JP2014238461A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that uses an electrostatic latent image and performs accurate color shift correction control.SOLUTION: An image forming apparatus includes: image forming means including a photoreceptor that is driven to rotate, scanning means for scanning the photoreceptor with light to form an electrostatic latent image on the photoreceptor, and process means for acting on the photoreceptor for image formation; forming means for forming an electrostatic latent image for correction for color shift correction on the photoreceptor; power source means corresponding to the process means; and detection means for detecting the output of the power source means when the electrostatic latent image for correction formed on the photoreceptor passes through a latent image detection area. In the direction of rotation of the photoreceptor, the shape on the downstream side of the electrostatic latent image for correction is determined according to the shape on the upstream side of the latent image detection area, or in the direction of rotation of the photoreceptor, the shape on the upstream side of the electrostatic latent image for correction is determined according to the shape on the downstream side of the latent image detection area.

Description

  The present invention relates to an image forming apparatus using an electrophotographic method, and more particularly to an image forming apparatus capable of forming an electrostatic latent image.

  In an electrophotographic image forming apparatus, a so-called tandem method is known in which an image forming unit for each color is independently provided for high-speed printing. In the tandem image forming apparatus, an image is sequentially transferred from an image forming unit of each color to an intermediate transfer belt, and further, the image is transferred collectively from the intermediate transfer belt to a recording medium.

  In such an image forming apparatus, color misregistration (positional misregistration) may occur when the images are overlapped due to mechanical factors in the image forming unit of each color. For this reason, Patent Document 1 discloses that each color toner image for color misregistration detection is formed on an intermediate transfer belt, and a relative positional deviation of each color toner image is detected by an optical sensor to perform correction. Yes. However, in the configuration described in Patent Document 1, it is necessary to form a toner image for color misregistration detection on the photosensitive member and the intermediate transfer belt, and to clean the formed toner image. Will reduce the usability.

  For this reason, Patent Document 2 discloses a configuration in which color misregistration is corrected by an electrostatic latent image formed on a photoconductor.

JP-A-7-234612 JP 2012-32777 A

  The image printed by the image forming apparatus is required to have a predetermined quality. Therefore, it is necessary to improve the detection accuracy of the color misregistration amount for the color misregistration correction control using the electrostatic latent image.

  The present invention provides an image forming apparatus that uses an electrostatic latent image and performs highly accurate color misregistration correction control.

According to one aspect of the present invention, an image forming apparatus includes: a rotationally driven photoconductor; a scanning unit that forms an electrostatic latent image on the photoconductor by scanning the photoconductor with light; Image forming means including a process means acting on the photoconductor, a forming means for forming a correction electrostatic latent image for color misregistration correction on the photoconductor, and a power supply means corresponding to the process means. Detecting means for detecting an output of the power supply means when the electrostatic latent image for correction formed on the photoreceptor passes through a latent image detection area;
And the downstream shape of the electrostatic latent image for correction is determined by the upstream shape of the latent image detection region in the rotational direction of the photosensitive member, or in the rotational direction of the photosensitive member. The shape on the upstream side of the electrostatic latent image for correction is determined by the shape on the downstream side of the latent image detection region.

  The accuracy of color misregistration correction control using an electrostatic latent image can be increased.

1 is a configuration diagram of an image forming unit of an image forming apparatus according to an embodiment. FIG. 3 is a diagram illustrating a high-voltage power supply system in an image forming unit according to an embodiment. FIG. 4 is a diagram illustrating latent image marks formed on an intermediate transfer belt. Explanatory drawing of a latent image mark detection. 6 is a timing chart of color misregistration correction control according to an embodiment. 5 is a flowchart of color misregistration correction control according to an embodiment. Explanatory drawing of the relationship between the latent image detection area | region and latent image mark by one Embodiment. Explanatory drawing of the relationship between the detection waveform of a latent image mark, and a detection error. Explanatory drawing of the relationship between the latent image detection area | region and latent image mark by one Embodiment. Explanatory drawing of the image data for forming the latent image mark by one Embodiment. The figure which shows the primary transfer power supply circuit by one Embodiment. FIG. 4 is a diagram illustrating a potential difference between a surface potential of a photoreceptor and a primary transfer roller. 6 is a timing chart of color misregistration correction control according to an embodiment. 5 is a flowchart of color misregistration correction control according to an embodiment. Explanatory drawing of the relationship between the latent image detection area | region and latent image mark by one Embodiment. Explanatory drawing of the detection voltage by one Embodiment.

  Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings. In the following drawings, components that are not necessary for the description of the embodiments are omitted from the drawings.

<First embodiment>
FIG. 1 is a configuration diagram of an image forming unit of the image forming apparatus according to the present embodiment. The letters a, b, c and d at the end of the reference numerals are members relating to the formation of toner images of yellow (Y), magenta (M), cyan (C) and black (Bk), respectively. It shows that there is. In the following description, when it is not necessary to distinguish between colors, reference numerals excluding the English letters a, b, c, and d at the end are used. The photoconductor 22 is an image carrier and is driven to rotate. The charging roller 23 charges the surface of the corresponding color photoconductor 22 to a uniform potential. The charging roller 23 is arranged with a predetermined crossing angle with respect to the corresponding photosensitive member 22. This reason will be described later. For example, the charging bias output by the charging roller 23 is −1200 V, and thereby the surface of the photoconductor 22 is charged to a potential (dark potential) of −700 V. The scanner unit 20 scans the surface of the photoconductor 22 with a laser beam corresponding to image data of an image to be formed, and forms an electrostatic latent image on the photoconductor 22. As an example, the potential (bright potential) of the portion where the electrostatic latent image is formed becomes −100 V by scanning with laser light. Each of the developing units 25 has a corresponding color toner, and the developing sleeve 24 supplies the toner to the electrostatic latent image on the photosensitive member 22 to develop the electrostatic latent image on the photosensitive member 22. As an example, the developing bias output from the developing sleeve 24 is −350 V, and the developing unit 25 attaches toner to the electrostatic latent image by this potential. The primary transfer roller 26 is an image carrier that transfers the toner image formed on the photoconductor 22 to an intermediate transfer belt 30 that is driven by rollers 31, 32, and 33. As an example, the transfer bias output from the primary transfer roller 26 is +1000 V, and the primary transfer roller 26 transfers toner to the intermediate transfer belt 30 by this potential. At this time, a color image is formed by superimposing the toner images of the respective photosensitive members 22 and transferring them onto the intermediate transfer belt 30.

  The secondary transfer roller 27 transfers the toner image on the intermediate transfer belt 30 to the recording medium 12 conveyed through the conveyance path 18. The fixing roller pairs 16 and 17 heat and fix the toner image transferred to the recording medium 12. Here, the toner that has not been transferred from the intermediate transfer belt 30 to the recording medium 12 by the secondary transfer roller 27 is collected in the waste toner container 36 by the cleaning blade 35. In addition, a detection sensor 40 is provided to face the intermediate transfer belt 30 in order to perform color misregistration correction by forming a conventional toner image.

  The scanner unit 20 may be configured to scan the photosensitive member 22 with an LED array or the like instead of a laser. Further, instead of providing the intermediate transfer belt 30, a direct transfer type image forming apparatus that directly transfers the toner image of each photoconductor 22 to the recording medium 12 may be used.

  Next, the reason why the rotation axis of the charging roller 23 is arranged obliquely with respect to the photosensitive member 22, that is, with a crossing angle will be described. As a method for charging the photosensitive member 22, a contact charging method is widely used because of advantages such as low ozone and low power consumption. However, in the contact charging method, when image formation is repeated for a long period of time, toner particles adhere to the surface of the charging roller 23, and uneven charging occurs. However, by disposing the rotation axis of the charging roller 23 with a crossing angle with respect to the rotation axis of the photosensitive member 22, the rotational directions of the charging roller 23 and the photosensitive member 22 are made different to create a circumferential speed difference. As a result, toner particles adhering to the surface of the charging roller 23 can be removed, and the photoreceptor 22 can be uniformly charged. This is the reason for providing the crossing angle.

  FIG. 2A is a diagram illustrating a high-voltage power supply system to each process unit of the image forming unit 10. Here, the process unit is a member that acts on the photosensitive member 22 for image formation, including any of the charging roller 23, the developing device 25, and the primary transfer roller 26. The charging power supply circuit 43 applies a voltage to the corresponding charging roller 23. Further, the developing power supply circuit 44 applies a voltage to the developing sleeve 24 of the corresponding developing device 25. Further, the primary transfer power supply circuit 46 applies a voltage to the corresponding primary transfer roller 26. In this manner, the charging power supply circuit 43, the development power supply circuit 44, and the primary transfer power supply circuit 46 function as a voltage application unit for the process unit.

  FIG. 2B is a configuration diagram of a charging power supply circuit 43 that applies a voltage to the charging roller 23. The transformer 62 boosts the AC signal from the drive circuit 61. A rectifier circuit 51 including diodes 1601 and 1602 and capacitors 63 and 66 rectifies and smoothes the boosted AC signal, and applies a DC voltage from the output terminal 53 to the charging roller 23. The comparator 60 controls the output voltage of the drive circuit 61 so that the voltage of the output terminal 53 divided by the detection resistors 67 and 68 is equal to the voltage setting value 55 set by the control unit 54. Note that a current having a magnitude corresponding to the voltage of the output terminal 53 flows through the charging roller 23, the photosensitive member 22, and the ground.

  In the present embodiment, the current detection circuit 50 is inserted in the charging power supply circuit 43 between the output circuit 500 on the secondary side of the transformer 62 and the ground point 57. A current flowing from the output terminal 53 to the current detection circuit 50 via the output circuit 500 of the transformer 62 flows from the operational amplifier 70 to the ground through the resistor 71. A detection voltage 56 proportional to the current flowing through the resistor 71, that is, the amount of current flowing through the output terminal 53 appears at the output terminal of the operational amplifier 70. The detection voltage 56 is input to the negative input terminal (inverting input terminal) of the comparator 74, and the comparator 74 outputs a binarized voltage 561 corresponding to the magnitude of the detection voltage 56 and the reference voltage (Vref) 75. To do.

  The binarized voltage 561 output from the comparator 74 is input to the CPU 321 in the control unit 54. The control unit 54 controls the entire image forming apparatus such as controlling the scanner unit 20 in order to form an electrostatic latent image on each photoconductor 22.

  Next, color misregistration correction control in this embodiment will be described. In this embodiment, detection of color misregistration, that is, position misregistration of each color, is performed for each color. In the present embodiment, an electrostatic latent image for correcting misregistration (hereinafter referred to as a latent image mark) is formed on the photosensitive member 22 by scanning with the scanner unit 20, and the latent image mark reaches a position facing the charging roller 23. Measure the time to do. The measured change in arrival time reflects the amount of displacement of the irradiation position by the scanner unit 20, that is, the amount of displacement of the image. It is known that the irradiation position of the scanner unit 20 varies due to a temperature change inside the apparatus due to continuous printing or the like. In this embodiment, it is possible to detect in real time a positional shift accompanying a temperature change inside the apparatus.

  First, a method for detecting a latent image mark will be described. FIG. 3 is a diagram illustrating a state in which the latent image mark 80 is formed on the photoconductor 22. The latent image mark 80 on the photosensitive member 22 formed by the scanner unit 20 is conveyed in the direction of the arrow as the photosensitive member 22 rotates. At this time, the developing sleeve 24 and the primary transfer roller 26 are separated from the photosensitive member 22. A configuration in which the applied voltage is turned off (zero) or a configuration in which a bias having a polarity opposite to that of a normal polarity is applied may be used.

  When the latent image mark 80 reaches the vicinity of the charging roller 23, the amount of current flowing from the photosensitive member 22 to the charging power supply circuit 43 via the charging roller 23 changes. FIG. 4A shows the change over time of the detection voltage 56 of the current detection circuit 50 when the latent image mark 80 passes through the position facing the charging roller 23. 4A shows that the detection voltage 56 starts to decrease when the latent image mark 80 reaches the vicinity of the charging roller 23, and increases when the latent image mark 80 starts to pass through the position opposite to the charging roller 23. Yes. By detecting the binarized voltage 561 obtained by binarizing the detection voltage 56 with the comparator 74, the timing when the leading edge of the latent image mark 80 reaches the charging roller 23 and the trailing edge of the latent image mark 80 are charged on the charging roller 23. It is possible to detect the timing of exiting. The leading edge of the latent image mark 80 is an end portion of the latent image mark 80 on the downstream side (front side in the traveling direction) of the photosensitive member 22 in the rotation direction, and the rear end is an upstream side (back side in the traveling direction). Part.

  Here, the reason why the detection voltage 56 decreases while the latent image mark 80 passes the position facing the charging roller 23 will be described. FIG. 4B shows the surface potential of the photoreceptor 22. In FIG. 4B, the horizontal axis indicates the surface position of the photosensitive member 22 in the rotation direction, and the region 93 indicates a region where the latent image mark 80 is formed. It is assumed that toner is not attached to the latent image mark 80. The vertical axis in FIG. 4B indicates the potential, the dark potential of the photosensitive member 22 is VD (for example, −700 V), the bright potential is VL (for example, −100 V), and the charging bias of the charging roller 23 is VC (for example, −100 V). 1200V). In the region 93 where the latent image mark 80 is formed, the potential difference 96 between the charging roller 23 and the photosensitive member 22 is larger than the potential difference 95 in other regions. For this reason, when the latent image mark 80 reaches the vicinity of the charging roller 23, the current flowing from the photosensitive member 22 toward the charging roller 23 increases. As the current increases, the voltage value at the output terminal of the operational amplifier 70, that is, the detection voltage 56 decreases.

  Note that the current flowing from the photosensitive member 22 toward the charging roller 23 is a discharge generated in the gap between the photosensitive member 22 and the charging roller 23 via the nip portion where the photosensitive member 22 and the charging roller 23 are in contact with each other. I can think of things. Here, the discharge is generated when the voltage applied to the gap between the surfaces of the photosensitive member 22 and the charging roller 23 exceeds the discharge breakdown voltage depending on the distance between the surfaces. In the following description, an area where current flows between the photoconductor 22 and the charging roller 23 is referred to as a latent image detection area. That is, if the region where current flows between the photosensitive member 22 and the charging roller 23 is only the nip portion between the photosensitive member 22 and the charging roller 23, this nip portion becomes a latent image detection region. On the other hand, if a current flows only by discharge between the photosensitive member 22 and the charging roller 23, the area where discharge occurs is the latent image detection area. Further, when both the current via the nip portion and the current due to the discharge are generated, a region where the nip portion and the region where the discharge is generated is a latent image detection region.

  FIG. 5 is a timing chart of color misregistration correction control according to this embodiment. Note that the control in FIG. 5 is performed for each color. The control unit 54 outputs a drive signal for driving the cam for separating the developing sleeve 24 at timing T1, and the developing sleeve 24 changes to a state separated from the photosensitive member 22 at timing T2. Further, the control unit 54 controls the transfer bias of the primary transfer roller 26 from the on state to the off state, that is, to zero at the timing T3. Further, the scanner unit 20 forms a plurality of latent image marks 80 on the photosensitive member 22 by laser light during a period of timing T4 to T6. In FIG. 5, the black square portion indicates the latent image mark 80. Then, between timings T5 and T7, the control unit 54 detects the latent image mark 80 with the binarized voltage 561. Note that the charging power supply circuit 43 outputs a charging bias to the charging roller 23 from the start of control to time T7.

  In the present embodiment, the misregistration of each color is corrected independently. Therefore, a reference value is acquired for each color in advance before performing the above-described color misregistration correction control. The acquisition of the reference value is desirably performed in a state where the amount of color misregistration between the colors is small, for example, after the actually formed toner image is detected by the detection sensor 40 or after performing conventional color misregistration correction control.

  Hereinafter, acquisition of a reference value for a certain color will be described. In order to acquire the reference value, the control unit 54 forms a plurality of latent image marks 80 on the photoconductor 22. The plurality of latent image marks 80 are formed in order to cancel the influence of uneven rotation speed of the photoconductor 22 and the like. In the following description, 20 latent image marks 80 are formed, but this is an example. As shown in FIG. 4A, two edges, rising and falling, are generated in the binarized voltage 561 by one latent image mark 80. Therefore, by forming 20 latent image marks 80, the control unit 54 detects 40 edges for each color, but the control unit 54 detects each edge detection time t (k) ( k = 1 to 40).

  The control part 54 calculates | requires and preserve | saves the reference value es by the following formula | equation (1) after detecting all the edges. Equation (1) is obtained by integrating the detection times of the intermediate positions of the edges of the latent image marks 80.

  FIG. 6 is a flowchart of color misregistration correction control. By starting the color misregistration correction, the control unit 54 forms the same number, for example, 20 latent image marks 80 on the photoconductor 22 as the reference value is acquired in S1. In S <b> 2, the control unit 54 detects each edge at the front end and the rear end of the latent image mark 80 based on a change in the detection current of the current detection circuit 50, and detects each edge with respect to the same reference timing as when the reference value is acquired. Time t (i) is measured. Subsequently, in S3, the control unit 54 calculates Δes by the following equation (2).

  In S4, the control unit 54 determines whether or not a value obtained by subtracting the reference value es from Δes is 0 or more. When the value obtained by subtracting the reference value es from Δes is 0 or more, this indicates that the irradiation timing of the laser light of the scanner unit 20 corresponding to the color is delayed from the reference value. Therefore, in that case, the control unit 54 advances the irradiation timing of the laser beam of the scanner unit 20 corresponding to the color in S5. The amount to be advanced corresponds to a value obtained by subtracting the reference value es from Δes. On the other hand, when the value obtained by subtracting the reference value es from Δes is less than 0, this indicates that the laser unit irradiation timing of the scanner unit 20 corresponding to the color is earlier than the reference value. Therefore, in that case, the control unit 54 delays the irradiation timing of the laser light of the scanner unit 20 corresponding to the color in S6. The amount to be delayed is also an amount corresponding to the difference between Δes and the reference value es. By performing the above processing for each color, it is possible to correct the positional deviation of the toner image of each color. Of course, when the difference between Δes and the reference value es is zero, the irradiation timing remains as it is.

  Next, the shape of the latent image mark 80 formed on the photoconductor 22 will be described. As already described, in this embodiment, the latent image mark 80 is detected by detecting a change in current that occurs when the latent image mark 80 passes through the position facing the charging roller 23. Therefore, the current change amount increases as the change amount per unit time of the area of the latent image mark 80 entering the latent image detection region increases. Similarly, the amount of current change increases as the amount of change per unit time of the area of the latent image mark 80 that exits from the latent image detection region increases. FIGS. 7A and 7B are views showing the relationship between the latent image detection area 81 and the latent image mark 80 formed on the photosensitive member 22. In order to obtain a good detection result, it is desirable that the latent image mark 80 is formed as wide as possible in the main scanning direction and has a width equal to or larger than the width of the latent image detection region 81 in the sub scanning direction. . However, even if the area of the latent image mark 80 is smaller than the latent image detection area 81, the current change amount is small, but detection is possible.

  FIG. 7A shows the case where the latent image detection area 81 has no inclination with respect to the latent image mark 80, and FIG. 7B shows the case where the latent image detection area 81 has an inclination with respect to the latent image mark 80. It shows the state when it has. In the present embodiment, the rotation axis of the charging roller 23 and the rotation axis of the photosensitive member 22 are not parallel but have a predetermined angle, so the relationship between the latent image mark 80 and the latent image detection area 81 is as shown in FIG. become. When an inclination occurs as shown in FIG. 7B, the area of the latent image mark 80 that enters the latent image detection area 81 per unit time is smaller than when the inclination does not occur as shown in FIG. 7A. Become. Therefore, in the case shown in FIG. 7B, the rising slope of the detection voltage 56 is smaller than in the case of FIG. Similarly, when the tilt occurs as shown in FIG. 7B, the area of the latent image mark 80 that escapes from the latent image detection area 81 per unit time as compared with the case where the tilt does not occur as shown in FIG. 7A. Will be less. Therefore, in the case shown in FIG. 7B, the falling slope of the detection voltage 56 is smaller than in the case of FIG.

  Here, the influence of the difference in the steepness (slope) of the change in the detection voltage 56 on the detection result will be described. In FIG. 8, the vertical axis indicates the detection voltage 56, and the horizontal axis indicates time. Th represents a reference voltage (Vref) 75 that is a threshold for position detection, and L represents a voltage fluctuation value. 8A shows the case where the positional relationship between the latent image detection area 81 and the latent image mark 80 is shown in FIG. 7A, and FIG. 8B shows the case shown in FIG. 7B. Yes. As can be seen from FIG. 4A, the actual detection voltage 56 does not become an ideal straight line, but fluctuates due to the voltage fluctuation L. As shown in FIG. 8, when the detection voltage 56 fluctuates, the timing at which the detection voltage 56 reaches the threshold value deviates from the ideal position. The detection timing error Δt increases as the detection voltage 56 rises slowly as described below.

First, assuming that the width of the latent image detection area 81 in the main scanning direction is S, the width in the sub-scanning direction is w, and the speed of the photosensitive member 22 is vd, as is apparent from FIG. The rise time t1 of the detection voltage 56 is obtained by w / vd. If the amplitude change of the detection voltage 56 is Vpp,
From the relationship of Vpp / t1 = L / Δt1, Δt1 is
Δt1 = w · L / (vd · Vpp) (3)
It becomes.

Next, the case of tilting as shown in FIG. 8B will be described. The size of the latent image detection area 81 and the speed of the photosensitive member 22 are the same as in FIG. As shown in FIG. 8B, after the latent image mark 80 starts to enter the latent image detection area 81 until the latent image detection area 81 is covered, that is, until the detection voltage 56 rises, Let S be the moving distance. In this case, S = K · sin θ + w · cos θ, and the time t2 in FIG. 8B is S / vd = (K · sin θ + w · cos θ) / vd. Therefore,
From the relationship of Vpp / t2 = L / Δt2, Δt2 is
Δt2 = (K · sin θ + w · cos θ) · L / (vd · Vpp) (4)
It becomes.

  From equation (4), it can be seen that the smaller the slope of the detected voltage, the greater the error for the same voltage fluctuation. As the error increases, the accuracy of correcting the misalignment can thereby be reduced. Note that the rise of the detection voltage 56, that is, the time when the area of the latent image mark 80 covering the latent image detection area 81 is increased as the latent image mark 80 enters the latent image detection area 81 is described. However, the same applies to the fall of the detection voltage 56. Therefore, by making the change in the detection time steep, it is possible to suppress a decrease in detection accuracy due to the influence of the timing error caused by the voltage fluctuation generated in the detection voltage 56, and thus the accuracy of correcting the image positional deviation. The decrease can also be suppressed.

  For this reason, in this embodiment, as shown in FIG. 9A, the latent image mark 80 is formed to be inclined according to the angle between the rotation axis of the charging roller 23 and the rotation axis of the photosensitive member 22. In FIG. 9A, the latent image mark 80 is formed so that the two sides of the latent image mark 80 are parallel to the rotation axis of the charging roller 23. The present invention is not limited to the parallel form, and the shape of the latent image mark 80 enters the latent image detection area 81 per unit time so that the rising slope of the detection voltage 56 is a required value. The area of the latent image mark 80 to be determined can be determined. Specifically, the area of the latent image mark 80 that enters the latent image detection area 81 per unit time, where the rising slope of the detection voltage 56 is a required value, is determined as a threshold value, and the unit enters per unit time. The shape of the latent image mark 80 can be determined so that the area is equal to or greater than the threshold value. Similarly, the shape of the latent image mark 80 can be determined by the area of the latent image mark 80 that escapes from the latent image detection area 81 per unit time so that the falling slope of the detection voltage 56 is a required value. it can. In other words, the area of the latent image mark 80 that exits from the latent image detection area 81 per unit time, with the slope of the detection voltage 56 falling as a required value, is determined as a threshold, and the area that exits per unit time is the threshold. As described above, the shape of the latent image mark 80 can be determined.

Subsequently, detection errors in the case of forming as shown in FIG. 9A will be described. As shown in FIG. 9A, after the latent image mark 80 starts to enter the latent image detection area 81 until the latent image detection area 81 is completely covered, that is, until the detection voltage 56 rises, Let S be the moving distance. In this case, S = w / cos θ, and the time t3 in FIG. 9B is S / vd = w / (vd · cos θ). Therefore,
From the relationship of Vpp / t3 = L / Δt3, Δt3 is
Δt3 = (L · w) / (vd · Vpp · cos θ) (5)
It becomes. θ is, for example, about 0.2 degrees, and therefore Equation (5) is approximately the same as the case of FIG. 7B shown in Equation (3).

  As described above, according to the present invention, even when the charging roller 23 is tilted with respect to the photosensitive member 22, it is possible to prevent deterioration in detection accuracy of color misregistration correction due to the tilt, and realize accurate color misregistration correction. it can.

  Next, formation of the latent image mark 80 will be described with reference to FIG. For example, as shown in FIG. 10A, a latent image mark 80 inclined by 20 dots in the sub-scanning direction is formed with respect to 5906 dots in the main scanning direction. FIG. 10B shows image data for forming the latent image mark 80 tilted as shown in FIG. Here, a portion where the scanning line is changed is referred to as a step portion. Therefore, 20 step portions are provided in FIG. Specifically, the scanning line is divided into 20 regions, and the scanning line is shifted in the sub-scanning direction by one dot for each division. Furthermore, in this embodiment, the exposure ratio of adjacent dots in the sub-scanning direction is adjusted at this step portion. That is, as shown in FIG. 10B, for the position where scanning is performed in the sub-scanning direction, the sum of the two exposure amounts is substantially equal to the exposure amount at the position where scanning is not performed in the sub-scanning direction. Like. This is to smooth the level difference of the image at the level difference portion and make the level difference difficult to see. Note that the image data illustrated in FIG. 10B when the rotation axes of the photosensitive member 22 and the charging roller 23 are not parallel is stored in the CPU 321 in advance.

<Second embodiment>
In this embodiment, the latent image mark 80 is detected by the primary transfer power supply circuit 46 that applies a voltage to the primary transfer roller 26. FIG. 11 is a configuration diagram of the primary transfer power supply circuit 46. In the present embodiment, the primary transfer power supply circuit 46 is configured to supply a voltage to all of the primary transfer rollers 26a to 26d in FIG. That is, the primary transfer power supply circuit 46 of this embodiment is a circuit in which the primary transfer power supply circuits 46a to 46d in FIG. In the primary transfer power supply circuit 46, the directions of the anodes and cathodes of the diodes 1601 and 1602 are opposite to those of the charging power supply circuit 43 in FIG. This is because the polarity of the applied potential is opposite to that of the charging power supply circuit 43. The output terminals 53a to 53d are output terminals to the primary transfer rollers 26a to 26d, respectively. As shown in FIG. 11, in this embodiment, the current detection circuit 150 is provided in common to the circuit that applies a voltage to the primary transfer roller 26 of each color, so that the detection voltage 56 is applied to the output terminals 53a to 53d. The voltage corresponds to the sum of the flowing currents.

  Next, color misregistration correction control in the present embodiment will be described focusing on differences from the first embodiment. In this embodiment, the latent image mark 80 is detected by a current detection circuit 150 that detects a current flowing through the primary transfer roller 26. For this reason, the primary transfer roller 26 needs to be in contact with the photoreceptor 22. Further, the developing sleeve 24 is also brought into contact with the photosensitive member 22 and the developing bias is turned off (zero), or the developing bias is set to a polarity opposite to that of the normal one, so that the toner is not placed on the latent image mark 80. Depending on the influence of the surrounding environment or the like, some toner may adhere, but even in such a case, the latent image mark 80 can be detected. Note that, similarly to the first embodiment, the developing sleeve 24 may be separated from the photosensitive member.

  12A shows the potential difference between the photosensitive member 22 and the primary transfer roller 26 when the toner is not attached to the latent image mark 80, and FIG. 12B shows the potential difference between the photosensitive member 22 and the primary transfer roller 26 when the toner is attached to the latent image mark 80. ing. In FIG. 12, the vertical axis indicates the potential, the dark potential of the photosensitive member 22 is VD (for example, −700 V), the bright potential is VL (for example, −100 V), and the transfer bias of the primary transfer roller 26 is VT (for example, +1000 V). It is said. When toner adheres, in the area 93 of the latent image mark 80, the potential difference 112 between the primary transfer roller 26 and the photosensitive member 22 becomes larger than the potential difference 111 when no toner adheres. The difference from the potential difference 110 is reduced. Therefore, the more toner that has adhered, the smaller the current change in the area of the latent image mark 80. However, if the toner amount is small, the current change can be detected.

  FIG. 13 is a timing chart of color misregistration correction control according to the present embodiment. First, at timing T <b> 1, the control unit 54 turns off the developing bias output from the developing power supply circuit 44 to the developing sleeve 24. During the period from timing T2 to timing T4, the control unit 54 forms the latent image mark 80 on the photoconductor 22 of each color by laser light. In this embodiment, since the current detection circuit 150 is common to each color, the latent image mark 80 is formed so that the timing at which the latent image mark 80 of each color comes to the position of the primary transfer roller 26 is different. The controller 54 detects the latent image mark 80 of each photoconductor during the period from timing T3 to T5. Note that the primary transfer power supply circuit 46 applies a transfer bias to the primary transfer roller 26 from the start of control to time T5.

  Also in the present embodiment, the reference value is acquired in advance before performing the positional deviation correction control. As in the first embodiment, the reference value is determined by forming a plurality of latent image marks 80 on each photoconductor 22 and measuring the detection time of each edge with respect to the reference timing. In the following description, 20 latent image marks 80 are formed on each photoconductor 22, but this is an example. In the present embodiment, yellow is used as a reference color, and a relative positional shift with respect to this reference color is corrected for colors other than the reference color. Accordingly, the reference values esYM, esYC, and esYBk for magenta, cyan, and black are obtained and stored according to the following (6), (7), and (8), respectively.

In the above equation (6), tm (k) is the detection time of the latent image mark 80 of the photosensitive member 22b corresponding to magenta, and ty (k) is the latent image mark 80 of the photosensitive member 22a corresponding to yellow. Detection time. Similarly, in the above formulas (7) and (8), tc (k) and tbk (k) are the detection times of the latent image mark 80 of the photoreceptor 22c corresponding to cyan and the photoreceptor 22d corresponding to black, respectively. It is. Note that ty (k) is the same as in equation (6).

  FIG. 14 is a flowchart of color misregistration correction control according to this embodiment. With the start of the color misregistration correction control, the control unit 54 forms the same number of latent image marks 80, for example, 20 latent image marks 80 on each photoconductor 22 in S11 as when the reference value is acquired. In S <b> 12, the control unit 54 detects the leading and trailing edges of the latent image mark 80 based on a change in the current value detected by the current detection circuit 150. More specifically, the control unit 54 measures the detection times ty (i), tm (i), tc (i), and tbk (i) of each edge with respect to the same reference timing as when the reference value is acquired. Subsequently, in S13, the control unit 54 calculates ΔesYM, ΔesYC, ΔesYBk by the following equations (9), (10), and (11).

  In S14, the control unit 54 determines whether or not a value obtained by subtracting the reference value esYM from ΔesYM is 0 or more. When the value obtained by subtracting the reference value esYM from ΔesYM is 0 or more, this indicates that the laser beam irradiation timing of the magenta scanner unit 20b is delayed with respect to the reference scanner unit 20a. Therefore, the control unit 54 advances the laser beam irradiation timing of the scanner unit 20b in S15. The amount to be advanced corresponds to a value obtained by subtracting the reference value esYM from ΔesYM. On the other hand, when the value obtained by subtracting the reference value esYM from ΔesYM is less than 0, this indicates that the irradiation timing of the laser light of the scanner unit 20b corresponding to magenta is delayed with respect to the reference scanner unit 20a. . Therefore, the control unit 54 delays the irradiation timing of the laser light of the scanner unit 20b in S16. The amount to be delayed is also an amount corresponding to the difference between ΔesYM and the reference value esYM. Of course, when the value obtained by subtracting the reference value esYM from ΔesYM is 0, it is not necessary to change the laser beam irradiation timing of the scanner unit 20b. The control unit 54 performs the same process as the process for magenta on the scanner unit 20c corresponding to cyan in S17 to S19, and performs the process on the scanner unit 20d corresponding to black in S20 to S22.

  Next, the shape of the latent image detection area of the photosensitive member 22 and the primary transfer roller 26 and the detection waveform of the latent image mark 80 will be described. In general, the primary transfer roller 26 is composed of a metallic core metal and a rubber layer, and the shape of the latent image detection area 81 as a nip portion is a core metal diameter, rubber roller diameter, rubber hardness, roller length. Further, it is determined by the urging force to the photosensitive member 22 or the like. Ideally, it is rectangular as shown in FIG. This is because, as described in the first embodiment, the latent image mark 80 simultaneously overlaps the entire main image scanning direction of the latent image detection area 81, and the detection voltage 56 rises steeply. However, in reality, the cored bar is bent by the urging force, and the upstream and downstream shapes of the latent image detection region 81 in the rotation direction of the photosensitive member 22 are concave as shown in FIG. That is, the width in the sub-scanning direction becomes narrower from the end to the center. In this case, since the latent image mark 80 enters from both ends of the latent image detection area 81, the latent image detection area 81 enters the latent image detection area 81 per unit time as compared with the ideal shape of the latent image detection area 81. The area of the latent image mark 80 is reduced. Therefore, the detection voltage 56 rises slowly. On the contrary, when the latent image mark 80 goes out of the latent image detection area 81, the falling of the detection voltage 56 becomes gradual. Therefore, as described in the first embodiment, the detection error increases and the color misregistration correction accuracy deteriorates.

  Therefore, in this embodiment, in order to make the rise and fall of the detection voltage 56 steep, as shown in FIG. 15C, the latent image mark 80 is arranged on the upstream side and the downstream side in the rotation direction of the photoconductor 22. The shape of is made convex. That is, the latent image mark 80 is formed so that the width of the central portion of the latent image mark 80 in the sub-scanning direction is increased so that the latent image mark 80 enters the entire latent image detection area 81 in the main scanning direction. . In other words, the shape on the downstream side (front side in the traveling direction) of the latent image mark 80 in the rotation direction of the photosensitive member 22 is determined to be the same as the shape on the upstream side of the latent image detection region 81. Similarly, the shape on the upstream side (the rear side in the traveling direction) of the latent image mark 80 in the rotation direction of the photosensitive member 22 is determined to be the same as the shape on the downstream side of the latent image detection region 81. With this configuration, the latent image mark 80 that enters or exits the latent image detection area 81 per unit time can be enlarged.

  Reference numerals 90 to 92 in FIG. 16 indicate temporal changes in the detection voltage 56 in the cases of FIGS. As shown in FIG. 16, in the case of FIG. 15B, the rise and fall of the detection voltage 56 are gentler than in the case of FIG. Further, since the area of the latent image detection region 81 shown in FIG. 15B is smaller than the area of the latent image detection region 81 of FIG. 15A, the potential after rising is low. On the other hand, by forming the latent image mark 80 as shown in FIG. 15C, the rising and falling edges of the detection voltage 56 are made steep, so that the detection accuracy of the position of the latent image mark 80 is degraded. Can be prevented. With this configuration, highly accurate color misregistration correction can be realized.

  It is also possible to detect the latent image mark 80 by providing a current detection circuit in the development power supply circuit 44. In the first embodiment, the current flowing through each photoconductor 22 is detected individually, and in the second embodiment, it is detected by a common circuit. However, the method of detecting current individually and the method of detecting with a common circuit can be applied without depending on the process unit used for current detection. In the first embodiment, each photoconductor 22 independently corrects the positional deviation, and in the second embodiment, the relative positional deviation with respect to the reference color is corrected. However, any method can be applied without depending on the process unit used for current detection.

  Further, in each of the above-described embodiments, the electrostatic latent image formed on the photoconductor 22 is detected by a change in the current flowing between the process unit and the photoconductor 22. In other words, a change in the potential of the surface of the photoconductor 22 is detected by a change in the current flowing between the process unit and the photoconductor 22. However, for example, when constant current control is used for primary transfer to the intermediate transfer belt 30, a change in the potential of the surface of the photoreceptor 22 is detected as a change in the voltage output from the primary transfer power supply circuit 46. That is, the latent image mark 80 can be detected not only by the output current of the power supply circuit to the process unit but also by the output voltage.

[Other Embodiments]
The present invention can also be realized by executing the following processing. That is, software (program) that realizes the functions of the above-described embodiments is supplied to a system or apparatus via a network or various storage media, and a computer (or CPU, MPU, etc.) of the system or apparatus reads the program. It is a process to be executed.

Claims (13)

A rotatingly driven photoreceptor, scanning means for forming an electrostatic latent image on the photoreceptor by scanning the photoreceptor with light, and process means acting on the photoreceptor for image formation. Image forming means;
Forming means for forming a correcting electrostatic latent image for color misregistration on the photosensitive member;
Power supply means corresponding to the process means;
Detecting means for detecting an output of the power supply means when the electrostatic latent image for correction formed on the photosensitive member passes through a latent image detection area;
With
In the rotational direction of the photoconductor, the downstream shape of the electrostatic latent image for correction is determined by the upstream shape of the latent image detection region, or the static image for correction is determined in the rotational direction of the photoconductor. The image forming apparatus according to claim 1, wherein the shape of the upstream side of the electrostatic latent image is determined by the shape of the downstream side of the latent image detection region.   The shape on the upstream side of the latent image detection region in the rotation direction of the photosensitive member is concave, and the shape on the downstream side of the electrostatic latent image for correction is convex. Image forming apparatus.   The shape of the downstream side of the latent image detection region in the rotation direction of the photosensitive member is concave, and the shape of the upstream side of the electrostatic latent image for correction is convex. The image forming apparatus described in 1.   In the rotational direction of the photoconductor, the downstream shape of the electrostatic latent image for correction is the same as the upstream shape of the latent image detection region, or in the rotational direction of the photoconductor The shape of the electrostatic latent image for correction is determined so that the shape on the upstream side of the electrostatic latent image for correction is the same as the shape on the downstream side of the latent image detection region. Item 4. The image forming apparatus according to any one of Items 1 to 3. A rotatingly driven photoreceptor, scanning means for forming an electrostatic latent image on the photoreceptor by scanning the photoreceptor with light, and process means acting on the photoreceptor for image formation. Image forming means;
Forming means for forming a correcting electrostatic latent image for color misregistration on the photosensitive member;
Power supply means corresponding to the process means;
Detecting means for detecting an output of the power supply means when the electrostatic latent image for correction formed on the photosensitive member passes through a latent image detection area;
With
The shape of the electrostatic latent image for correction is determined according to the direction of the rotation axis of the process means.   6. The image forming apparatus according to claim 5, wherein the electrostatic latent image for correction has a shape having a side parallel to a rotation axis of the process means. A rotatingly driven photoreceptor, scanning means for forming an electrostatic latent image on the photoreceptor by scanning the photoreceptor with light, and process means acting on the photoreceptor for image formation. Image forming means;
Forming means for forming a correcting electrostatic latent image for color misregistration on the photosensitive member;
Power supply means corresponding to the process means;
Detecting means for detecting an output of the power supply means when the electrostatic latent image for correction formed on the photosensitive member passes through a latent image detection area;
With
The shape of the electrostatic latent image for correction is such that the electrostatic latent image for correction that enters the latent image detection area per unit time while the electrostatic latent image for correction enters the latent image detection area. It is determined by the area of the image or the area of the electrostatic latent image for correction that escapes from the latent image detection area per unit time while the electrostatic latent image for correction escapes from the latent image detection area. An image forming apparatus.   8. The shape of the electrostatic latent image for correction is determined so that the area of the electrostatic latent image for correction that enters the latent image detection area per unit time is equal to or larger than a threshold value. The image forming apparatus described in 1.   The shape of the electrostatic latent image for correction is determined such that the area of the electrostatic latent image for correction that escapes from the latent image detection area per unit time is equal to or greater than a threshold value. The image forming apparatus according to 8.   The process means is a charging means for charging the photoconductor, a developing means for developing an electrostatic latent image formed on the photoconductor with toner to form a toner image on the photoconductor, and formed on the photoconductor. The image forming apparatus according to claim 1, wherein the image forming apparatus is any one of a transfer unit that transfers a toner image to a recording medium or an image carrier.   The image forming apparatus according to claim 1, wherein the latent image detection area is an area where the photoconductor and the process unit are in contact with each other.   By detecting a change in current detected by the detection means with a threshold value, a deviation amount from the reference value of the electrostatic latent image for correction is detected, and a position of an image formed on the photoconductor is controlled based on the deviation amount. The image forming apparatus according to claim 1, further comprising a control unit that performs the control.   By detecting a change in voltage detected by the detection means with a threshold value, a shift amount is detected from a reference value of the electrostatic latent image for correction, and a position of an image formed on the photoconductor is controlled based on the shift amount. The image forming apparatus according to claim 1, further comprising a control unit.