JP2014115346A - Image forming apparatus - Google Patents

Abstract

Provided is a technique for correcting color misregistration using an electrostatic latent image for color misregistration correction without forming a toner image for color misregistration while maintaining the usability of the image forming apparatus.
An image forming apparatus according to the present invention forms an electrostatic latent image pattern for color misregistration correction on a photosensitive drum, and the electrostatic latent image pattern faces a specific process unit on the photosensitive drum. Is measured for each image forming unit. The image forming apparatus further determines the timing at which the electrostatic latent image for color misregistration correction of each color is detected at the detection position in the next color misregistration correction based on the measured moving time for each image forming unit. Thus, the start timing of forming the electrostatic latent image for color misregistration correction of each color is controlled.
[Selection] Figure 8

Description

  The present invention relates to an image forming apparatus using an electrophotographic system.

  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 a tandem type image forming apparatus, images are sequentially superimposed and transferred from an image forming unit of each color to an intermediate transfer belt, and further, the images are transferred from the intermediate transfer belt to a recording medium in a lump, thereby transferring the images to the recording medium. Form.

  In such an image forming apparatus, due to mechanical factors of the image forming unit, there is a possibility that color misregistration (position misregistration) in which the positions of the images of the respective colors deviate from each other may occur when the images are superimposed and transferred. In particular, when each image forming unit independently has a laser scanner (optical scanning device) and a photosensitive drum, color misregistration occurs if the positional relationship between the laser scanner and the photosensitive drum is different for each image forming unit. there is a possibility. This can occur when the laser scanning position on the photosensitive drum is not synchronized.

  In order to correct such color misregistration, color misregistration correction control is generally performed in the image forming apparatus. In Patent Document 1, a toner image for detection of each color is transferred from a photosensitive drum to an image carrier (such as an intermediate transfer belt), and the relative position of the detection toner image in the main scanning direction and the conveyance direction is measured using an optical sensor. While detecting, color misregistration correction control is performed based on the detection result.

JP-A-7-234612

  However, as described above, when color misregistration correction control is performed by detecting a detection toner image with an optical sensor, the detection toner image used for color misregistration correction control from the photosensitive drum to the image carrier (intermediate transfer belt). (100% density) is transferred. In this case, since it is necessary to clean the transferred detection toner image after the color misregistration correction control, the usability of the image forming apparatus is lowered.

  The present invention has been made in view of the above problems. The present invention provides a technique for performing color misregistration correction by using an electrostatic latent image for color misregistration correction without forming a toner image for color misregistration correction while maintaining the usability of the image forming apparatus. With the goal.

  The present invention can be realized as an image forming apparatus, for example. An image forming apparatus according to an aspect of the present invention includes a photosensitive member that is rotationally driven, a process unit that is disposed in the vicinity of the photosensitive member, and that acts on the photosensitive member, and irradiates the photosensitive member with light. Light irradiation means for forming an electrostatic latent image on the body, each of which is provided corresponding to a plurality of image forming means for forming images of different colors on the photosensitive member, and a plurality of process means. A power supply unit that functions as a power source for the process unit, and a common position that is provided for a plurality of image forming units, and the electrostatic latent image for color misregistration correction that is formed on the photosensitive member faces the process unit. In accordance with the change in the output of the power supply means that occurs when passing the detection position corresponding to the detection position, the detection means for detecting the timing when the electrostatic latent image for color misregistration correction reaches the detection position and the light irradiation means are controlled. , Electrostatic latent image shape for color misregistration correction Measuring unit for measuring each of the plurality of image forming units with the time from the start timing to the timing detected by the detecting unit as a moving time, and each color by the detecting unit based on the moving time measured by the measuring unit The timing for starting the formation of electrostatic latent images for color misregistration correction for each color is controlled so that the timing at which the electrostatic latent image for color misregistration correction is detected is different for each of the plurality of image forming means. And a control means.

  According to the present invention, there is provided a technique for performing color misregistration correction using an electrostatic latent image for color misregistration correction without forming a toner image for color misregistration correction while maintaining the usability of the image forming apparatus. it can.

1 is a diagram illustrating a configuration example of a tandem (4-drum system) image forming apparatus according to a first embodiment. FIG. The figure which shows the structural example of the high voltage power supply device which concerns on 1st Embodiment. The figure which shows the circuit structural example containing the charging high voltage power supply circuit and current detection circuit in the high voltage power supply device which concerns on 1st Embodiment. The block diagram which shows the functional block which the engine control part 54 which concerns on 1st Embodiment has. , , 6 is a flowchart illustrating a procedure of reference value generation processing executed in the image forming apparatus according to the first embodiment. FIG. 4 is a diagram illustrating an example of a color misregistration correction toner pattern formed on an intermediate transfer belt and an example of an electrostatic latent image formed on a photosensitive drum. FIG. 6 is a graph showing an example of a detection voltage output by a current detection circuit, and a diagram showing a change in the surface potential of the photosensitive drum when toner is not attached to the electrostatic latent image and when it is not attached. 6 is a timing chart showing operation timings of the image forming apparatus corresponding to the reference value generation processing according to the first embodiment. , , 5 is a flowchart illustrating a procedure of color misregistration correction control executed in the image forming apparatus according to the first embodiment. 6 is a timing chart showing operation timings of the image forming apparatus corresponding to color misregistration correction control according to the first embodiment. The figure which shows the circuit structural example containing the charge high voltage power supply circuit and current detection circuit in the high voltage power supply device which concerns on 2nd Embodiment. The figure which shows an example of the output voltage of a current detection circuit filtered by the high pass filter. 10 is a timing chart showing operation timings of the image forming apparatus corresponding to the reference value generation processing according to the second embodiment. 9 is a timing chart showing operation timings of the image forming apparatus corresponding to color misregistration correction control according to the second embodiment. 10 is a timing chart showing operation timings of the image forming apparatus corresponding to the reference value generation processing according to the third embodiment. 10 is a timing chart showing operation timings of the image forming apparatus corresponding to color misregistration correction control according to the third embodiment. The figure which shows an example of the output voltage of a current detection circuit filtered by the high pass filter. The figure which shows the circuit structural example containing the charging high voltage power supply circuit and current detection circuit in the high voltage power supply device which concerns on a modification.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. The following embodiments do not limit the invention according to the claims, and all combinations of features described in the embodiments are not necessarily essential to the solution means of the invention.

[First Embodiment]
<Overall configuration of image forming apparatus>
First, an overall configuration of an image forming apparatus according to a first embodiment of the present invention will be described with reference to FIG. FIG. 1 shows a configuration of an image forming apparatus 10 of a tandem system (4-drum system).

  In the image forming apparatus 10, first, the registration sensor 111 detects the leading end of the recording medium 12 fed onto the conveyance path by the pickup roller 13. Thereafter, when the leading end of the recording medium 12 reaches a position slightly passing through the conveying roller pairs 14 and 15, the conveyance of the recording medium 12 is temporarily stopped.

  The scanner units 20a to 20d include reflection mirrors, laser diodes (light emitting elements), and the like, and irradiate laser beams 21a to 21d to photosensitive drums 22a to 22d, which are examples of rotationally driven photosensitive members, respectively. At this time, the photosensitive drums 22a to 22d are previously charged by the charging rollers 23a to 23d. For example, a voltage of −1200 V is applied to each of the charging rollers 23 a to 23 d. The charging rollers 23a to 23d charge the photosensitive drums 22a to 22d so that the surfaces of the photosensitive drums 22a to 22d have a potential of −700 V (hereinafter referred to as “background potential”), for example. In the present embodiment, the charging rollers 23a to 23d are examples of charging means.

  When the surfaces of the charged photosensitive drums 22a to 22d are irradiated with the laser beams 21a to 21d, the potential of the irradiated portions changes from −700V to −100V, for example. By changing the surface potential of the photosensitive drums 22a to 22d with the laser beams 21a to 21d, electrostatic latent images are formed on the surfaces of the photosensitive drums 22a to 22d. As described above, the scanner units 20a to 20d are an example of a light irradiation unit that irradiates the photosensitive drums 22a to 22d (photosensitive members) with light to form an electrostatic latent image on the sightseeing drum.

  The developing sleeves 24a to 24d supply toner from the developing units 25a to 25d to the surfaces of the photosensitive drums 22a to 22d, respectively, by applying a voltage of −350V, for example. Accordingly, the developing sleeves 24a to 24d adhere toner (developer) to the electrostatic latent images formed on the surfaces of the photosensitive drums 22a to 22d. As a result, the electrostatic latent image is developed, and toner images (developer images) are formed on the surfaces of the photosensitive drums 22a to 22d. The developing units 25a to 25d contain different color toners, for example, yellow (Y), magenta (M), cyan (C), and black (Bk) toners. Each is housed. Therefore, Y, M, C, and Bk toner images are formed on the surfaces of the photosensitive drums 22a to 22d, respectively. Thus, the letter a of each reference symbol corresponds to Y color, b corresponds to M color, c corresponds to C color, and d corresponds to Bk color. In the present embodiment, the developing sleeves 24a to 24d are examples of developing units.

  The primary transfer rollers 26a to 26d output a positive voltage of, for example, +1000 V, and transfer the toner images of the respective colors formed on the photosensitive drums 22a to 22d onto the surface of the intermediate transfer belt 30. The toner images of a plurality of colors formed on the photosensitive drums 22a to 22d are sequentially superimposed and transferred onto the intermediate transfer belt 30. As a result, a multicolor toner image (multicolor image) is formed on the intermediate transfer belt 30. In the present embodiment, the primary transfer rollers 26a to 26d are an example of a transfer unit.

  Hereinafter, a group of members that are directly related to the formation of toner images of each color, such as the scanner unit 20, the photosensitive drum 22, the charging roller 23, the developing sleeve 24, the developing device 25, and the primary transfer roller 26, are respectively referred to as “ This is referred to as an “image forming unit”. (The image forming unit may not include the scanner unit 20.) The image forming apparatus 10 forms four images for forming toner images of Y color, M color, C color, and Bk color, respectively. Department. When the names of the members are described with the subscripts a to d omitted, each of the four image forming units is shown. The four image forming units are an example of a plurality of image forming units that form images of different colors on the photosensitive drum 22 (photoconductor).

  A member that is disposed in the vicinity of the periphery of the photosensitive drum 22 and acts on the photosensitive drum 22 for image formation is referred to as a “process unit”. The charging roller 23, the developing sleeve 24, and the primary transfer roller 26 are examples of process units (process means). In this way, each of a plurality of types of members can be associated with the process unit.

  The intermediate transfer belt 30 is an endless belt that is rotationally driven by rollers 31, 32, and 33. The toner image transferred from the photosensitive drums 22a to 22d onto the intermediate transfer belt 30 is provided with a secondary transfer roller 27 as the intermediate transfer belt 30 rotates (movement of the peripheral surface in the direction of the arrow 50). It is conveyed to a position (secondary transfer position). In accordance with the timing at which the toner image on the intermediate transfer belt 30 is conveyed to the secondary transfer position, the conveyance of the recording medium 12 that has been temporarily stopped is resumed. The recording medium 12 is conveyed so that the recording medium 12 also reaches the secondary transfer position at the timing when the toner image reaches the secondary transfer position. At the secondary transfer position, the toner image is transferred onto the recording medium 12 from the intermediate transfer belt 30 by the secondary transfer roller 27. A toner image (toner pattern) formed on the intermediate transfer belt 30 is disposed downstream of the photosensitive drums 22a to 22d in the conveyance direction of the toner image on the intermediate transfer belt 30 (in the direction of the arrow 50). A detection sensor 40 for detection is provided. As will be described later, the detection sensor 40 is used to detect and correct color misregistration (position misregistration) that occurs when toner images of a plurality of colors are superimposed.

  Thereafter, the recording medium 12 is conveyed on the conveyance path to the position where the fixing roller pairs 16 and 17 are provided. The fixing roller pairs 16 and 17 fix the toner image on the recording medium 12 by applying heat and pressure to the toner image transferred on the recording medium 12. The fixed recording medium 12 is discharged (output) to the outside of the image forming apparatus 10.

  Here, the toner that is not transferred from the intermediate transfer belt 30 to the recording medium 12 by the secondary transfer roller 27 and remains on the intermediate transfer belt 30 is collected in a waste toner container 36 by the cleaning blade 35.

  In the present embodiment, the scanner unit 20 is employed as an optical system for irradiating the photosensitive drum 22 with light. However, the present invention is not limited to this, and from the viewpoint that color misregistration may occur, for example, an LED An array may be employed. In this embodiment, the image forming apparatus 10 is an image forming apparatus provided with the intermediate transfer belt 30, but may be an image forming apparatus of another type. For example, the image forming apparatus 10 includes a recording medium conveyance belt (endless belt), and directly transfers a toner image formed on each photosensitive drum 22 to a recording medium conveyed by the recording medium conveyance belt. An adopted image forming apparatus may be used. In this case, the recording medium conveyance belt conveys a recording medium on which toner images of a plurality of colors formed by a plurality of image forming units are transferred from the plurality of photosensitive drums 22 in a superimposed manner. Further, when performing color misregistration correction using a toner pattern for color misregistration detection, the toner pattern is formed on the recording medium conveyance belt instead of the intermediate transfer belt 30, and the toner pattern is detected by a sensor (detection sensor). 40 etc.).

<Configuration of high-voltage power supply device>
Next, the configuration of the high-voltage power supply device in the image forming apparatus 10 will be described with reference to FIG. 2A. 2A includes charging high voltage power supply circuits 43a to 43d, development high voltage power supply circuits 44a to 44d, primary transfer high voltage power supply circuits 46a to 46d, and a secondary transfer high voltage power supply circuit 48.

  The charging high-voltage power supply circuits 43a to 43d can form a background potential on the surface of the photosensitive drums 22a to 22d by applying a voltage to the charging rollers 23a to 23d, and can form an electrostatic latent image by laser light irradiation. Put it in a state. The development high-voltage power supply circuits 44a to 44d apply a voltage to the development sleeves 24a to 24d, thereby placing toner on the electrostatic latent images on the photosensitive drums 22a to 22d to form toner images. The primary transfer high-voltage power supply circuits 46 a to 46 d apply a voltage to the primary transfer rollers 26 a to 26 d to transfer the toner images on the photosensitive drums 22 a to 22 d to the intermediate transfer belt 30. The secondary transfer high-voltage power supply circuit 48 applies a voltage to the secondary transfer roller 27 to transfer the toner image on the intermediate transfer belt 30 to the recording medium 12.

  In the present embodiment, a common current detection circuit 49 is connected to the charging high-voltage power supply circuits 43a to 43d. The current detection circuit 49 is a circuit that detects currents (output currents 69 a to 69 d) that flow through the charging rollers 23 a to 23 d, converts the detected currents into voltages, and outputs the detected voltages 56. When the electrostatic latent image formed on the photosensitive drum 22 moves along with the rotation of the photosensitive drum 22 and passes through a position facing the charging roller 23, the current flowing through the charging roller 23 changes. At that time, a change in the current flowing through the charging roller 23 can be detected based on the detection voltage 56 output from the current detection circuit 49. Further, it can be detected from the change in the current flowing through the charging roller 23 (change in the detection voltage 56) that the electrostatic latent image formed on the photosensitive drum 22 has passed the position facing the charging roller 23 and the timing thereof. That is, the timing when the electrostatic latent image reaches the position facing the charging roller 23 can be detected, and this timing corresponds to the detection timing of the electrostatic latent image in the present embodiment.

<Configuration of charging high-voltage power supply circuit and current detection circuit>
Next, a circuit configuration example including the charging high-voltage power supply circuits 43a to 43d and the current detection circuit 49 in the high-voltage power supply device of FIG. 2A will be described with reference to FIG. 2B. In the present embodiment, the charging high-voltage power supply circuits 43a to 43d are an example of a power supply unit that is provided corresponding to a plurality of process units and functions as a power source for the plurality of process units. In the present embodiment, an example in which a plurality of power supply units that function as power supplies for a plurality of process units is provided corresponding to the plurality of process units will be described. However, a plurality of power supply units are not necessarily provided. There is no. For example, a common charged high-voltage power supply circuit (power supply means) may be provided for a plurality of process units.

  In the charging high-voltage power supply circuit 43a shown in FIG. 2B, the transformer 62a boosts the voltage amplitude of the AC signal generated by the drive circuit 61a by several tens of times. The rectifier circuit 51a including the diodes 64a and 65a and the capacitors 63a and 66a rectifies and smoothes the AC signal boosted by the transformer 62a into a negative DC voltage, and outputs it to the output terminal 53a. The comparator 60a controls the output voltage of the drive circuit 61a so that the voltage of the output terminal 53a divided by the detection resistors 67a and 68a is equal to the voltage setting value 55a set by the engine control unit 54. Here, since the output terminal 53a is connected to the charging roller 23a, an output current 69a flows from the ground point through the photosensitive drum 22a and the charging roller 23a by the negative DC voltage that is output. The charging high-voltage power supply circuits 43b to 43d have the same configuration as the charging high-voltage power supply circuit 43a and operate in the same manner, and therefore, the same reference numerals differing only in the last alphabetic character are attached, and the description thereof is omitted. To do.

  The current detection circuit 49 is inserted between the secondary side circuits 52 a to 52 d of the transformers 62 a to 62 d and the ground point 57. For this reason, the output currents 69 a to 69 d are superimposed after passing through the secondary side circuits 52 a to 52 d and all flow into the current detection circuit 49. Here, since the input terminal of the operational amplifier 70 has high impedance and almost no current flows, almost all of the current obtained by superimposing the output currents 69 a to 69 d flows to the resistor 71. Further, since the inverting input terminal of the operational amplifier 70 is connected to the output terminal via the resistor 71 (negatively fed back), it is virtually grounded to the reference voltage 73 connected to the non-inverting input terminal. As a result, when the current flowing through the resistor 71 changes, the detection voltage 56 of the output terminal of the operational amplifier 70, not the inverting input terminal of the operational amplifier 70, changes. In other words, the detection voltage 56 proportional to the current obtained by superimposing the output currents 69 a to 69 d is output from the output terminal of the operational amplifier 70. The capacitor 72 is for stabilizing the inverting input terminal of the operational amplifier 70.

  Further, the detection voltage 56 is input to the inverting input terminal of the comparator (comparator) 74, and the threshold voltage 75 (predetermined threshold voltage Vref) is input to the non-inverting input terminal of the comparator 74. The comparator 74 compares the detection voltage 56 with the threshold voltage 75 and outputs a binarized detection voltage 76 to the engine control unit 54 as a comparison result. Specifically, when the detection voltage 56 falls below the threshold voltage 75, the output of the comparator 74 becomes Hi (high level), and the voltage of Hi is input to the engine control unit 54. On the other hand, when the detection voltage 56 is equal to or higher than the threshold voltage 75, the output of the comparator 74 is Lo (low level), and the Lo voltage is input to the engine control unit 54.

  The threshold voltage 75 is between the minimum value of the detection voltage 56 when the electrostatic latent image for color misregistration correction passes the position facing the process unit (charging roller 23a) and the value of the detection voltage 56 before passing. Is set to the value of Here, a position facing the process unit is defined as a “detection position” for detecting the electrostatic latent image. As will be described later with reference to FIG. 5, in this embodiment, when the electrostatic latent image formed on the photosensitive drum 22a passes the detection position, it falls to the detection voltage 56 output from the current detection circuit 49. A rise occurs. As a result, the detection voltage 56 changes across the threshold voltage 75 to change the level of the detection voltage 76 (Hi → Lo, Lo → Hi). The engine control unit 54 causes the detection voltage 56 to fall and rise. Will be detected. In the present embodiment, the engine control unit 54 determines an intermediate timing between the detection timing of the rising edge (edge) and the detection timing of the falling edge (edge) of the electrostatic latent image based on the detection voltage 76. It is specified as the detection timing. Note that the method is not limited to the method used in the present embodiment, and for example, the rising timing or the falling timing itself of the detection voltage 56 may be specified as the detection timing of the electrostatic latent image.

  The detection voltage 76 corresponds to a current detection signal indicating a detection result of the current (output currents 69a to 69d) flowing through the charging rollers 23a to 23d (and the photosensitive drums 22a to 22d) by the current detection circuit 49. Based on this current detection signal, the engine control unit 54 passes the positions (detection positions) where the electrostatic latent images formed on the photosensitive drums 22a to 22d (on the photosensitive member) face the charging rollers 23a to 23d. The timing to perform is detected. That is, the engine control unit 54 detects a change in output from the charging high-voltage power supply circuits 43a to 43d to the charging rollers 23a to 23d (process unit) that occurs when the color misregistration correcting electrostatic latent image passes through the detection position. The timing at which the electrostatic latent image reaches the detection position is detected. The engine control unit 54 also sets the electrostatic latent image for color misregistration correction at the detection position based on the comparison result between the detection voltage 56 (output voltage from the current detection circuit 49) 56 and the threshold voltage 75 by the comparator 74. Detect the arrival timing. As described above, in the present embodiment, the engine control unit 54 functions as an example of the detection unit, and the current detection circuit 49 and the comparator 74 can also be included in the detection unit.

<Hardware Configuration of Engine Control Unit 54>
Next, the configuration of the engine control unit 54 will be described. As shown in FIG. 2B, the engine control unit 54 includes a CPU 81, an ASIC 82, a RAM 83, and an EEPROM 84, and comprehensively controls the operation of the image forming apparatus 10. The CPU 81 uses the RAM 83 as a main memory and a work area, reads various control programs stored in the EEPROM 84 into the RAM 83, and executes them to control the engine mechanism unit described above. The ASIC 82 performs, for example, control of each motor, high-voltage power supply control of the developing bias, and the like in various print sequences based on an instruction from the CPU 81. Note that the ASIC 82 may execute part or all of the functions of the CPU 81. Conversely, the CPU 81 may execute some or all of the functions of the ASIC 82. Also, some of the functions of the engine control unit 54 may be executed by other hardware corresponding to the engine control unit 54.

  FIG. 2C is a block diagram illustrating functional blocks included in the engine control unit 54. 2C, the engine control unit 54 includes a patch forming unit 87, a process unit control unit 88, and a color misregistration correction control unit 89 as functional blocks. A sensor 85 and an actuator 86 connected to the engine control unit 54 indicate hardware included in the image forming apparatus 10.

  The sensor 85 corresponds to various sensors included in the image forming apparatus 10 such as the registration sensor 111 and the current detection circuit 49. The actuator 86 corresponds to various actuators provided in the image forming apparatus 10 such as a drum driving motor and a developing sleeve separation motor. The engine control unit 54 performs various processes by controlling the actuator 86 based on information obtained from the sensor 85. For example, the actuator 86 functions as a drive source that drives a cam for separating the developing sleeves 24a to 24d from the photosensitive drums 22a to 22d.

  The patch forming unit 87 controls the scanner units 20a to 20d to form electrostatic latent image patterns (latent image marks) described later on the surfaces of the photosensitive drums 22a to 22d. As will be described later with reference to FIGS. 3A to 3C and the like, the process unit control unit 88 generates and detects an electrostatic latent image for reference value generation processing and color misregistration correction control based on the electrostatic latent image. Control the operation and settings of each process unit. The color misregistration correction control unit 89 calculates a value corresponding to the color misregistration correction amount based on the timing detected by the detection voltage 76, and reflects the calculated value, thereby calculating the color. Deviation correction control is performed.

  Note that the above description does not limit what hardware is used to realize the function of the engine control unit 54. The functions of the engine control unit 54 described above may be realized by any hardware such as the CPU 81, the ASIC 82, and other hardware, and the processing for realizing the functions is distributed to each hardware and executed. May be.

<Procedure for overall color misregistration correction control>
Next, color misregistration correction control according to the present embodiment, which is executed by the image forming apparatus 10, will be described.

  First, the engine control unit 54 executes a reference value generation process for generating a reference value that is a target in color misregistration correction control. The reference value is not obtained by the reference value generation process, but may be stored in advance in, for example, the EEPROM 84 (FIG. 2B). The engine control unit 54 may execute the above-described color misregistration correction control with reference to the EEPROM 84 as appropriate. In this case, a reference value determined in advance by measurement at the design stage or manufacturing stage of the image forming apparatus 10 can be used as the reference value.

  In the reference value generation process, the engine control unit 54 first forms a toner pattern (toner mark) for color misregistration detection by a toner image on the intermediate transfer belt 30, and based on the detection result of the toner pattern by the detection sensor 40. Perform color misregistration correction. Accordingly, the engine control unit 54 sets the state of the device to a state (reference state) in which the color misregistration amount is at least small. Next, the engine control unit 54 forms an electrostatic latent image (electrostatic latent image pattern) for color misregistration correction on the photosensitive drums 22a to 22d in the reference state. Furthermore, the engine control unit 54 determines the movement time until the formed electrostatic latent image moves to a position (detection position) opposite to the charging rollers 23a to 23d as the photosensitive drums 22a to 22d rotate. Measurement is performed for each image forming unit (each color). The measurement is performed by detecting a change in charging current (current detected by the current detection circuit 49). Based on the measurement result, the engine control unit 54 generates and sets a reference value for color misregistration correction control to be executed later.

  Thereafter, the engine control unit 54 again performs an electrostatic latent image for correcting color misregistration (electrostatic latent image) at a specific timing (for example, when the temperature in the image forming apparatus 10 changes due to continuous printing or the like). Color misregistration correction control is executed by forming an image pattern) and performing the same measurement as described above. Specifically, the engine control unit 54 starts the electrostatic latent image for color misregistration correction from the timing at which the formation of the electrostatic latent image on the photosensitive drums 22a to 22d is started. Each image forming unit (each color) is measured with the time until the timing at which the latent image moves to the detection position (that is, the timing detected at the detection position) as the movement time. Further, the engine control unit 54 calculates a difference value between the movement time corresponding to the Y color (reference color) and the movement time corresponding to another color. Here, the deviation (change) of the difference value obtained in the current measurement from the reference value reflects the amount of color deviation. Therefore, the engine control unit 54 corrects the color misregistration by controlling the plurality of image forming units so that the calculated difference value approaches the set reference value when image formation (printing) is executed.

  In the present embodiment, the engine control unit 54 adjusts (controls) the timing at which the scanner units 20a to 20d irradiate the photosensitive drums 22a to 22d with the laser beams 21a to 21d (light irradiation timing), thereby correcting the color misregistration. Do. Note that the control of the image forming unit for color misregistration correction is not limited to the light irradiation timing control as described above. For example, the color misregistration correction may be performed by controlling the rotational speed of the photosensitive drums 22a to 22d and adjusting the (mechanical) position of the reflecting mirrors included in each of the scanner units 20a to 20d.

<Reference value generation processing>
First, reference value generation processing according to the present embodiment will be described with reference to the flowcharts shown in FIGS. 3A to 3C.

(Procedure for color misregistration correction based on toner pattern)
In step S <b> 301, the engine control unit 54 causes the image forming unit to form a color misregistration detection toner pattern (color misregistration correction toner image) on the intermediate transfer belt 30. Here, FIG. 4 shows a state where a toner pattern for detecting color misregistration is formed on the intermediate transfer belt 30.

  In FIG. 4A, toner patterns 400 and 401 are used to detect a color misregistration amount in the paper transport direction (sub-scanning direction) and are formed as lines along the sub-scanning direction. The toner patterns 402 and 403 are used to detect a color misregistration amount in a direction (main scanning direction) orthogonal to the paper transport direction, and are formed as lines inclined by 45 degrees with respect to the main scanning direction and the sub scanning direction. Yes. 4A, tsf1 to 4, tmf1 to 4, tsr1 to 4, and tmr1 to 4 indicate the detection timing of each toner pattern, and an arrow 50 indicates the moving direction of the peripheral surface of the intermediate transfer belt 30. Show. For detection of the color misregistration correction toner pattern (toner image), an optical sensor (detection sensor 40) that receives reflected light when the toner image is irradiated with light and outputs a voltage corresponding to the amount of received light. Can be applied.

First, in the sub-scanning direction, the moving speed of the peripheral surface of the intermediate transfer belt 30 is v [mm / s], and the Y color is used as a reference color. In addition, in the toner patterns 400 and 401 for detecting the amount of color misregistration in the paper conveyance direction, the theoretical distance between each of the M, C, and Bk patterns and the Y color (reference color) pattern is respectively set. dsM, dsC, dsBk. For example, the color misregistration amount δesM of the M color when the Y color is used as the reference color is obtained by the following equation (1).
δesM = v × {(tsf2−tsf1) + (tsr2−tsr1)} / 2−dsM
(1)
Note that the color misregistration amounts δesC and δesBk of the C color and the Bk color are similarly obtained.

Further, regarding the main scanning direction, in FIG. 4A, the color misregistration amounts of the respective colors for the left and right toner patterns along the arrow 50 are δemf and δemr. For example, with respect to δemf, the color misregistration amount δemfM of the M color (left side) when the Y color is used as the reference color is obtained by the following equation (2).
δemfM = v × {(tmf2−tsf2) − (tmf1−tsf1)} (2)
The color shift amounts δemfC and δemfBk for the C color and Bk color (left side) and the color shift amounts δemrM, δemrC, and δemrBk for the M color, C color, and Bk color (right side) are also obtained in the same manner.

  The engine control unit 54 can determine the direction of deviation from the sign (positive or negative) of the above calculation result. Further, the engine control unit 54 corrects the writing position from δemf and the main scanning width (main scanning magnification) from δemr−δemf. If there is an error in the main scanning width (main scanning magnification), the engine control unit 54 considers not only δemf but also the amount of change in the image frequency (image clock) that has changed with the correction of the main scanning width. The write position is calculated.

  The engine control unit 54 corrects the color misregistration by changing the image forming condition, that is, the output timing of the laser beam from the scanner unit 20a so as to eliminate (at least reduce) the color misregistration of the calculated color misregistration amount. Do. For example, if the amount of color misregistration in the sub-scanning direction is −4 lines, the engine control unit 54 instructs the video controller (not shown) to advance the output timing of laser light by +4 lines. As described above, the color misregistration correction is performed so that the color misregistration amount becomes at least small by the processing of S301, and the corrected state of the image forming apparatus 10 is used as a reference for the subsequent color misregistration correction control based on the electrostatic latent image. .

  Next, returning to the description of FIG. 3A, in S302, the engine control unit 54 controls the photosensitive drum 22a in order to suppress the influence when the rotational speed (moving speed of the circumferential surface) of the photosensitive drums 22a to 22d varies. The rotational phase relationship (rotational position relationship) between ˜22d is adjusted to a predetermined state. Specifically, the engine control unit 54 adjusts the rotation phases of the photosensitive drums 22b to 22d corresponding to the other colors with respect to the rotation phase of the photosensitive drum 22a corresponding to the reference color. When the photosensitive drum drive gears are provided on the rotation shafts of the photosensitive drums 22a to 22d, the phase relationship of the drive gears of the photosensitive drums 22a to 22d is substantially adjusted.

  Thereafter, before starting the process of S303, the engine control unit 54 waits until the rotational phase difference between the photosensitive drums 22a to 22d reaches a predetermined phase difference. In this predetermined phase difference state, the rotational phase of each photosensitive drum 22 has a constant phase difference (including a case of zero) with respect to the rotational phase of each of the plurality of other photosensitive drums 22. Note that step S302 indicates a rotational phase relationship (rotation) between the photosensitive drums 22a to 22d when the influence of fluctuations in the rotational speeds (surrounding surface movement speeds) of the photosensitive drums 22a to 22d is small or due to the configuration of the image forming apparatus 10. If it is difficult to match the positional relationship, it may be omitted.

(Formation procedure of electrostatic latent image)
Next, in S303 to S311, the engine control unit 54 performs a loop process for j = 1 to 4, thereby a plurality of (in this embodiment, 20) electrostatic latent images corresponding to each of the four colors. The process of forming each is executed. For j = 1 to 4, four colors of Y, M, C, and Bk are assigned in order, and each color is sequentially processed. In this embodiment, the case where j = 1 to 4 is sequentially associated with the Y color, the M color, the C color, and the Bk color has been described. However, the correspondence between j and the toner color is as follows. It may be different and can be arbitrarily determined.

  First, the engine control unit 54 starts a timer (j) corresponding to the j-th color in S304. Moreover, the engine control part 54 performs a loop process about i = 1-20 further in S305-S310. i = 1 to 20 corresponds to the number of electrostatic latent image formations, and this loop process repeatedly forms an electrostatic latent image on the photosensitive drum 22. In S306, the engine control unit 54 outputs a laser signal to the scanner unit 20 corresponding to the j-th color, and forms an electrostatic latent image for color misregistration correction on the photosensitive drum 22 based on the laser signal.

  Here, FIG. 4B shows a state where the electrostatic latent image 80 is formed on the photosensitive drum 22a corresponding to the Y color. The electrostatic latent image 80 is formed to have as wide a width as possible in an image area in the main scanning direction (an area in which an image formed in the area can be transferred to a recording medium), and 30 in the paper conveyance direction. It is formed to have a width of about the line. The width of the electrostatic latent image 80 in the main scanning direction is at least half the width of the image region in the main scanning direction in order to obtain a good detection result when detecting the electrostatic latent image. It is desirable to be In addition, the detection accuracy of the electrostatic latent image can be further improved by expanding the width of the electrostatic latent image 80 to an area exceeding the width of the image area and to an area where the electrostatic latent image can be formed. Is possible.

  Returning to the description of FIG. 3A, if it is determined in S307 that i <20, the engine control unit 54 performs a standby process of the standby time Wait1 in S308, and sets a time interval for repeatedly forming an electrostatic latent image. adjust. The waiting time Wait1 is set to a time at which the electrostatic latent image can be detected with high accuracy. For example, the time required to draw about 30 lines in the paper conveyance direction, which is the same as the time when the electrostatic latent image is drawn in S306, etc. Set to

  On the other hand, if the engine control unit 54 determines in step S307 that i <20 is not satisfied (that is, i = 20), formation of electrostatic latent images corresponding to different colors (laser light generation) is performed in the next loop processing. Output). In this case, the engine control unit 54 performs a waiting process for a waiting time Wait2 different from the waiting time Wait1 in S309. Thereby, the engine control unit 54 adjusts a time interval for forming electrostatic latent images corresponding to different colors (outputting laser light). The waiting time Wait2 in S309 is a time when it is possible to prevent the timings for detecting the electrostatic latent images corresponding to the respective colors from overlapping each other (as will be described later) even if the assumed maximum color misregistration occurs. Is set. The standby time Wait2 is set to a longer time than the standby time Wait1, for example.

  Here, in the transport direction of the electrostatic latent image formed on the photosensitive drum 22, on the upstream side of the charging roller 23 used for detection of the electrostatic latent image, as shown in FIGS. 1 and 2A, A developing sleeve 24 and a primary transfer roller 26 are provided. When the electrostatic latent image is formed and the electrostatic latent image is detected by the charging roller 23, the developing sleeve 24 and the primary transfer roller 26 are moved to the photosensitive drum 22 so as not to affect their operations. It operates so that the action of is at least small. For example, the developing sleeve 24 and the primary transfer roller 26 are separated from the photosensitive drum 22. Alternatively, the voltage applied to the developing sleeve 24 and the primary transfer roller 26 is set to zero (off), so that the developing sleeve 24 and the primary transfer roller 26 can perform normal image formation using toner. Compared to the above, the operation to the photosensitive drum 22 is at least reduced.

  The development bias high-voltage power supply circuit (development high-voltage power supply circuit 44) applies a bias voltage having a polarity opposite to that at the time of normal image formation to the corresponding development sleeve 24, whereby the electrostatic latent image on the photosensitive drum 22 is applied. The toner may not be attached to the toner. Also, a so-called jumping development method is adopted in which the photosensitive drum 22 and the developing sleeve 24 are brought into a non-contact state with respect to the developing sleeves 24a to 24d, and a voltage obtained by superimposing an AC bias on a DC bias is applied to the developing sleeve 24. In this case, it is only necessary to turn off the voltage application.

  Such a state of the developing sleeve 24 and the primary transfer roller 26 is continued until a series of processes in the flowcharts of FIGS. 3A to 3C is completed. This is the same when executing a series of processes in the flowcharts of FIGS. 7A to 7C described later.

(Detection procedure of electrostatic latent image)
When the process shown in the flowchart of FIG. 3A ends, the engine control unit 54 starts the electrostatic latent image detection process shown in the flowchart of FIG. 3B. The engine control unit 54 performs a loop process for j = 1 to 4 in S321 to S326. Note that four colors Y, M, C, and Bk are sequentially assigned to j = 1 to 4, and the respective colors are sequentially processed. The engine control unit 54 further performs loop processing for i = 1 to 40 in S322 to S325 in the loop processing.

  In S323, the engine control unit 54 executes a process for detecting a plurality of electrostatic latent images formed on the photosensitive drums 22a to 22d by the process according to the flowchart of FIG. 3A. The engine control unit 54 outputs the rising edge and the falling edge of the detection voltage 56 output from the current detection circuit 49 corresponding to the output currents 69a to 69d flowing through the charging rollers 23a to 23d, from the comparator 74. Detection is based on the detection voltage 76. Thereby, the engine control unit 54 detects (identifies) the timing at which the electrostatic latent image formed on each photosensitive drum 22 reaches each detection position. The output currents 69a to 69d and the detection voltage 56 change according to the surface potential of the photosensitive drums 22a to 22d.

  Specifically, the detection voltage 56 output from the current detection circuit 49 is binarized by the comparator 74 as described above with reference to FIG. 2B, and the binarized voltage is used as the detection voltage 76. 74 is output to the engine control unit 54. The engine control unit 54 monitors the detection voltage 76 and detects the timing at which the detection voltage 76 changes from Lo to Hi as a rising edge of the detection voltage 56. Further, the engine control unit 54 detects the timing at which the detection voltage 76 changes from Hi to Lo as the falling edge of the detection voltage 56. The engine control unit 54 repeats such processing in the loop processing of S321 to S326 and S322 to S325, so that 20 electrostatic latent images formed on the photosensitive drums 22a to 22d respectively are charged with the charging roller 23a. The timing when it reaches ˜23d is detected.

  When the rising edge or the falling edge is detected in S323, the engine control unit 54 determines the value of the timer (j) corresponding to the jth color at the time of edge detection in S324 as the measured value t (j) (i). Is temporarily stored in the RAM 83. By the process of S324, a plurality of measurement values corresponding to the detection result of the rising edge or the falling edge are stored.

(Change in surface potential of photosensitive drum)
FIG. 5A is a graph showing temporal changes in the detection voltage 56 output by the current detection circuit 49 when the electrostatic latent image 80 reaches the charging roller 23a corresponding to the process unit. The detection voltage 56 changes according to the surface potential of the photosensitive drum 22a as described above. In the graph shown in FIG. 5A, the vertical axis indicates the detection voltage 56 output from the current detection circuit 49, and the horizontal axis indicates time. In FIG. 5A, when the electrostatic latent image 80 approaches the position of the charging roller 23a as the photosensitive drum 22 rotates (movement of the peripheral surface), the detection voltage 56 decreases. When the voltage passes through the position of the charging roller 23a, the detection voltage 56 has a characteristic of increasing.

  Here, the reason why the detection voltage 56 decreases as shown in FIG. 5A will be described with reference to FIGS. 5B and 5C show the surface potential of the photosensitive drum 22a when the toner is attached to the electrostatic latent image 80 formed in the region 93 on the photosensitive drum 22a and when the toner is not attached. It is a schematic diagram which shows. 5B and 5C, the horizontal axis indicates the position of the surface of the photosensitive drum 22a in the movement direction (conveyance direction), and the region 93 indicates the position where the electrostatic latent image 80 is formed. ing. The vertical axis represents the surface potential of the photosensitive drum 22a. The dark potential of the photosensitive drum 22a is VD (for example, -700V), the bright potential is VL (for example, -100V), and the charging bias potential of the charging roller 23a is VC (for example, -1000V).

  As shown in FIGS. 5B and 5C, in the region 93 where the electrostatic latent image 80 is formed, the potential difference 96 between the potential (VC) of the charging roller 23a and the surface potential of the photosensitive drum 22a is the other than that. It becomes larger than the potential difference 95 in the region of. For this reason, when the electrostatic latent image 80 reaches the charging roller 23a, the current flowing through the charging roller 23a and the photosensitive drum 22a increases. As the current increases, the level of the output terminal voltage (detection voltage 56) of the operational amplifier 70 decreases as shown in FIG.

  As described above, the current value (that is, the detection voltage 56) detected by the current detection circuit 49 reflects the surface potential of the photosensitive drum 22a. 5B and 5C show an example of the difference between the surface potential of the photosensitive drum 22a and the potential (VC) of the charging roller 23a. However, a similar phenomenon occurs with respect to the difference between the surface potential of the photosensitive drum 22a and the transfer bias potential (transfer voltage) or the development bias potential (development voltage). For this reason, the electrostatic latent image 80 is detected not only using the charging roller 23a as in this embodiment, but also using the developing sleeve 24a or the primary transfer roller 26 (as will be described later as Modification 1). Is possible.

  In the present embodiment, as described above, in the reference value generation process, when color misregistration is detected after S302, the developing sleeve 24a is separated from the photosensitive drum 22a, and no toner is placed on the electrostatic latent image 80. The color shift is detected by detecting the current flowing through the charging roller 23a. However, as described below, the color misregistration can be detected even when toner is placed on the electrostatic latent image 80.

  FIG. 5C shows the surface potential of the photosensitive drum 22a and the potential of the charging roller 23a (VC) when toner is placed on (attached to) the electrostatic latent image 80 formed in the region 93 on the photosensitive drum 22a. ) And the potential difference. In FIG. 5C, the potential difference 97 between the charging roller 23a and the photosensitive drum 22a when the toner is placed on the electrostatic latent image 80 in the region 93 is shown in FIG. 5B when the toner is not placed. It can be seen that it is smaller than the potential difference 96. That is, the difference between the potential difference 95 in the region other than the region 93 and the potential difference 97 in the region 93 is small. This means that the amount of change in the detection voltage 56 when the electrostatic latent image 80 (with toner) passes the position of the charging roller 23a is reduced. However, even when toner is placed on the electrostatic latent image 80, it is possible to sufficiently detect the change in the detection voltage 56 (that is, the change in the current flowing through the charging roller 23a).

  When the toner is placed on the electrostatic latent image 80, it is necessary to clean the toner on the photosensitive drum 22 and the intermediate transfer belt 30 after detecting the color misregistration. However, by reducing the toner amount (concentration), the cleaning becomes simple and the cleaning can be executed in a short time. For example, at least in the color misregistration correction executed in S301, the density may be set lower than that of the detection toner image (density 100%) formed on the intermediate transfer belt 30 or the like.

(Procedure for calculating the time difference in moving time of the electrostatic latent image)
When the process shown in the flowchart of FIG. 3B ends, the engine control unit 54 starts the process shown in the flowchart of FIG. 3C.

  First, in S331, the engine control unit 54 calculates reference values δesYM, δesYC, and δesYBk for color misregistration correction control to be executed later based on the detection result of the electrostatic latent image formed on each photosensitive drum 22. calculate. Specifically, the engine control unit 54 determines the movement time (that is, the photosensitive time) from when the electrostatic latent image of each color is formed on the photosensitive drum 22 until it reaches the position (detection position) facing the charging roller 23. The movement time until the electrostatic latent image formed on the drum 22 moves to the detection position is calculated. Further, the engine control unit 54 sets other colors (M color, C color, and Bk color corresponding to the second color) to the moving time of the electrostatic latent image of Y color (reference color corresponding to the first color). The difference value of the moving time of the electrostatic latent image is calculated as a reference value. However, the engine control unit 54 improves the measurement accuracy by averaging the 20 difference values calculated corresponding to the above-described 20 electrostatic latent image formations.

In the present embodiment, as described above, the engine control unit 54 determines an intermediate timing (midpoint) between the falling edge detection timing and the rising edge detection timing of the detection voltage 56 based on the detection voltage 76. It is specified as the detection timing of the electrostatic latent image. Specifically, the engine control unit 54 calculates (specifies) the intermediate timing between the falling edge and the rising edge of the detection voltage 56 detected in S323 as the electrostatic latent image detection timing. The detection timing of the electrostatic latent image corresponds to the above-described timing at which the electrostatic latent image reaches the detection position. For example, the detection timings of the k-th (k = 1 to 20) electrostatic latent images for Y, M, C, and Bk colors (j = 1 to 4) are t (j) (2k−1), respectively. + T (j) (2k)) / 2
Is required. Note that t (j) (i) corresponds to an elapsed time after the electrostatic latent image is formed on the photosensitive drum 22. For this reason, the value obtained by the above equation is a measured value of the movement time of the electrostatic latent image from the timing at which the electrostatic latent image is formed on the photosensitive drum 22 to the timing at which the electrostatic latent image is detected. Equivalent to.

In S331, the engine control unit 54 calculates reference values δesYM, δesYC, and δesYBk using the following equations (3) to (5).
δesYM = Σ {k = 1 to 20} {(t (2) (2k−1) + t (2) (2k)) / 2} −Σ {k = 1 to 20} {(t (1) (2k− 1) + t (1) (2k)) / 2} (3)
δesYC = Σ {k = 1 to 20} {(t (3) (2k−1) + t (3) (2k)) / 2} −Σ {k = 1 to 20} {(t (1) (2k− 1) + t (1) (2k)) / 2} (4)
δesYBk = Σ {k = 1 to 20} {(t (4) (2k−1) + t (4) (2k)) / 2} −Σ {k = 1 to 20} {(t (1) (2k− 1) + t (1) (2k)) / 2} (5)
In Expressions (3) to (5), for each of the plurality of image forming units, measurement values of a plurality of movement times corresponding to the plurality of electrostatic latent images are obtained, and the Y color (reference color) and the other colors (M The difference value of the movement time is calculated between each of the two colors (color, C color, and Bk color). In equations (3) to (5), a value obtained by averaging the calculated difference values is obtained based on the formation of 20 consecutive electrostatic latent images.

  Finally, in S332, the engine control unit 54 uses the difference values (time differences) δesYM, δesYC, and δesYBk of the movement time of the electrostatic latent image calculated in S331 as targets for the color misregistration correction control to be executed later. And is stored in the EEPROM 84. When performing the color misregistration correction control, the engine control unit 54 forms an electrostatic latent image (electrostatic latent image pattern) for color misregistration correction on the photosensitive drum 22 in the same manner as the reference value generation process described above. The movement time from the formation of the electrostatic latent image to the movement to the detection position is measured again. Further, the engine control unit 54 calculates a difference value of the movement time between the Y color and the other colors so that the difference value approaches the reference value obtained by the above-described reference value generation process (that is, Finally, the image forming unit is controlled so that the error between the difference value and the reference value is eliminated and become equal). Specifically, the engine control unit 54 performs color misregistration correction by controlling the timing at which the scanner unit 20 irradiates the photosensitive drum 22 with laser light. Note that the color misregistration correction can also be realized, for example, by controlling the rotation speed of the photosensitive drums 22a to 22d.

<Timing chart of reference value generation processing>
FIG. 6 is a timing chart showing the operation timing of the image forming apparatus 10 in the reference value generation processing based on the flowcharts of FIGS. 3A to 3C. Here, the reference value generation process described above will be further described with reference to FIG.

  First, at time T1, the engine control unit 54 outputs a drive signal for driving a cam for separating the developing sleeves 24a to 24d. Accordingly, at time T2, the developing sleeves 24a to 24d operate so as to be separated from the state in contact with the photosensitive drums 22a to 22d. Next, at time T3, the engine control unit 54 controls the primary transfer high pressure from the on state to the off state. The operations from time T1 to time T3 are executed before the start of S303 in FIG. 3A.

  As described above, instead of separating the developing sleeve 24 at time T1, the voltage output from the developing high-voltage power supply circuits 44a to 44d is set to zero, or a voltage having a polarity opposite to that of the normal voltage is output. You may control. Further, the primary transfer rollers 26a to 26d may be controlled so as to be separated from the photosensitive drums 22a to 22d instead of turning off the primary transfer high pressure.

  Next, in the period of time T4 to T6, the engine control unit 54 sequentially outputs laser signals 90a to 90d for forming an electrostatic latent image for color misregistration correction on the photosensitive drum 22 corresponding to each color. Output to 20a to 20d. In the present embodiment, the engine control unit 54 outputs the laser signals 90a to 90d for a period corresponding to about one turn of the photosensitive drum 22 with the timing shifted for each color.

  After time T4, the electrostatic latent image formed at the position (exposure position) where the laser beam output from the scanner unit 20 is incident on the photosensitive drum 22 is applied to the charging roller 23a as the photosensitive drum 22 rotates. Move to the opposite position (charging position). During the period from time T5 to time T7, the electrostatic latent image on each photosensitive drum 22 sequentially passes through the position facing the charging roller 23, thereby causing a change in the current detection signal. The current detection signal corresponds to the detection voltage 56 (or detection voltage 76) output from the current detection circuit 49. In FIG. 6, during the periods 95a to 95d, a change occurs in the current detection signal as the electrostatic latent images formed based on the laser signals 90a to 90d pass through the positions facing the charging rollers 23a to 23d, respectively. Shows the period.

  As shown in FIG. 6, when the engine control unit 54 sequentially outputs the laser signals 90a to 90d, the engine control unit 54 allows sufficient waiting times 94a to 94d (Wait2 in S309 in FIG. 3A) between signals corresponding to different colors. Provided. Thus, the periods 95a to 95d in which the current detection signals change based on the laser signals 90a to 90d are prevented from overlapping each other. Accordingly, as shown in FIG. 2B, a circuit configuration including a common current detection circuit 49 can be applied to a plurality of process units (charging rollers 23a to 23d).

  Finally, at time T8, the engine control unit 54 controls the charging high voltage for the charging rollers 23a to 23d from the on state to the off state, and ends the process. In this way, after the periods 95a to 95d in which the current detection signal changes are detected, the reference value calculation process described with reference to FIG. 3C is performed.

<Color misregistration correction control>
Next, color misregistration correction control according to the present embodiment will be described with reference to flowcharts shown in FIGS. 7A to 7C. 7A and 3A, S602 and S302, S604a to S604d and S304, S605a to S605d and S305, S606a to S606d and S306, S608a to S608d and S308, and S610a to S610d and S310 are the same processes. . Moreover, S621 to S626 in FIG. 7B are the same as S321 to S326 in FIG. 3B. For simplification of description, description of processing similar to that in FIGS. 3A and 3B is omitted.

  As shown in FIG. 7A, when the processing of S602 is completed, the engine control unit 54 executes the processing for each color in parallel. For the Y color, the engine control unit 54 forms an electrostatic latent image 20 times in S604a to S610a as in FIG. 3A. Further, the engine control unit 54 waits for the time of (ew−δesYM), (ew × 2-δesYC), and (ew × 3-δesYBk) in S603b to S603d for M color, C color, and Bk color, respectively. Process. After that, the engine control unit 54 performs the processing of S604b to S604d and below (20 electrostatic latent image formations) as in the case of Y colors S604a to S610a.

  Here, ew is the time required to form (20) electrostatic latent images for one color, and corresponds to the time required for the processing of S605a to S610a. Further, δesYM, δesYC, and δesYBk are values stored in the EEPROM 84 in the above-described reference value generation process (S332 in FIG. 3C) or the previous color misregistration correction control (S642 in FIG. 7C).

  δesYM, δesYC, and δesYBk correspond to the difference value of the moving time corresponding to each color with reference to the moving time of the electrostatic latent image corresponding to the Y color to the detection position. Unless the state of the image forming apparatus 10 has changed from the state when the reference value is generated (reference state) (that is, color misregistration has occurred), the static processing on the photosensitive drums 22a to 22d is performed by the standby processing of S603b to S603c. The moving time of the electrostatic latent image is equal between the image forming units (for each color). For this reason, when the measured value of the moving time of the electrostatic latent images formed on the photosensitive drums 22a to 22d becomes a different value for each color, it can be determined that color misregistration has occurred, and the difference between the measured values is Accordingly, the color misregistration amount can be determined. In addition, by the standby processing in S603b to S603d, the interval between the electrostatic latent images of the respective colors can be adjusted to an interval that can be continuously detected at the detection position.

  Thereafter, the engine control unit 54 performs the same processing as S321 to S326 (FIG. 3B) in S621 to S626 (FIG. 7B). When the process of S626 is completed, the engine control unit 54 advances the process to S631 (FIG. 7C).

  In S631, the engine control unit 54, based on the measurement result stored in S624, moves the difference value (time difference) of the movement time to the detection position of the electrostatic latent image formed on the photosensitive drum 22 as in S331. dδesYM, dδesYC, and dδesYBk are calculated. The engine control unit 54 temporarily stores the dδesYM, dδesYC, and dδesYBk calculated in S632 in the RAM 83.

  Next, in S633, S636, and S629, the engine control unit 54 determines the magnitude of color misregistration (color misregistration amount) currently occurring. Specifically, the engine control unit 54 determines the color shift of the M color with respect to the Y color as follows, and corrects the generated color shift. In S633, the engine control unit 54 determines whether dδesYM <0. When dδesYM <0 (that is, when the detection timing of the electrostatic latent image of M color is advanced when Y is used as a reference), the engine control unit 54 performs the M color detection by the scanner unit 20b in S635. The timing of laser beam emission is delayed. On the other hand, when dδesYM ≧ 0 (that is, when the detection timing of the electrostatic latent image of M color is delayed when using Y color as a reference), the engine control unit 54 performs M by the scanner unit 20b in S634. The timing of emitting the color laser beam is advanced. In this way, the engine control unit 54 can reduce color misregistration between the Y color and the M color.

  In S636 to S641, the engine control unit 54 performs the same processing as that for the M color (S633 to S635) for the C color and the Bk color, so that the Y color and the C color are changed between the Y color and the C color. It is possible to reduce a color shift between the Bk colors. By the processing of S633 to S641, the difference value of the moving time of the electrostatic latent image formed on the photosensitive drum 22 can be brought close to the reference value targeted in the current color misregistration correction control. That is, the color misregistration state of the image forming apparatus 10 can be brought close to the target reference state and finally returned to the reference state.

  Finally, in S642, the engine control unit 54 updates the reference value used in the current color misregistration correction control based on the state of the image forming apparatus 10 after the color misregistration correction in S633 to S641. Specifically, the engine control unit 54 stores δesYM, δesYC and δesYBk stored in the EEPROM 84 during the execution of S332 (FIG. 3C) or the previous S642, and dδesYM, dδesYC temporarily stored in the RAM 83 in S632. And dδesYBk, the reference values δesYM, δesYC, and δesYBk are updated.

<Timing chart for color misregistration correction control>
FIG. 8 is a timing chart showing the operation timing of the image forming apparatus 10 in the color misregistration correction control based on the flowcharts of FIGS. 7A to 7C. Here, the color misregistration correction control described above will be further described with reference to FIG. In FIG. 8, the same components as those in FIG. 6 are denoted by the same reference numerals, and the description thereof is omitted.

  In the reference value generation process (FIG. 6), the engine control unit 54 provides standby times 94a to 94d between signals corresponding to different colors when sequentially outputting the laser signals 90a to 90d. On the other hand, in the actual color misregistration correction control (FIG. 8), the engine control unit 54 continuously outputs the laser signals 91a to 91d without providing such a waiting time. This is because in S603b to S603d, the engine control unit 54 performs standby processing based on δesYM, δesYC, and δesYBk obtained by the reference value generation processing or the previous color misregistration correction control.

  In the color misregistration correction control according to the present embodiment, the timing at which the color misregistration correction electrostatic latent image of each color is detected at the detection position is different for each image forming unit based on the already measured movement time. Thus, the start timing of forming the electrostatic latent image for color misregistration correction of each color is controlled. Thereby, it is possible to efficiently execute the color misregistration correction control and to shorten the processing time required for the color misregistration correction control. Here, as shown in FIG. 8, the time lengths of the laser signals 91a to 91d and the current detection signals 96a to 96d are the same as the laser signals 90a to 90d and the current detection signals 95a to 95d shown in FIG. Therefore, as an example, the processing time required for the color misregistration correction control is controlled by controlling the timing of starting the formation of an electrostatic latent image for color misregistration correction for each color so that the interval between the current detection signals 96a to 96d becomes zero. Can be shortened. In addition, it is possible to shorten the processing time required for the reference value generation processing.

  As described above, the image forming apparatus 10 according to the present embodiment forms an electrostatic latent image pattern for color misregistration correction on the photosensitive drum 22, and the electrostatic latent image pattern is specified on the photosensitive drum 22. The moving time until moving to a position facing the process unit is measured for each image forming unit. In this embodiment, the charging roller 23 is employed as the process unit. Further, the image forming apparatus 10 variably controls the output interval of the laser signal in the next color misregistration correction control based on the measurement result of the movement time obtained by the reference value generation process or the previous color misregistration correction control. Specifically, the image forming apparatus 10 is configured so that the timing at which the electrostatic latent image for color misregistration correction of each color is detected at the detection position is different for each image forming unit based on the measured moving time. Controls the start timing of forming an electrostatic latent image for color misregistration correction for each color.

  According to the present embodiment, it is possible to efficiently execute the color misregistration correction control and shorten the processing time required for the color misregistration correction control. In addition, it is possible to shorten the waiting time caused by the color misregistration correction control that can be performed before and after the printing process or in the middle of the printing process.

  The image forming apparatus 10 according to the present embodiment further includes a movement time corresponding to the first color (reference color) and a movement time corresponding to the second color among a plurality of colors corresponding to the plurality of image forming units. Color misregistration is corrected by controlling the plurality of image forming units so that the difference value approaches a preset reference value.

  According to the present embodiment, since color misregistration correction control is performed based on the electrostatic latent image pattern formed on the photosensitive drum 22, it is not necessary to develop the electrostatic latent image pattern with toner, and the developed toner image Is not required to be transferred onto the intermediate transfer belt 30. In this way, it is possible to improve the usability of the image forming apparatus by performing color misregistration correction using the electrostatic latent image pattern without forming a color misregistration correction toner image.

  Further, in this embodiment, since the color misregistration can be corrected without developing the electrostatic latent image with toner, the time can be shortened as compared with the case where a toner pattern for color misregistration correction is formed on the intermediate transfer belt 30. Color misregistration correction can be executed. There is also a method of transferring an electrostatic latent image pattern for color misregistration correction onto an intermediate transfer belt. However, in order to maintain the electrostatic latent image pattern on the intermediate transfer belt, it is necessary to increase the electrical resistance value of the belt, which tends to cause image defects. According to this embodiment, such an image defect does not occur.

  In addition, according to the present embodiment, for example, color misregistration correction can be performed with high accuracy as compared with a method of performing color misregistration correction without using toner and predicting the color misregistration amount from the amount of change in temperature in the apparatus. Is possible.

[Second Embodiment]
The second embodiment is characterized in that the detection voltage 56 output from the current detection circuit 49 is filtered by a high-pass filter. This high-pass filter suppresses the influence of the change in the direct current component of the charging current, which may occur due to variations in performance and durability of the photosensitive drum 22 and the charging roller 23, on the detection accuracy of the electrostatic latent image. . Hereinafter, for simplification of description, description of parts common to the first embodiment is omitted.

FIG. 9 is a diagram showing an example of a circuit configuration including the charging high-voltage power supply circuits 43a to 43d and the current detection circuit 49 according to the second embodiment of the present invention. In FIG. 9, the same reference numerals are given to the same components as those in FIG. 2B, which is a circuit configuration example of the first embodiment, and description thereof is omitted below. The differences from the first embodiment are as follows.
A point in which a capacitor 77 is provided between the output terminal of the operational amplifier 70 of the current detection circuit 49 from which the detection voltage 56 is output and the comparator 74.
A point that a resistor 78 is provided between the inverting input terminal 90 and the non-inverting input terminal 91 of the comparator 74.
A point where a resistor 79 is provided between the inverting input terminal 90 and the power supply (power supply voltage Vcc).
Here, the power supply voltage Vcc is higher than the threshold voltage Vref2 applied to the non-inverting input terminal 91 of the comparator 74. In the circuit configuration shown in FIG. 9, the capacitor 77 and the resistors 78 and 79 function as a high-pass filter that removes a DC component from the detection voltage 56 output from the current detection circuit 49 and inputs only the AC component to the comparator 74. To do.

  FIG. 10 is a diagram illustrating a change in voltage at each terminal when an electrostatic latent image for color misregistration detection is detected in the circuit configuration of FIG. 9 as in the first embodiment. 10, a waveform 501 is a waveform of the detection voltage 56 of the current detection circuit 49, a waveform 502 is a waveform of a voltage applied to the inverting input terminal 90 of the comparator 74, and a waveform 503 is a non-inverting input of the comparator 74. A waveform of a voltage applied to the terminal 91 and a waveform 504 indicate the waveforms of the detection voltage 76 output from the comparator 74, respectively. In FIG. 10, the horizontal axis represents time, and the coordinates of the horizontal axis are aligned in all of the waveforms 501 to 504. In addition, the vertical axis represents voltage, and for the waveforms 501 and 504, the scale of the vertical axis is adjusted for comparison.

  In FIG. 10, a period before time T10 represents a period before the electrostatic latent image for color misregistration correction is detected. During this period, the detection voltage 56 (waveform 501) of the current detection circuit 49 is constant without changing. The voltage at the inverting input terminal 90 of the comparator 74 (waveform 502) divides the power supply voltage Vcc and the threshold voltage Vref2 at the non-inverting input terminal 91 (waveform 503) by the resistors 78 and 79. Voltage. Therefore, the voltage at the inverting input terminal 90 of the comparator 74 (waveform 502) becomes constant at a voltage higher than the voltage at the non-inverting input terminal 91 of the comparator 74 (waveform 503). Lo is output as the waveform 504).

  Next, the portion after the time T10 in FIG. 10 (on the right side) shows a state when the electrostatic latent image for color misregistration correction is started to be detected. After time T10, when the detection voltage 56 (waveform 501) of the current detection circuit 49 reaches a position (electrically resistant position) where the electrostatic latent image for color misregistration correction faces the charging roller 23 on the photosensitive drum 22, When the electrostatic latent image disappears from the charging position, the level rises and returns to the original level. As described with reference to the flowchart of FIG. 3A, the electrostatic latent image is formed repeatedly (20 times) on the photosensitive drum 22 at regular intervals, so that the detection voltage 56 (waveform 501) of the current detection circuit 49 is Each time the electrostatic latent image passes through the charging position, the level is repeatedly lowered and raised. Here, the waveform 501 changes only in the direction in which the voltage decreases when an electrostatic latent image is detected. On the other hand, the voltage (waveform 502) of the inverting input terminal 90 of the comparator 74 after passing through the high-pass filter changes its center value at a speed determined by the time constant of the high-pass filter after time T10.

  In FIG. 10, times T11 to T18 indicate timings when the electrostatic latent image reaches the charging position and the detection voltage 76 output from the comparator 74 changes from Lo to Hi. Times T <b> 21 to T <b> 28 indicate timings at which the electrostatic latent image deviates from the charging position and the detection voltage 76 output from the comparator 74 changes from Hi to Lo. Times T31 to T38 indicate intermediate times calculated by the engine control unit 54 between each of the times T11 to T18 and each of the times T21 to T28. These times T31 to T38 are specified by the engine control unit 54 as the electrostatic latent image detection timing.

  As described above, the voltage (waveform 502) of the inverting input terminal 90 of the comparator 74, which is the output of the high-pass filter, is the center value of the waveform 502 immediately after time T10 when the electrostatic latent image detection is started. Changes. For this reason, as shown in FIG. 10, the intermediate time T31 calculated by the engine control unit 54 deviates from the intermediate time T41 in the waveform 501 that should be detected. For this reason, during the period in which the center value of the waveform 502 changes depending on the time constant of the high-pass filter, at the detection timing of the electrostatic latent image specified by the engine control unit 54 (for example, times T31, T32, and T33). Note that some error may occur.

  For example, in the circuit configuration of FIG. 2B that does not include a high-pass filter as in this embodiment, if the change in the DC component of the charging current is large, the threshold voltage 75 to be compared with the detection voltage 56 output from the current detection circuit 49. May be difficult to set properly. Furthermore, if the change in the direct current component of the charging current becomes larger than the change in the charging current when the electrostatic latent image is detected, the change in the detection voltage 56 when the electrostatic latent image passes through the charging position is There is a possibility that the comparator 74 cannot detect the electrostatic latent image. In such cases,

  According to the present embodiment, the high-pass filter filters the DC component of the charging current that may change due to variations in performance during manufacturing, durability deterioration, and the like of the photosensitive drum 22 and the charging roller 23. Thus, although an error may occur in the specified detection timing at the start of detection of the electrostatic latent image, the influence of the change in the DC component of the charging current on the detection accuracy of the electrostatic latent image is described above. It can be reduced by the action of the high-pass filter. In particular, when the current detection circuit 49 detects the current flowing in the plurality of charging rollers 23a to 23d as described above, the ratio of the DC component in the detected current tends to increase. In such a case, it is beneficial to use a high-pass filter as in this embodiment.

<Timing chart of reference value generation processing>
FIG. 11 is a timing chart showing the operation timing of the image forming apparatus 10 in the reference value generation processing according to the present embodiment. In FIG. 11, the same parts as those in FIG. 6 are denoted by the same reference numerals, and the description thereof is omitted.

  The difference from the first embodiment (FIG. 6) is that the engine control unit 54 outputs dummy laser signals 190a to 190d before outputting the laser signals 90a to 90d, respectively. As described above, within a certain period from the start of detection of a plurality of continuous electrostatic latent images until the output of the high-pass filter is stabilized, the detection timing of the electrostatic latent image specified by the engine control unit 54 is somewhat. Error may occur. Therefore, in the present embodiment, the engine control unit 54 uses the dummy laser signals 190a to 190d within such a fixed period, and the current detection signals 195a to 195d corresponding to those signals are used for generating a reference value. do not do. That is, only the current detection signals 95a to 95d corresponding to the laser signals 90a to 90d output following the dummy laser signals 190a to 190d are used for generating the reference value.

<Timing chart for color misregistration correction control>
FIG. 12 is a timing chart showing the operation timing of the image forming apparatus 10 in the color misregistration correction control after the reference value generation processing according to the present embodiment. In FIG. 12, the same parts as those in FIG. 11 are denoted by the same reference numerals, and the description thereof is omitted.

  In the present embodiment, the engine control unit 54 variably controls the output timing of the laser signals 90a to 90d based on the previous measurement result, similarly to the color misregistration correction control (FIG. 8) of the first embodiment. Yes. Therefore, as in the first embodiment, it is possible to continuously obtain the current detection signals 95a to 95d corresponding to the laser signals 90a to 90d without providing a waiting time between laser signals of different colors. . Therefore, in the color misregistration correction control after the reference value generation process, as shown in FIG. 12, the engine control unit 54 performs dummy only for the laser signal 90a corresponding to the Y color on which the electrostatic latent image is first formed. A laser signal 190a is added. As a result, the current detection signal 195a corresponding to the dummy laser signal 190a appears only before the current detection signal 95a.

  As described above, according to the present embodiment, in addition to the effects of the first embodiment, the charging current direct current that may be generated due to variations in performance, durability deterioration, or the like of the photosensitive drum 22 and the charging roller 23 is manufactured. The influence of the change in the component on the detection accuracy of the electrostatic latent image can be suppressed. That is, high-pass filtering can increase the detection accuracy of the electrostatic latent image, and can improve the accuracy of color misregistration correction.

[Third Embodiment]
In the third embodiment, the engine control unit 54 causes the photosensitive drum 22 to form a plurality of electrostatic latent images respectively corresponding to the plurality of image forming units within a period in which the photosensitive drum 22 rotates once. It is said. Specifically, a circuit configuration similar to that of the second embodiment (FIG. 9) is used, and the electrostatic latent image for color misregistration correction corresponding to each color is corresponded to within the period in which the photosensitive drum 22 rotates once. Each drum 22 is formed twice. Hereinafter, for simplification of description, description of portions common to the first and second embodiments is omitted.

<Timing chart of reference value generation processing>
FIG. 13 is a timing chart showing operation timings of the image forming apparatus 10 in the reference value generation processing according to the third embodiment of the present invention. In FIG. 13, the same parts as those in FIG. 11 are denoted by the same reference numerals, and the description thereof is omitted.

  As shown in FIG. 13, at times T4 to T6, which corresponds to the time required for one rotation (one rotation) of the photosensitive drum, the engine control unit 54 displays the electrostatic latent images for four colors on the photosensitive drums 22a to 22d, respectively. In order to form twice, laser signals 91a to 91d and 92a to 92d are output. Similar to the first and second embodiments, when the reference value generation process is executed, the current color misregistration amount is not yet known. For this reason, it is necessary to prevent the current detection signals 196a to 196d and 197a to 197d corresponding to the laser signals 91a to 91d and 92a to 92d, respectively, from overlapping in time. Therefore, the engine control unit 54 sequentially outputs the laser signals 91a to 91d and 92a to 92d with sufficient intervals 94a to 94g. Similarly to the second embodiment, the engine control unit 54 responds to a certain time until the output of the high-pass filter is stabilized before the laser signals 91a to 91d and 92a to 92d are output. -191d and 192a-192d are output.

  In the present embodiment, the engine control unit 54 outputs the laser signals 91a to 91d and 92a to 92d for four colors during one rotation of the photosensitive drum 22, thereby causing the electrostatic drums 22a to 22d to generate electrostatic latent images. Form an image. Furthermore, the engine control unit 54 sequentially detects these electrostatic latent images based on the current detection signals 196a to 196d and 197a to 197d, and generates a reference value. As described above, in the first and second embodiments, the reference value generation processing is performed using the time of one round of the photosensitive drum 22 for each color, whereas in the present embodiment, one of the photosensitive drums 22 is used. A reference value generation process for four colors is performed in a round time. Therefore, according to the present embodiment, it is possible to reduce the time required for generating the reference value, although the detection accuracy of the electrostatic latent image may be disadvantageous as compared with the first and second embodiments. It is.

<Timing chart for color misregistration correction control>
FIG. 14 is a timing chart showing the operation timing of the image forming apparatus 10 in the color misregistration correction control after the reference value generation processing according to the present embodiment. In FIG. 14, the same parts as those in FIG. 13 are denoted by the same reference numerals, and the description thereof is omitted.

  In the present embodiment, similarly to the color misregistration correction control (FIGS. 8 and 12) of the first and second embodiments, the output timings of the laser signals 291a to 291d and 292a to 292d are set based on the previous measurement result. Variable control. Therefore, as in the first and second embodiments, the current detection signals 296a to 296d and 297a to 297d can be continuously obtained without providing a waiting time between laser signals of different colors. Therefore, in the color misregistration correction control after the reference value generation process, the engine control unit 54 performs dummy only for the laser signal 291a corresponding to the Y color on which the electrostatic latent image is first formed, as shown in FIG. A laser signal 191a is added. As a result, the current detection signal 196a corresponding to the dummy laser signal 191a appears only before the current detection signal 296a.

  Here, when comparing the laser signal used in the reference value generation process (FIG. 13) and the color misregistration correction control (FIG. 14) in the present embodiment, the laser signal used in the latter is longer. That is, the laser signals 291a to 291d and 292a to 292d have longer time lengths than the laser signals 91a to 91d and 92a to 92d. This is because in the color misregistration correction control, the laser signal 291a to 291d and 292a to 292d are output by varying the output timing, and the waiting time (dummy laser signal) is reduced, so the time length of the laser signal is increased. It is. Thereby, the current detection signals 296a to 296d and 297a to 297d are also lengthened. Thus, in this embodiment, the detection accuracy of the electrostatic latent image in the color misregistration correction control is improved.

  As described above, according to the present embodiment, the time required for generating the reference value can be shortened, although the detection accuracy of the electrostatic latent image can be disadvantageous as compared with the first and second embodiments. It is possible.

  The reference value generation processing in the first embodiment (FIG. 6), the second embodiment (FIG. 11), and the third embodiment (FIG. 13) is performed on the photosensitive drum 22 by the scanner unit 20. This is common in that a reference value is generated based on a difference in time from when an image is formed until the electrostatic latent image reaches a position (charging position) facing the charging roller 23. On the other hand, there is a difference between the processing time and the detection accuracy of the electrostatic latent image at the charging position. The same applies to the color misregistration correction control of the first embodiment (FIG. 8), the second embodiment (FIG. 12), and the third embodiment (FIG. 13).

  The reference value generation processing and the color misregistration correction control in the first to third embodiments can be used in any combination. For example, the reference value generation process may be executed according to the second embodiment, and the color misregistration correction control may be executed according to the third embodiment. Such a combination uses a process with excellent measurement accuracy because the reference value generation process has a sufficient processing time, whereas the color misregistration correction control may be executed before or after the printing process. The aim is to adopt processing with a short processing time.

[Fourth Embodiment]
In the second and third embodiments, a dummy laser output signal is used in order to cope with an error occurring in the detection timing of the electrostatic latent image caused by the circuit configuration including the high-pass filter described with reference to FIG. The current detection signal corresponding to it is not used. In the fourth embodiment, in order to shorten the time required for the reference value generation process and the color misregistration correction control, compared to the second and third embodiments, a static color correction static image that is first formed on the photosensitive drum 22 is used. It is characterized by adjusting the pattern of the electrostatic latent image. Hereinafter, for simplification of description, description of portions common to the first to third embodiments is omitted.

  In the present embodiment, as in the second embodiment, the circuit configuration shown in FIG. 9 is used as the circuit configuration including the charging high-voltage power supply circuits 43a to 43d and the current detection circuit 49. FIG. 15 is a diagram illustrating a change in voltage of each terminal when an electrostatic latent image for color misregistration detection is detected in the circuit configuration of FIG. In FIG. 15, waveforms 511 to 514 correspond to the waveforms 501 to 504 of FIG. 10, and show the waveform of the voltage of the same part in the circuit configuration shown in FIG. 9.

  In the present embodiment, among the plurality of continuous electrostatic latent images for color misregistration detection, the length of the first electrostatic latent image along the conveyance direction on the photosensitive drum 22 is set to the other subsequent electrostatic latent image. Make it longer than the latent image. In FIG. 15, as an example, the length of the first electrostatic latent image is set to 1.75 times the length of the other electrostatic latent images. Thereby, compared with the waveform 502 of FIG. 10, the center value of the waveform 512 can be stabilized in a shorter time. That is, in the present embodiment, the length of the laser signal corresponding to the first electrostatic latent image may be set to a length that stabilizes the output of the high-pass filter. In this case, the engine control unit 54 does not use only the time T31 corresponding to the first electrostatic latent image detection timing, but uses the second and subsequent electrostatic latent image detection timings (after the time T32). Color misregistration may be detected.

  As described above, according to the present embodiment, the length of the electrostatic latent image pattern for color misregistration correction is changed. As a result, even when high-pass filtering is applied to the detection voltage 56 output from the current detection circuit 49, the time until the filtered voltage is stabilized can be shortened, and the time required for color misregistration correction control can be reduced. It can be shortened. Further, as in the second embodiment, high-pass filtering can improve the detection accuracy of the electrostatic latent image and can improve the accuracy of color misregistration correction.

[Modification]
Below, the modification applicable with respect to the above-mentioned embodiment is further demonstrated. In each of the above-described embodiments, the charging rollers 23a to 23d have been described as examples of process units that are current detection targets for color misregistration detection and correction. However, it is also possible to execute current detection for another process unit, and to perform reference value generation processing and color misregistration correction control. That is, the primary transfer rollers 26a to 26d or the developing sleeves 24a to 24d may be a process unit that is a current detection target.

  FIGS. 16A and 16B are diagrams showing circuit configuration examples including a primary transfer high-voltage power supply circuit 46a, a development high-voltage power supply circuit 44a, and a current detection circuit 49, respectively. In FIGS. 16A and 16B, the primary transfer high-voltage power supply circuits 46b to 46d and the development high-voltage power supply circuits 44b to 44d are not shown for the sake of simplification. Is provided.

  The circuit configuration illustrated in FIG. 16A illustrates a configuration in which the process units that are current detection targets for color misregistration detection and correction are the primary transfer rollers 26a to 26d. Compared to FIG. 2B in the above-described embodiment, it is the same except that the directions of the anodes and cathodes of the diodes 64 and 65 are reversed. Further, a transfer bias (transfer voltage) of, for example, +1000 V is output from the output terminal 53. In the above-described embodiment, instead of the charging high-voltage power supply circuit 43a, the above-described color misregistration correction control is executed for the primary transfer high-voltage power supply circuit 46a shown in FIG. An effect can be obtained. In this case, the above-described color misregistration correction control may be executed with the primary transfer roller 26a in contact with the photosensitive drum 22a instead of the charging roller 23a.

  The circuit configuration illustrated in FIG. 16B illustrates a configuration in which the process units that are current detection targets for color misregistration detection and correction are the developing sleeves 24a to 24d. In the circuit configuration, for example, a developing bias (developing voltage) of −400 V is output from the output terminal 53. Compared to FIG. 2B in the above-described embodiment, the difference is that a reverse development bias output circuit is added. When detecting the electrostatic latent image, the reverse development bias output circuit is operated by setting the output of CLK2 to OFF and the output of CLK1 to ON. Further, during normal image formation, the output of CLK1 is set off and the output of CLK2 is set on.

  When the development high-voltage power supply circuits 44a to 44d are operated, it is desirable that the output voltage be higher than VL so that the toner does not adhere to the photosensitive drum. Correspondingly, for example, in the circuit configuration of FIG. 16B, if the output of CLK2 is set to OFF and the output of CLK1 is set to ON, a reverse polarity voltage (reverse bias) is output. Good. Even when the reverse development bias output circuit does not exist, for example, when VL is a negative voltage of −100 V, the output of the development high voltage power supply circuits 44a to 44d is a negative voltage and the absolute value is smaller than VL. You may set to voltage (for example, -50V).

  In the above-described embodiment, the same effect as in the above-described embodiment can be obtained by executing the above-described color misregistration correction control for the development high-voltage power circuit 44a shown in FIG. 16A instead of the charging high-voltage power circuit 43a. It is possible to obtain.

Claims (13)

A photosensitive member that is driven to rotate, a process unit that is disposed in the vicinity of the photosensitive member and that acts on the photosensitive member, and irradiates light on the photosensitive member to form an electrostatic latent image on the photosensitive member. A plurality of image forming means for forming images of different colors on the photosensitive member, respectively,
A power supply means provided corresponding to a plurality of the process means and functioning as a power supply for the plurality of process means;
When the electrostatic latent image for color misregistration correction, which is provided in common to the plurality of image forming units and formed on the photoconductor, passes a detection position corresponding to a position facing the process unit. Detecting means for detecting a timing at which the color misregistration correcting electrostatic latent image has reached the detection position in response to a change in the output of the power supply means,
Each of the plurality of image forming means is controlled by using the time from the timing when the light irradiation means is controlled to start the formation of the electrostatic latent image for color misregistration correction to the timing detected by the detecting means as a moving time. Measuring means for measuring, and
Based on the movement time measured by the measurement unit, the timing at which the detection unit detects the color latent image for color misregistration correction is different for each of the plurality of image forming units. An image forming apparatus comprising: control means for controlling timing for starting formation of the electrostatic latent image for color misregistration correction for each color. The control means includes the plurality of images so that a difference value between the movement time corresponding to the first color and the movement time corresponding to the second color among the plurality of colors approaches a preset reference value. The image forming apparatus according to claim 1, wherein the color misregistration is corrected by controlling the forming unit. The detection means includes
A current detection circuit that detects an output current output to the process means corresponding to the power supply means, converts the detected current into a voltage, and outputs the voltage; and
A comparator that compares an output voltage from the current detection circuit with a predetermined threshold voltage, and based on a comparison result between the output voltage and the threshold voltage by the comparator, The image forming apparatus according to claim 1, wherein a timing at which a latent image reaches the detection position is detected. A high-pass filter for filtering the output voltage from the current detection circuit to remove at least a DC component of the output voltage;
The image forming apparatus according to claim 3, wherein the comparator compares the output voltage filtered by the high-pass filter with the threshold voltage. The control means includes
For each of the plurality of image forming units, the light irradiation unit forms a plurality of continuous electrostatic latent images for color misregistration correction on the photoconductor, and a plurality of continuous color misregistration correction static images. Among the electrostatic latent images, the length of the first electrostatic latent image for color misregistration correction along the conveyance direction of the photosensitive member is made longer than the subsequent electrostatic latent image for color misregistration correction. The image forming apparatus according to claim 4. For each of the plurality of image forming units, the control unit outputs a plurality of the electrostatic latent images for color misregistration correction that are continuously formed on the photoconductor by the light irradiation unit over a period in which the photoconductor rotates once. The plurality of image forming means such that a value obtained by averaging the plurality of difference values obtained corresponding to the plurality of color misregistration correcting electrostatic latent images approaches the preset reference value. The image forming apparatus according to claim 2, wherein color misregistration is corrected by controlling the color shift.   The control unit is configured to apply a plurality of color misregistration correction electrostatic latent images respectively corresponding to the plurality of image forming units on the photoconductor by the light irradiation unit within a period in which the photoconductor rotates once. The image forming apparatus according to claim 1, wherein the image forming apparatus is formed. A sensor for detecting a toner pattern formed on the endless belt;
By transferring the color misregistration correction toner pattern formed by the plurality of image forming units to the endless belt, and controlling the plurality of image forming units based on the detection result of the toner pattern by the sensor. Correction means for correcting color misregistration;
After the correction by the correction unit, the difference between the movement time corresponding to the first color and the movement time corresponding to the second color, which is measured by the measurement unit and measured by the measurement unit. The image forming apparatus according to claim 2, further comprising a setting unit that sets a value as the reference value. The process means includes
Charging means for charging the photosensitive member, developing means for attaching toner to the electrostatic latent image in order to develop the electrostatic latent image with toner and form a toner image on the photosensitive member, or the photosensitive member The image forming apparatus according to claim 1, wherein the image forming apparatus is a transfer unit that transfers the toner image formed on the endless belt to an endless belt. The endless belt is
The plurality of color images formed by the plurality of image forming units are transferred from the plurality of photoconductors in an overlapping manner to form a multicolor image, or formed by the plurality of image forming units. 10. The image forming apparatus according to claim 8, wherein the image forming apparatus is a recording medium conveying belt that conveys a recording medium on which the plurality of color images transferred from the plurality of photosensitive members are transferred. The control means includes
9. The color misregistration is corrected by controlling a timing at which the light irradiation unit irradiates light to the photoconductor or by controlling a rotation speed of the photoconductor. The image forming apparatus according to any one of the above.   The image forming apparatus according to claim 1, wherein a plurality of the power supply units are provided corresponding to the plurality of process units.   The control means includes a time for forming the electrostatic latent image for color misregistration correction, a movement time corresponding to a first color among the plurality of colors, and a movement time corresponding to a second color. The image forming apparatus according to claim 1, wherein the timing for starting the formation of the electrostatic latent image for color misregistration correction of each color is controlled based on the difference value.

Images