an inertia component on the drum shaft 809.

5 (c) is a diagram showing the time variation of the torque variation component [Delta] T _L of the load torque T _L including acceleration torque. FIG. 5 (c), in the form in consideration of the acceleration torque generated when driven by the speed variation of the transfer belt 8 shows a transient change in the torque fluctuation component [Delta] T _L generated on the drum shaft 809. Compared with the fluctuation of the load torque T _{L and} the fluctuation of the acceleration torque, the width of the transient change of the load torque T _L and the torque fluctuation component ΔT _L shown in FIG. 5A and FIG. Te, the width of the transient change in the torque fluctuation component [Delta] T _L including acceleration torque is increased.

Since the torque fluctuation component ΔT _L is basically small enough to be ignored, in order to increase the followability due to factors other than the assist torque T _AS , it is considered to increase the maximum transfer portion friction torque T _FMAX or decrease the acceleration torque. It is done. Since the maximum transfer portion friction torque _TFMAX is closely related to the toner transfer process in the primary transfer, it is not easy to change the maximum transfer portion friction torque _TFMAX . On the other hand, lowering the acceleration torque can be realized relatively easily by reducing the equivalent drum inertia J. The magnitude of the inertia component of the brushless DC motor (hereinafter referred to as BLDC motor) 805 added to the drum shaft 809 is greatly affected by the gear ratio between the reduction gear 810 and the motor shaft gear 811. The inertia component of the BLDC motor 805 is a value obtained by multiplying the square of the gear ratio by the motor shaft inertia. As a result, the rotor inertia of the BLDC motor 805 becomes much larger than the inertia component of the photosensitive drum 2 on the drum shaft 809. For this reason, the BLDC motor 805 of this embodiment uses an inner rotor type low inertia type. As a result, the equivalent drum inertia J can be significantly reduced. As a result, the acceleration torque is also greatly reduced.

As described above, in this embodiment, by applying the assist torque T _AS , the DC component of the load torque _TL acting on the drum shaft 809 is canceled, and the BLDC motor 805 having a low inertia component is selected. As a result, the photosensitive drum 2 can be sufficiently driven by the transfer belt 8 by the transfer portion friction torque _TF . In this embodiment, the BLDC motor (drum driving means) 805 is used as an assist torque generating source. However, the present invention is not particularly limited as long as a constant torque can be generated.

(Drive configuration of photosensitive drum and transfer belt)
Next, the electrical and mechanical drive configurations of the photosensitive drum 2 and the transfer belt 8 for realizing the driven drive will be described. FIG. 6 is an explanatory diagram of a driving configuration of the photosensitive drum 2. The drum shaft 809 of the photosensitive drum 2 is mechanically connected to the reduction gear shaft 808. A reduction gear 810 is fixed to the reduction gear shaft 808 by a joining mechanism (not shown). The reduction gear 810 meshes with the motor gear 811 of the BLDC motor 805. The rotational torque from the BLDC motor 805 is transmitted to the photosensitive drum 2 through the motor gear 811, the reduction gear 810, the reduction gear shaft 808 and the drum shaft 809 to rotate the photosensitive drum 2.

A rotary encoder (rotational speed detector) 807 is fixed to the reduction gear shaft 808. The rotary encoder 807 rotates integrally with the reduction gear shaft 808, the drum shaft 809, and the photosensitive drum 2. Based on the encoder signal 820 of the rotary encoder 807, a rotation speed detection value representing the rotation speed of the photosensitive drum 2 can be obtained. The rotation speed detection value can be used for detection of assist torque T _AS described later (assist torque derivation flow) and detection of eccentricity of the drum shaft 809.

  The host CPU 801 collectively controls the start timing and stop timing of each process (charging, exposure, development, and primary transfer high voltage) in the image forming process, calculation of a color misregistration amount described later, and other various setting values. The controller 802 receives a command signal (drive on / off, target speed, register setting value, etc.) from the host CPU 801 and outputs various control signals (drive on / off, PWM value, etc.) to the motor driver IC 803. In addition, since the angular velocity feedback control is performed by the encoder signal 820 of the rotary encoder 807 when the assist torque is derived, the controller 802 includes a PID controller therein. The motor driver IC 803 controls the drive circuit 804 based on the control signal from the controller 802 and the rotational position signal from the rotational position detection unit 806, and switches the phase current to be supplied to the BLDC motor 805 and adjusts the current amount. I do.

  FIG. 7 is an explanatory diagram of the drive configuration of the transfer belt 8. The transfer belt 8 is rotated by rotating at least one of the plurality of rollers 10, 11 and 21. Specifically, the transfer belt 8 is rotated by rotating a drive roller 11 installed so as to contact the inside of the transfer belt 8. The driving roller 11 is fixed to the roller shaft 912. On the roller shaft 912, a reduction gear 910 and a rotary encoder (rotational speed detector) 907 are fixedly disposed. The reduction gear 910 meshes with the motor gear 911 of the BLDC motor (belt driving means) 905. The rotational torque from the BLDC motor 905 is transmitted to the drive roller 11 via the motor gear 911, the reduction gear 910, and the reduction gear shaft 912, and rotates the drive roller 11. The transfer belt 8 is rotated by the rotation of the driving roller 11. The rotary encoder 907 rotates integrally with the reduction gear 910, the roller shaft 912, and the drive roller 11. Based on the encoder signal 920 of the rotary encoder 907, a rotation speed detection value representing the rotation speed of the transfer belt 8 can be obtained.

  As with the photosensitive drum 2, the driving roller 11 is rotated by reducing the rotational speed of the BLDC motor 905 by a reduction gear 910. The electrical configuration including the host CPU 801, the controller 902, the motor driver IC 903, the drive circuit 904, and the rotational position detection unit 904 is the same as the electrical configuration of the photosensitive drum 2, and thus description thereof is omitted. The transfer belt 8 during the image forming process is driven by angular velocity feedback control based on the encoder signal 920 of the rotary encoder 907. The controller 902 has a PID controller. The PID controller provides angular velocity feedback so that the difference between the target speed commanded by the host CPU 801 (hereinafter referred to as process speed) and the rotational speed detection value obtained by converting the encoder signal 920 of the rotary encoder 907 into the process speed becomes small. Take control.

  The transfer belt 8 and the BLDC motor 905 that rotates the transfer belt 8 constitute a transfer device that transfers the toner image transferred onto the transfer belt 8 to the sheet S. The plurality of photosensitive drums 2 come into contact with the transfer belt 8, and each of the plurality of photosensitive drums 2 is rotated by the rotation of the transfer belt 8 by a BLDC motor (belt driving unit) 905. A BLDC motor (drum driving means) 805 applies torque to the plurality of photosensitive drums 2 and rotates the transfer belt 8 following the rotation.

(Control sequence)
((Assist torque derivation))
Next, a method for deriving the assist torque T _AS will be described.
In general, when the main power supply is turned on, the image forming apparatus 1 first enters a state called an adjustment mode. In the adjustment mode, temperature adjustment of the fixing roller of the fixing device 23 and various image adjustments are performed. Only when the adjustment mode is finished, the printing operation is possible. In the present embodiment, a sequence for deriving the assist torque T _AS in the adjustment mode is provided. The image forming apparatus 1 is generally operable at a plurality of process speeds in order to cope with a plurality of paper types including cardboard. Therefore, the assist torque T _AS is derived corresponding to each of the plurality of process speeds.

The assist torque T _AS is derived by measuring the load torque _TL acting on the drum shaft 809 during the image forming process. In this embodiment, the load acting on the drum shaft 809 is obtained from the torque value generated by the BLDC motor 805. The motor driver IC 803 uses a driver IC that determines the amount of phase current flowing to the BLDC motor 805 based on the PWM signal. The PWM signal is a pulse width modulation signal. The amount of phase current is adjusted by changing the duty ratio of the PWM signal. The duty ratio of the PWM signal is a quotient obtained by dividing the high-level time width (ON time) of a rectangular wave signal having a constant period (PWM period) by the PWM period. When the duty ratio is large, the amount of current flowing through the phase increases. Conversely, when the duty ratio is small, the amount of current flowing through the phase decreases. Here, it can be said that the magnitude of the phase current is equivalent to the torque generated by the motor 805. Since the magnitude of the phase current is proportional to the duty ratio, the torque generated by the motor 805 is also proportional to the duty ratio.

In order to detect a load torque acting on the drum shaft 809 during the image forming process, the photosensitive drum 2 is controlled to a target process speed. The rotational speed of the photosensitive drum 2 is controlled by a PID controller provided in the controller 802. The PID controller is configured to perform angular velocity feedback control of the photosensitive drum 2 based on the encoder signal 820 of the rotary encoder 807 when assist torque is derived. However, the controller 802 is configured to output a PWM signal having a predetermined duty ratio corresponding to the derived assist torque T _AS to the motor driver IC 803 during the image forming process.

The assist torque derivation flow will be described. In order to derive the assist torque T _AS , first, the primary transfer roller 6 is in a state of being detached from the transfer belt 8. Second, the assist torque T _AS is derived during the image forming process in order to derive the load torque _TL acting on the drum shaft 809 due to the effects of charging, development, and interference between the toner and the cleaning blade 40. The torque fluctuation component of the load torque T _L in the image forming process (AC component) [Delta] T _L is sufficiently small relative to the steady component of the load torque T _L (DC component), the assist torque T _AS The derivation may be performed when the photosensitive drum 2 is idling.

The operation of the host CPU 801 when deriving the assist torque T _AS during the adjustment mode will be described. First, the host CPU 801 sends a command to remove the primary transfer roller 6 to a driver IC (not shown) of a stepping motor that moves the primary transfer roller 6 up and down. Second, the host CPU 801 controls various devices that perform an image forming process such as the exposure device 5, the charger 3, and the developing device 7. Third, the upper CPU 801 issues a drive command for the photosensitive drum 2.

  FIG. 8 is a flowchart showing an assist torque deriving sequence of the controller 802. The controller (torque adjustment means) 802 executes an assist torque derivation sequence based on a program stored in the ROM 812. When assist torque derivation is started during the adjustment mode, the controller 802 inputs an assist torque derivation command signal (process speed setting value, assist derivation ON command, etc.) from the host CPU 801 (S1101). The controller 802 selects a process speed (S1102). The controller 802 outputs a control signal to the motor driver IC 803 and starts the angular velocity feedback control of the photosensitive drum 2 at the selected process speed (S1103). The controller 802 determines whether or not T1 time (predetermined time) has elapsed since the start of driving of the photosensitive drum 2 (S1104). The time T1 is set to be equal to or longer than the time from when the photosensitive drum 2 starts to be driven until the toner transferred from the developing unit 7 to the photosensitive drum 2 reaches the cleaning blade 40. Further, the time T1 is defined based on the time from the start of driving of the photosensitive drum 2 to the time when the rotational speed of the photosensitive drum 2 is stabilized.

When the time T1 has elapsed (YES in S1104), the controller 802 starts sampling the duty ratio of the PWM signal, and stores the sampled duty ratio values P ₁ , P ₂ ,..., P _N in the RAM 813. (S1105). Controller 802, the value _P 1 of the sampled duty _ratio, P 2, · · ·, the number of _{P N} is determined whether the host vehicle has reached the predetermined sampling number N (S1106). When the number of sampled duty ratio values P ₁ , P ₂ ,..., P _N reaches a predetermined sampling number N (YES in S1106), the controller 802 stops sampling (S1107). When the sampling is completed, the upper CPU 801 stops the optical scanning device 5 and stops the high-voltage power supply to the charger 3, the developing device 7 and the primary transfer roller 6. The controller 802 rotates the photosensitive drum 2 once or twice, and then outputs a drive stop command for the photosensitive drum 2 to the motor driver IC 803 (S1108). The reason for rotating the photosensitive drum 2 once or twice is to remove the toner on the photosensitive drum 2 by the cleaning blade 40.

ここで、Ｐ_ａｖｅ：ＰＷＭデューティの平均値、Ｐ_N：Ｎ個目のサンプリングデータ、Ｎ：サンプリング数である。制御器８０２は、平均値Ｐ_ａｖｅをＲＡＭ８１３に保存する（Ｓ１１１０）。デューティ比の平均値Ｐ_ａｖｅは、アシストトルクＴ_ＡＳに相当する値である。すなわち、これで、選択された一つのプロセス速度に対するアシストトルクＴ_ＡＳが導出されたことになる。その後、制御器８０２は、残りのプロセス速度のそれぞれについて同様にＳ１１０２からＳ１１１０の動作を繰り返し行う（Ｓ１１１１）。全てのプロセス速度に対するアシストトルクＴ_ＡＳが導出されたら、アシストトルク導出シーケンスを終了する。 The controller 802 calculates an average value P _ave of the sampled duty ratio values P ₁ , P ₂ ,..., P _N based on the equation (3) (S1109).

Here, P _{ave is} the average value of PWM duty, P _{N is the} Nth sampling data, and N is the number of samplings. The controller 802 stores the average value P _ave in the RAM 813 (S1110). The average value P _ave of the duty ratio is a value corresponding to the assist torque T _AS . That is, the assist torque T _AS for one selected process speed is derived. Thereafter, the controller 802 repeats the operations from S1102 to S1110 for each of the remaining process speeds (S1111). When the assist torque T _AS for all process speeds is derived, the assist torque deriving sequence is terminated.

In this embodiment, in order to derive the assist torque T _AS , the assist torque deriving sequence is executed in a state where the contact between the photosensitive drum 2 and the transfer belt 8 in the primary transfer portion PT is released. However, the method of the present embodiment is not particularly limited as long as the same amount of torque as the direct current component of the load torque _TL acting on the photosensitive drum 2 can be derived.

((Color shift detection and driven detection))
Next, a color misregistration detection method and a driven detection method will be described. First, the control system 900 for color misregistration detection and follow-up detection and the color misregistration detection pattern (toner pattern) 111 will be described. FIG. 9 is a block diagram of the control system 900. The host CPU 801 includes a timing detection unit 1101, a pattern interval calculation unit 1102, a color misregistration calculation unit 1103, a color misregistration correction unit 1104, a driven state detection unit 1105, and a target speed adjustment unit 1106. The color misregistration detection sensor 100 detects a color misregistration detection pattern 111 formed on the transfer belt 8 and outputs an analog signal (detection signal) 121 to the comparator 1108. The comparator 1108 binarizes the waveform (detection waveform) of the analog signal 121 and outputs a binary signal (detection signal) 122 to the timing detection unit 1101. The timing detection unit 1101 detects the change timing of the binary signal 122 and outputs the detected timing information (time information) 123 to the pattern interval calculation unit 1102. The pattern interval calculation unit (calculation unit) 1102 calculates the distance between the color misregistration detection patterns 111 based on the timing information 123, and uses the calculated distance as the pattern interval information (vs. reference color) 124 to the color misregistration calculation unit 1103. Output. A color misregistration calculation unit (calculation unit) 1103 calculates a color misregistration amount based on pattern interval information (vs. reference color) 124. The color misregistration correction unit 1104 calculates correction information 125 from the calculated color misregistration amount, and outputs the correction information 125 to the image formation control unit 1109.

  The pattern interval calculation unit 1102 also calculates pattern interval information (adjacent color) 126 based on the timing information 123 and outputs it to the driven state detection unit (surface speed difference calculation unit) 1105. The driven state detection unit 1105 detects the driven state of the photosensitive drum 2 with respect to the transfer belt 8 based on the pattern interval information (adjacent color) 126. The target speed adjustment unit 1106 adjusts the target speed setting value 127 of the photosensitive drum 2 according to the detected driven state, and outputs the adjusted target speed setting value 127 to the controller 802. The clock signal 128 output from the crystal oscillator 1107 is used for synchronization of various processing operations executed by the host CPU 801, color misregistration calculation described later, and time measurement at the time of detection of follow-up.

  Here, the analog signal (detection waveform) 121 output from the color misregistration detection sensor 100 will be described with reference to FIG. FIG. 10 is an explanatory diagram of the analog signal 121 output from the color misregistration detection sensor 100. The color misregistration detection pattern 111 is formed on the transfer belt 8. The regular reflection light RL received by the light receiving unit 102 of the color misregistration detection sensor 100 is the smallest when the region where the color misregistration detection pattern 111 and the detection region (not shown) of the color misregistration detection sensor 100 overlap is the largest. This is because when the light IL emitted from the light emitting unit 101 of the color misregistration detection sensor 100 hits the color misregistration detection pattern 111 which is a toner image, it is irregularly reflected by the irregular surface of the color misregistration detection pattern 111, and the light receiving unit 102. This is because the amount of regular reflection light RL received by the light source decreases.

  As the area where the detection area (not shown) of the color misregistration detection sensor 100 overlaps with the color misregistration detection pattern 111 becomes smaller, the light quantity of the regular reflection light RL increases. Therefore, the analog signal (detection waveform) 121 output from the color misregistration detection sensor 100 is a triangular wave signal as shown in FIG. The analog signal 121 is input to the comparator 1108 and compared with the threshold signal (voltage) 129 to be binarized to become a binary signal 122, and the binary signal 122 is input to the host CPU 801. The set value of the threshold signal 129 used at this time is stored in the ROM 812 in advance. The upper CPU 801 reads the set value from the ROM 812 and outputs a threshold signal 129 corresponding to the set value to the comparator 1108.

  The comparator 1108 outputs the binary signal 122 to the timing detection unit 1101 of the host CPU 801. The timing detection unit 1101 detects the edge of the binary signal 122 from the comparator 1108, and obtains the interval Tl between the falling edge and the rising edge and the interval Th between the rising edge and the falling edge. The timing detection unit 1101 derives the interval Tl and the interval Th by counting the edge interval of the binary signal 122 with the period of the clock signal 128 of the crystal oscillator 1107.

  Next, the color misregistration detection pattern 111 will be described with reference to FIG. FIG. 11 is a diagram showing a color misregistration detection pattern 111 (Ph_Y1 to Ph_K10, Ps_Y1 to Ps_K2) and a binary signal 122 (S_Y1 to S_K12). The color misregistration detection pattern 111 includes a plurality of sets. The plurality of sets of color misregistration detection patterns 111 are formed on the transfer belt 8 side by side in the rotation direction R2 (hereinafter, sometimes referred to as a conveyance direction R2 or a sub-scanning direction R2). In FIG. 11, the sub-scanning direction R <b> 2 is drawn by being bent for the sake of space, but is actually a linear direction along the rotation direction R <b> 2 of the transfer belt 8. In FIG. 11, some of the plurality of sets of color misregistration detection patterns 111 are omitted. One set includes a pattern formed in yellow (Ph_Y, Ps_Y), a pattern formed in magenta (Ph_M, Ps_M), a pattern formed in cyan (Ph_C, Ps_C), and a pattern formed in black (Ph_K, Ps_K) one by one. The color misregistration detection pattern 111 includes two types of pattern sets. One is a pattern set (a total of 10 sets from Ph_1set to Ph_10set) for detecting color misregistration in the sub-scanning direction R2. The other is a pattern set (two sets of Ps_1set and Ps_2set) for detecting color misregistration in the main scanning direction MS.

  First, a method for calculating the color misregistration amount in the sub-scanning direction R2 using the binary signal 122 will be described. In the pattern set Ph_nset for detecting color misregistration in the sub-scanning direction R2, the widths Phy, Phm, Phc, and Phk in the sub-scanning direction R2 of the patterns Ph_Yn, Ph_Mn, Ph_Cn, and Ph_Kn (n = 1 to 10) of the respective colors are It is formed in the same way. Further, the distances L1, L2, and L3 between the central positions of the adjacent patterns Ph_Yn, Ph_Mn, Ph_Cn, and Ph_Kn in the sub-scanning direction R2 are also formed with the same reference distance Lrf1. However, when color misregistration occurs in the sub-scanning direction R2, the distances L1, L2, and L3 differ from the reference distance Lrf1 by the amount of color misregistration.

  In this embodiment, yellow is the reference color. The calculation of the magenta color misregistration amount for yellow in Ph — 1set is performed as follows. The timing detection unit 1101 of the host CPU 801 measures a time interval Tm1 between the center of S_Y1 that is the binary signal 122 of Ph_Y1 (intermediate between the falling edge and the rising edge) and the center of S_M1 that is the binary signal 122 of Ph_M1. To do. The timing detection unit 1101 outputs the measured time interval Tm1 as timing information (time information) 123 to the pattern interval calculation unit 1102. The pattern interval calculation unit 1102 calculates the distance L1 between Ph_Y1 and Ph_M1 by multiplying the measured time interval Tm1 by the process speed of the image forming apparatus 1. The pattern interval calculation unit 1102 outputs the calculated distance L1 to the color misregistration calculation unit 1103 as pattern interval information (vs. reference color) 124. The color misregistration calculation unit 1103 calculates the amount of magenta color misregistration for yellow by comparing the calculated distance L1 with the reference distance Lrf1 stored in the ROM 812 in advance.

  The calculation of the amount of cyan color shift with respect to yellow is performed as follows. The timing detection unit 1101 measures a time interval Tc1 between the center of S_Y1 and the center of S_C1, which is the binary signal 122 of Ph_C1. The pattern interval calculation unit 1102 calculates the distance L1 + L2 between Ph_Y1 and Ph_C1 by multiplying the measured time interval Tc1 by the process speed. Since the distance L1 + L2 between Ph_Y1 and Ph_C1 is twice the distance L1 between Ph_Y1 and Ph_M1, the color misregistration calculation unit 1103 compares the value obtained by doubling the reference distance Lrf1 with the calculated distance L1 + L2 for yellow. The amount of cyan color misregistration is calculated.

  The calculation of the color misregistration amount of black with respect to yellow is performed as follows. The timing detection unit 1101 measures a time interval Tk1 between the center of S_Y1 and the center of S_K1, which is the binary signal 122 of Ph_K1. The pattern interval calculation unit 1102 calculates the distance L1 + L2 + L3 between Ph_Y1 and Ph_K1 by multiplying the measured time interval Tk1 by the process speed. Since the distance L1 + L2 + L3 between Ph_Y1 and Ph_K1 is three times the distance L1, the color misregistration calculation unit 1103 compares the calculated distance L1 + L2 + L3 with the value obtained by multiplying the reference distance Lrf1 by three, and the color misregistration amount of black with respect to yellow Is calculated.

  The distance L1, the distance L1 + L2, and the distance L1 + L2 + L3 are pattern distances for calculating the color misregistration amount in the sub-scanning direction R2.

The formulas for calculating the color misregistration amount in the sub-scanning direction R2 are summarized as follows.
Magenta color misregistration amount = Lrf1-L1 (4)
Cyan color misregistration amount = 2Lrf1- (L1 + L2) (5)
Black color misregistration amount = 3Lrf1- (L1 + L2 + L3) (6)

  The color misregistration calculation unit 1103 performs the above calculation for each of the pattern sets Ph_1set to Ph_10set of the color misregistration detection pattern 111. The color misregistration calculation unit 1103 obtains the color misregistration amount of each color in the final sub-scanning direction R2 by averaging the calculation results in each pattern set Ph_nset (n = 1 to 10).

  Next, a method for calculating the color misregistration amount in the main scanning direction MS using the binary signal 122 will be described. There are two types of color misregistration detection patterns in the main scanning direction MS. One pattern set Ps_1set includes color misregistration detection patterns Ps_Y1, Ps_M1, Ps_C1, and Ps_K1 formed with an inclination of 45 ° with respect to the transport direction R2. Another pattern set Ps_2set has color misregistration detections Ps_Y2, Ps_M2, Ps_C2, and Ps_K2 formed with an inclination of −45 ° with respect to the transport direction R2. The widths Psy, Psm, Psc, and Psk in the sub-scanning direction R2 of the color misregistration detection patterns Ps_Yn, Ps_Mn, Ps_Cn, and Ps_Kn (n = 1 to 2) of the respective colors are formed to be the same. Further, the distances L4, L5, L6, L7, L8 and L9 between the central positions of the patterns Ps_Yn, Ps_Mn, Ps_Cn and Ps_Kn adjacent in the sub-scanning direction R2 are also formed with the same reference distance Lrf1.

  The distances L4, L5, L6, L7, L8, and L9 are pattern distances for calculating the amount of color misregistration in the main scanning direction MS.

The formulas for calculating the color misregistration amount in the main scanning direction MS are summarized as follows.
Magenta color misregistration amount = ((Lrf1-L4)-(Lrf1-L7)) / 2 (7)
Cyan color misregistration amount = ((Lrf1-L5)-(Lrf1-L8)) / 2 (8)
Black color misregistration amount = ((Lrf1-L6)-(Lrf1-L9)) / 2 (9)

  According to the above equations (7), (8), and (9), the color misregistration amount in the main scanning direction MS can be calculated without being affected by the color misregistration in the sub scanning direction R2. Explaining with Expression (7), Expression Lrf1-L4 represents the change amount (color misregistration amount) of the relative position between yellow and magenta generated in Ps_1set. The expression Lrf1-L4 alone includes the amount of change in the relative position between yellow and magenta caused by the color shift in the sub-scanning direction R2. Next, the expression Lrf1-L7 represents the change amount (color shift amount) of the relative position between yellow and magenta generated in Ps_2set. If color misregistration occurs only in the sub-scanning direction R2 without color misregistration in the main scanning direction MS, the distance L4 and the distance L7 change similarly. That is, the calculated value of the expression Lrf1-L7 and the calculated value of Lrf1-L4 are the same. Accordingly, since the value obtained by subtracting the calculated value of the formula Lrf1-L7 from the calculated value of the formula Lrf1-L4 is 0, the influence of the color shift in the sub-scanning direction R2 on the calculated result of the formula (7) may be excluded. it can. On the other hand, when color misregistration occurs in the main scanning direction MS, for example, in FIG. 11, when magenta deviates in the positive direction of the main scanning direction MS with respect to yellow, the distance L4 becomes small and Lrf1-L4 becomes positive. Conversely, since the distance L7 increases, Lrf1-L7 becomes a negative value. Therefore, by dividing the value obtained by subtracting the value of Lrf1-L7 from the value of Lrf1-L4 by 2, it is possible to calculate the color misregistration amount in the main scanning direction MS without the influence of the color misregistration in the sub-scanning direction R2.

  Next, the reason why the color misregistration detection patterns in the main scanning direction MS are two sets Ps_1set and one set Ss_set of Ps_2set, whereas the color misregistration detection patterns Ph_1set to Ph_10set in the sub-scanning direction R2 are ten sets will be described. To do. The surface speed of the transfer belt 8 on which the color misregistration detection pattern 111 is formed has an AC speed fluctuation (a fluctuation having a sufficiently long period with respect to one set of color misregistration detection patterns) due to uneven thickness of the transfer belt 8 or the like. is there. This AC speed fluctuation lowers the color misregistration detection accuracy in the sub-scanning direction R2, and thus the detection accuracy is improved by forming the color misregistration detection pattern Ph_nset in the sub-scanning direction R2 a plurality of times and averaging the detection results. ing.

  Next, a method for detecting the driven state of the photosensitive drum 2 that rotates according to the rotation of the transfer belt 8 will be described. Also in the follow-up detection, S_Yn, S_Mn, S_Cn, S_Kn (binary signals 122 of the patterns Ph_Yn, Ph_Mn, Ph_Cn, Ph_Kn (n = 1 to 10) for detecting the color misregistration amount in the sub-scanning direction R2 described above. n = 1-10) is used. In the case of color misregistration detection, distances L1, L2, and L3 between S_Yn that is the binary signal 122 of the reference color yellow and the binary signals S_Mn, S_Cn, and S_Kn of other colors in the same pattern set were calculated. On the other hand, in the case of driven detection, the distance (for example, Lyy) of the binary signals (for example, Ph_Y1 and Ph_Y2) of the same color misregistration detection patterns of adjacent pattern sets (for example, Ph_1set and Ph_2set) is calculated. To do.

  The follow detection will be described with reference to FIG. FIG. 12 is a diagram showing the color misregistration detection pattern 111 (Ph_Y1 to Ph_K10, Ps_Y1 to Ps_K2) and the binary signal 122 (S_Y1 to S_K12), as in FIG. For example, the center position of the yellow color misregistration detection pattern Ph_Y1 in the pattern set Ph_1set and the center position of the yellow color misregistration detection pattern Ph_Y2 in the adjacent pattern set Ph_2set and the distance Lyy are calculated as follows. The timing detection unit 1101 of the host CPU 801 measures a time interval Tyy1 between the center of S_Y1 that is the binary signal 122 of Ph_Y1 and the center of S_Y2 that is the binary signal 122 of Ph_Y2. The timing detection unit 1101 outputs the measured time interval Tyy1 as timing information (time information) 123 to the pattern interval calculation unit 1102. The pattern interval calculation unit 1102 calculates the distance Lyy1 between adjacent yellow Ph_Y1 and Ph_Y2 by multiplying the measured time interval Tyy1 by the process speed of the image forming apparatus 1. The pattern interval calculation unit 1102 outputs the calculated distance Lyy1 to the driven state detection unit 1105 as pattern interval information (adjacent color) 126.

  Similarly, the distance Lmm1 of the adjacent magenta pattern, the distance Lcc1 of the adjacent cyan pattern, and the distance Lkk1 of the adjacent black pattern are also obtained from the time intervals Tmm1, Tmm2, and Tmm3 of the binary signal 122. The distances Lyyn, Lmmn, Lccn, and Lkkn (n = 1 to 9) of adjacent same color patterns from Ph_1set to Ph_10set are calculated. The distances Lyyn, Lmmn, Lccn, and Lkkn (n = 1 to 9) calculated for each of the nine colors are the reference distances (nominal names) if the transfer belt 8 rotates at a nominal speed (process speed). Distance) Same as Lrf2.

  The distances Lyyn, Lmmn, Lccn, and Lkkn (n = 1 to 9) are pattern distances for detecting the driven state of the photosensitive drum 2 and the transfer belt 8.

  FIG. 13 is a diagram showing changes in the distances Lyyn, Lmmn, Lccn, and Lkkn (n = 1 to 9) of adjacent same color patterns for detection of follow-up. When the surface speeds of the photosensitive drum 2 and the transfer belt 8 are the same, that is, when the photosensitive drum 2 and the transfer belt 8 are in a driven state, a result as shown in the graph of FIG. In FIG. 13A, the vertical axis indicates the distance between adjacent identical color patterns, and the distance for nine sets (exemplifying the distances Lyy1 to Lyy9 of adjacent yellow patterns) is plotted in the name of Plot_0. Has been. A broken line indicates the reference distance Lrf2. The fact that the plot of Plot_0 fluctuates in an alternating current (AC) manner is the influence of the fluctuation in the surface speed of the transfer belt 8 described above.

  By the way, when the driven state collapses and a speed difference is generated between the photosensitive drum 2 and the transfer belt 8, the distances Lyyn, Lmmn, Lccn, and Lkkn of adjacent same color patterns are offset. For example, when the transfer belt 8 is fast with respect to the photosensitive drum 2, the transfer interval is widened, so that the distances Lyyn, Lmmn, Lccn and Lkkn of the same color pattern adjacent to the reference distance Lrf2 become large. On the other hand, when the transfer belt 8 is slower than the photosensitive drum 2, the transfer interval is narrowed, so that the distances Lyyn, Lmmn, Lccn and Lkkn of the same color pattern adjacent to the reference distance Lrf2 are reduced.

  FIG. 13B is a diagram showing changes in the distances Lyyn, Lmmn, Lccn, and Lkkn (n = 1 to 9) of the same color pattern when the photosensitive drum 2 and the transfer belt 8 are not driven. In FIG. 13B, Plot_f indicates the result when the transfer belt 8 is fast relative to the photosensitive drum 2, and Plot_s indicates the result when the transfer belt 8 is slow relative to the photosensitive drum 2. Plot_f_ave indicated by a broken line represents an average value of 9 points of Plot_f. Plot_s_ave indicated by a broken line represents an average value of 9 points of Plot_s. As can be seen from FIG. 13B, the average value Plot_f_ave and the average value Plot_s_ave are different from the reference distance Lrf2 compared to Plot_0 in the driven state. The driven state detection unit (calculating unit) 1105 determines whether or not the driven state is maintained by calculating a difference between the average value Plot_ave of the nine distances Lyy1 to Lyy9 of the adjacent same color pattern and the reference distance Lrf2. To do.

Whether or not the driven state is maintained is determined by the following equation (10).
| Lrf2−Plot_ave | ≧ A (10)
Here, Plot_ave is an average value of distances between adjacent same-color patterns, and A is a predetermined value for determining the driven state. When the average value Plot_ave is offset by a predetermined value or more with respect to the reference distance Lrf2, it is determined that the driven state has collapsed. The predetermined value A in this embodiment is 10 μm (micrometer). The reference distance Lrf2 corresponds to the moving distance of the surface of the transfer belt 8 per predetermined time. The average value Plot_ave corresponds to the moving distance of the surface of the photosensitive drum 2 per predetermined time. Therefore, Lrf2−Plot_ave on the left side of Expression (10) corresponds to the difference in the moving distance between the transfer belt 8 and the surface of the photosensitive drum 2 per predetermined time. That is, the value of Lrf2−Plot_ave on the left side of Equation (10) is a value corresponding to the difference between the surface speed of the transfer belt 8 and the surface speed of the photosensitive drum 2 (surface speed difference). Therefore, the driven state detection unit 1105 determines whether or not the driven state is maintained based on the difference between the surface speed of the transfer belt 8 and the surface speed of the photosensitive drum 2 (surface speed difference).

  FIG. 14 is a flowchart showing a color misregistration detection and follow-up detection sequence. A host CPU (control unit) 801 executes a color misregistration detection and follow-up detection sequence based on a program stored in the ROM 812. The upper CPU 801 starts the color misregistration detection and follow-up detection sequence when the main power source of the image forming apparatus 1 is turned on or after the image of the specified number or more is formed. When the sequence is started, the upper CPU 801 forms a color misregistration detection pattern 111 on the transfer belt 8 (S1701). Next, the timing detection unit 1101 of the host CPU 801 starts pattern detection (S1702). The timing detection unit 1101 measures the detection timing from the binary signal 122 of the color misregistration detection pattern 111. The timing detection unit 1101 determines whether or not all patterns have been detected (S1703). The timing detection unit 1101 counts the number of edges of the detected waveform of the binary signal 122, and ends the pattern detection when the count reaches a predetermined number (96 in this embodiment) (YES in S1703).

  If detection of all patterns has not been completed (NO in S1703), the timing detection unit 1101 determines whether or not a predetermined time has elapsed since the start of the formation of the color misregistration detection pattern 111 (S1704). In this embodiment, the predetermined time is set to 10 seconds. However, the predetermined time is not limited to 10 seconds, and may be set appropriately according to the image forming apparatus. If the predetermined time has not elapsed (NO in S1704), the process returns to S1703. If the count does not reach the predetermined number even after the predetermined time has elapsed (YES in S1704), the color misregistration detection pattern 111 may not be formed correctly, the color misregistration detection sensor 100 may be broken, or the like. Therefore, an alert indicating that the color misregistration detection and follow-up detection sequence cannot be executed is displayed on the operation unit (not shown) (S1705).

  When the detection of all patterns is completed (YES in S1703), the pattern interval calculation unit 1102 calculates the pattern distance for calculating the color misregistration amount and the pattern distance for detecting the driven state (S1706). . The color misregistration calculation unit 1103 calculates a color misregistration amount (deviation amount) from the calculated pattern distances L1 to L9 (S1707). The driven state detection unit 1105 determines whether the driven state is maintained from the calculated pattern distances Lyyn, Lmmn, Lccn, and Lkkn (n = 1 to 9) using Expression (10) (S1708). When the average value Plot_ave obtained from the pattern distances Lyyn, Lmmn, Lccn, and Lkkn (n = 1 to 9) is equal to or larger than a predetermined value (NO in S1708), the driven state of the photosensitive drum 2 and the transfer belt 8 is not maintained. to decide. If the average value Plot_ave is less than the predetermined value (YES in S1708), it is determined that the driven state of the photosensitive drum 2 and the transfer belt 8 is maintained. If it is determined that the driven state is maintained (YES in S1708), the color misregistration correction unit 1104 calculates correction information (color misregistration correction amount) 125 from the calculated color misregistration amount (detection result), and performs correction. The information 125 is output to the image formation control unit 1109 (S1709).

  Color misregistration correction based on the correction information 125 will be briefly described. The optical scanning device 5 has a certain time interval from when the image formation start command signal is received from the host CPU 801 to when the image formation is actually started. Color misregistration correction in the sub-scanning direction R2 is performed by adjusting this time interval. The color misregistration correction unit 1104 outputs image formation timing correction information for adjusting this time interval to the image formation control unit 1109 as correction information 125 in the sub-scanning direction R2. In addition, the optical scanning device 5 has a certain time interval from when the synchronization signal is received from the BD 305 to when the image is written in the main scanning direction MS. Color misregistration correction in the main scanning direction MS is performed by adjusting this time interval. The color misregistration correction unit 1104 outputs light beam emission timing correction information for adjusting the time interval to the image formation control unit 1109 as the correction information 125 in the main scanning direction MS.

  Return to the description of the sequence. When the correction information 125 is output to the image formation control unit 1109 in S1709, the color misregistration detection and follow-up detection sequence ends. On the other hand, if it is determined in S1708 that the driven state is broken (followed state NG) (NO in S1708), it is determined that the color misregistration amount calculated in S1707 has not been detected correctly and is discarded (S1710). ). That is, the driven state detection unit (determination unit) 1105 determines whether or not to perform color misregistration correction based on the color misregistration amount based on the surface speed difference between the photosensitive drum 2 and the transfer belt 8. The driven state detection unit 1105 determines whether or not the driven state NG is determined three times consecutively (S1711). If the result of the driven state NG is determined three times consecutively (YES in S1711), it is determined that it is impossible to return to the driven state and an error is determined. If the result of the driven state NG is not continuous three times (NO in S1711), the target speed adjustment unit 1106 adjusts the target speed set value 127 of the photosensitive drum 2 according to the driven state detected in S1708 (S1712). The target speed of the photosensitive drum 2 is the process speed of S1102 in FIG. The target speed adjustment unit 1106 outputs the adjusted target speed set value 127 to the controller 802. The controller 802 executes the assist torque derivation sequence described above (steps after S1103 in the flowchart of FIG. 8) (S1713).

  The reason why the target speed of the photosensitive drum 2 is adjusted in the driven state NG will be described. When the image forming apparatus 1 is shipped, when the photosensitive drum 2 is rotated at the process speed, the surface speed of the photosensitive drum 2 and the surface speed of the transfer belt 8 are adjusted to be the same. However, if the diameter of the photosensitive drum 2 changes due to surface abrasion due to durability, the surface speed of the photosensitive drum 2 does not become the same as the surface speed of the transfer belt 8 even if the photosensitive drum 2 is rotated at the process speed. In the driven state NG, that is, in the state where the peripheral speed difference between the surface speed of the photosensitive drum 2 and the surface speed of the transfer belt 8 is large, even if the photosensitive drum 2 is rotated at the process speed, the surface speed of the photosensitive drum 2 is the surface of the transfer belt 8. It is in a state where it is no longer the same as the speed. Therefore, the target speed of the photosensitive drum 2 is adjusted by the surface speed of the photosensitive drum 2 and the surface speed of the transfer belt 8, and the surface speed of the photosensitive drum 2 is adjusted to the surface speed of the transfer belt 8.

The target speed of the photosensitive drum 2 is adjusted using the following equation (11).
V_tar = ((Lrf2-Plot_ave) / Lrf2) * Vp + Vp (11)
Here, V_tar represents a newly set target speed, and Vp represents a process speed. Expression (11) represents that the target speed V_tar of the photosensitive drum 2 is adjusted with respect to the process speed Vp by the change rate of the average value Plot_ave of the distance of the adjacent same color pattern with respect to the reference distance Lrf2.

The controller 802 derives the assist torque T _AS by rotating the photosensitive drum 2 at the target speed V_tar (S1713). The controller 802 outputs a PWM signal having a predetermined duty ratio corresponding to the derived assist torque T _AS to the motor driver IC 803 to rotate the photosensitive drum 2 (S1714). The upper CPU 801 returns to S1701 again to perform color misregistration detection and follow-up determination sequence. By adjusting the target speed of the photosensitive drum 2, the surface speeds of the photosensitive drum 2 and the transfer belt 8 approach the same value, so that the speed difference that affects color misregistration detection can be reduced. By rotating the photosensitive drum 2 at the adjusted target speed, the color misregistration calculation unit 1103 calculates a new color misregistration amount (new color misregistration correction amount). The image formation control unit 1109 performs color misregistration correction using a new color misregistration amount.

  The upper CPU 801 executes a color misregistration detection and follow-up detection sequence when the number of image formations reaches a predetermined number. An example of the timing for executing the color misregistration detection and follow-up detection sequence will be described with reference to FIG. FIG. 15 is a flowchart showing an image forming sequence. The upper CPU 801 executes an image forming sequence based on a program stored in the ROM 812. When an image output job is submitted, the upper CPU 801 determines whether or not the number of image formations after the previous color misregistration detection and follow-up detection sequence has reached a predetermined number (1000 in this embodiment). Judgment is made (S1801). When the number of formed images has reached the predetermined number (YES in S1801), the upper CPU 801 executes a color misregistration detection and follow-up detection sequence (S1802). After completion of the color misregistration detection and follow-up detection sequence, the upper CPU 801 forms an image based on the input image output job (S1803). On the other hand, if the number of images formed has not reached the predetermined number (NO in S1801), the upper CPU 801 forms an image without executing the color misregistration detection and follow-up detection sequences (S1803). After image formation, the upper CPU 801 determines whether or not there is a next image formation job (S1804). If there is a next image forming job (YES in step S1804), the process returns to step S1801 to confirm again whether or not the number of image forming sheets has reached a predetermined number. If there is no next image forming job (NO in S1804), the image forming sequence is terminated.

  According to the present embodiment, by executing the color misregistration detection and follow-up detection sequence, even if the driven state between the photosensitive drum 2 and the transfer belt 8 is broken, the occurrence of a detection error in the color misregistration detection is prevented. Can do. Therefore, color misregistration correction can be executed more accurately.

  According to this embodiment, it is possible to prevent erroneous detection of the color misregistration amount due to the influence of the surface speed difference between the photosensitive member and the intermediate transfer member.

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus 2 (2a, 2b, 2c, 2d) ... Photosensitive drum 9 (9a, 9b, 9c, 9d) ... Image forming part (image forming means)
8. Transfer belt 100 ... Color misregistration detection sensor (detection means)
111: Color misregistration detection pattern 801: Upper CPU (control means)
805... BLDC motor (drum drive means)
905... BLDC motor (belt drive means)

Claims (4)

A plurality of photosensitive drums;
A plurality of image forming means for forming toner images of different colors on the plurality of photosensitive drums;
An endless transfer belt that is stretched by a plurality of rollers and onto which toner images formed on the plurality of photosensitive drums are transferred, and a transfer belt by rotating at least one of the plurality of rollers A belt drive means for rotating the transfer device, and a transfer device for transferring the toner image transferred onto the transfer belt to a recording medium;
Detecting means for detecting a shift amount between a plurality of color misregistration detection patterns formed on the transfer belt in order to correct a shift between the toner images of a plurality of colors;
The plurality of photosensitive drums are in contact with the transfer belt, and each of the plurality of photosensitive drums is rotated by the rotation of the transfer belt by the belt driving unit,
Drum driving means for applying torque to the plurality of photosensitive drums to be driven to rotate by rotation of the transfer belt;
Control means,
The control means includes
Controlling the plurality of image forming means to form a plurality of color misregistration detection patterns on the transfer belt;
Calculating a deviation amount between the color misregistration detection patterns detected by the detection means;
When the amount of deviation between the color misregistration detection patterns detected by the detection means is equal to or greater than a predetermined value, torque is applied to the plurality of photosensitive drums so that the plurality of photosensitive drums are applied to the transfer belt. The drum driving unit is controlled to rotate following the rotation, and then the color misregistration detection pattern of the plurality of colors is formed again, and the color misregistration correction amount is based on the detection result of the color misregistration detection pattern of the plurality of colors. And correcting misregistration between the toner images of the plurality of colors based on the color misregistration correction amount,
When the shift amount between the color misregistration detection patterns detected by the detection unit is less than a predetermined value, the shift between the toner images of the plurality of colors is based on the shift amount less than the predetermined value. An image forming apparatus characterized by correcting the above.   The image forming apparatus according to claim 1, wherein the control unit discards the shift amount when the shift amount is equal to or greater than the predetermined value. A torque adjusting means for adjusting the torque for rotating the plurality of photosensitive drums by the drum driving means;
When the control unit discards the misregistration amount, a new color misregistration correction amount is calculated by the control unit while the plurality of photosensitive drums are rotated by the drum driving unit with the torque adjusted by the torque adjusting unit. The image forming apparatus according to claim 2, wherein the control unit corrects a shift between the toner images of the plurality of colors based on the new color shift correction amount.   The image forming apparatus according to claim 3, wherein the torque adjusting unit adjusts the torque by adjusting an amount of current flowing through the drum driving unit.

Images