JP2017203938A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent displacement of an image when an image forming rate is switched while reducing the stopping time of image formation.SOLUTION: An image forming apparatus 100 operable at a plurality of image forming rates comprises: pattern detection means 500 that detects a registration correction pattern at the first image forming rate; writing timing determination means 141 that determines a timing to write an electrostatic latent image in a photoreceptor 101 with a light beam emitted from a light source on the basis of a result of detection performed by the pattern detection means; and storage means 140 that stores in advance the amount of correction for correcting the writing timing. When operating at the preset first image forming rate, the image forming apparatus forms an image on the basis of the writing timing determined by the writing timing determination means, and when operating at the second image forming rate, corrects the writing timing on the basis of the amount of correction and a speed ratio before forming the image.SELECTED DRAWING: Figure 12

Description

  The present invention relates to an image forming apparatus operable at a plurality of image forming speeds.

  An electrophotographic image forming apparatus such as a copying machine or a laser beam printer scans a light beam emitted from a light source on a photosensitive member to form an electrostatic latent image, and the electrostatic latent image is converted into a plurality of color toners. To develop a multi-color toner image. The toner images of a plurality of colors are transferred and superimposed on the carrier belt. The toner images of a plurality of colors on the carrier belt are transferred and fixed on a recording medium to form a color image. In order to prevent color misregistration of a plurality of color toner images, a registration correction pattern is formed on the carrier belt, and the color misregistration amount is detected by detecting the registration correction pattern. Color misregistration correction and image position correction are performed by adjusting the formation position of the electrostatic latent image based on the detected color misregistration amount (Patent Document 1). Since color misregistration usually occurs due to expansion / contraction of parts, the image forming apparatus periodically sets a registration correction mode for detecting a color misregistration by forming a registration correction pattern and detecting the formation position of the pattern. Run it.

  In the image forming mode in which image formation is performed based on the input image data, the image forming apparatus adjusts the timing of writing the electrostatic latent image by the light beam during each scanning period of the light beam based on the detection result of the registration correction pattern. To do. The writing timing is the light beam emission start timing based on the image data. That is, the time from when the light detector receiving the light beam outputs the synchronization signal to when the light beam for writing the electrostatic latent image on the photosensitive member is output from the light source is adjusted according to the image data. By performing such adjustment, the writing position of the electrostatic latent image in the main scanning direction, which is the direction in which the light beam scans the photosensitive member, can be made substantially constant. The writing timing is controlled by the CPU or the like with the resolution of the frequency unit of the clock signal.

  On the other hand, in order to improve the image quality or to secure the fixability of paper types that require a high amount of heat to melt the toner (thick paper, etc.), it is possible to operate in a mode in which image formation is performed at a slower image formation speed than usual. Image forming apparatus. In recent years, in order to perform good image formation on various types of paper used by users, the types of image forming speeds have been increased, and image forming speeds used in the same image forming apparatus. The range of is expanding. In addition, there are an increasing number of cases where various paper types are printed in one job, such as printing a booklet in which the paper type of the cover is different from that of the middle paper. Changing the image forming speed requires changing the rotational speed of the photoreceptor. In some cases, the scanning speed of the light beam needs to be changed in accordance with the change in the rotation speed of the photosensitive member.

  However, every time the image forming speed changes, if the image forming apparatus executes the registration correction mode, the downtime increases and the productivity decreases. Therefore, even after the image forming speed is changed, the image is formed based on the registration correction amount before the change. This is because the amount of color misregistration as a distance does not change even when the image forming speed is changed immediately after registration correction. As a result, the time required for the registration correction mode is reduced. For example, after the registration correction mode is executed at a normal image forming speed to obtain the writing start timing, the registration correction mode is not executed at an image forming speed slower than normal. When an image is formed at a slower image forming speed than normal, the writing timing obtained by multiplying the writing timing obtained in the registration correction mode at the normal image forming speed by the ratio of the image forming speed is used. This reduces the image formation stop time.

JP 2003-5490 A

  However, strictly speaking, the writing timing obtained in the registration correction mode includes a delay time until the synchronization signal output from the photodetector is input to the CPU and a delay time of the CPU circuit. Further, the writing timing includes a delay time from when the CPU outputs an image signal (hereinafter referred to as a video signal) to the light source driving unit until when a light beam is emitted from the light source. These delay times are constant regardless of the image forming speed. For this reason, if the writing timing of the image forming speed slower than normal is determined by multiplying the writing timing of the normal image forming speed when executing the registration correction mode by the ratio of the image forming speed, an error occurs in the writing timing. In particular, in the case of a high speed in which one scanning period of the light beam in the main scanning direction is several hundreds μs, it is easily affected by a signal transmission delay time.

  In order to prevent image shift due to the signal transmission delay time, the registration correction mode may be executed every time the image forming speed is changed. However, if the registration correction mode is executed before the image formation every time the image formation speed is changed, the image formation stop time occurs, so the productivity of the image formed by the image forming apparatus decreases.

  Accordingly, the present invention provides an image forming apparatus capable of preventing image misalignment when the image forming speed is switched while reducing the stop time of image formation.

In order to solve the above problems, an image forming apparatus operable at a plurality of image forming speeds according to the present embodiment is provided.
A photoreceptor,
A light source that emits a light beam;
Deflecting means for deflecting the light beam so that the light beam emitted from the light source scans the surface of the photoreceptor in the main scanning direction;
Transfer means for developing an electrostatic latent image on the surface of the photoconductor formed by the light beam with toner and transferring the developed toner image to a carrier or a recording medium conveyed by the carrier;
Pattern detecting means for detecting a registration correction pattern transferred to the carrier by the transfer means when the image forming apparatus operates at a preset first image forming speed;
Based on the detection result of the pattern detection means, the electrostatic latent image is written onto the photosensitive member by the light beam emitted from the light source when the image forming apparatus operates at the first image forming speed. Writing timing determining means for determining timing;
Storage means for preliminarily storing a correction amount for correcting the writing timing when the image forming apparatus operates at a second image forming speed different from the first image forming speed;
With
When the image forming apparatus operates at the first image forming speed, an image is formed based on the writing timing determined by the writing timing determining unit,
When the image forming apparatus operates at the second image forming speed, the writing timing determined by the writing timing determining means is set to the correction amount stored in the storage means and the second image forming speed. The correction is made based on the speed ratio of the first image forming speed, and an image is formed based on the corrected writing start timing.

  According to the present invention, it is possible to prevent image shift when the image forming speed is switched while reducing the stop time (down time) of image formation.

1 is a cross-sectional view of an image forming apparatus. Explanatory drawing of an optical scanning device. Explanatory drawing of the delay time of a BD signal and a video signal. Explanatory drawing of the image position shift by delay time. The timing diagram which shows the relationship between delay time and the center image start timing. FIG. 6 is a timing diagram in the main scanning direction of a light beam with respect to a sheet. The timing diagram which shows the relationship between the correction | amendment of the center image starting timing based on speed ratio, and a correction amount. 6 is a flowchart of a control operation in a main scanning registration image formation speed correction mode. The block diagram of a control system. The figure which shows a registration correction pattern. The figure which shows the registration correction pattern and pattern sensor which were formed on the support body belt. 6 is a flowchart of image forming control operations executed by a CPU. The figure which shows the registration correction pattern of only two colors.

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

(Image forming device)
First, the image forming apparatus 100 will be described. FIG. 1 is a cross-sectional view of the image forming apparatus 100. The image forming apparatus 100 is an electrophotographic digital full-color printer that forms an image on a recording medium (hereinafter referred to as a sheet) using a plurality of colors of toner. The image forming apparatus 100 can operate at a plurality of image forming speeds. The image forming apparatus 100 includes four image forming units 10 (10Y, 10M, 10C, and 10K). The image forming unit 10Y forms a yellow image using yellow toner. The image forming unit 10M forms a magenta image using magenta toner. The image forming unit 10C forms a cyan image using cyan toner. The image forming unit 10K forms a black image using black toner. The suffixes Y, M, C, and K of the reference symbols indicate yellow, magenta, cyan, and black, respectively. In the following description, the subscripts Y, M, C, and K may be omitted if not particularly necessary. The four image forming units 10 have the same structure except for the color of the developer (toner).

  The image forming unit 10 includes a photosensitive drum (image carrier) 101 as a photosensitive member. The photosensitive drum 101 rotates around the rotation shaft 1 in the direction indicated by the arrow R1 in FIG. 1 during image formation. Around the photosensitive drum 101, a charging device 102, an optical scanning device 200, a developing device 103, a primary transfer device 104, and a drum cleaning device 4 are arranged. Below the photosensitive drum 101, an endless belt intermediate transfer member (hereinafter referred to as a carrier belt) 105 is disposed. The carrier belt 105 is stretched around the driving roller 11, the driven roller 13, and the secondary transfer counter roller 21. The carrier belt 105 rotates in the direction indicated by the arrow R2 in FIG. 1 during image formation. A primary transfer device 104 serving as a transfer unit is disposed to face the photosensitive drum 101 with a carrier belt 105 interposed therebetween. The secondary transfer roller 106 is disposed to face the secondary transfer counter roller 21 with the carrier belt 105 interposed therebetween.

  In the present embodiment, the carrier belt 105 is an intermediate transfer body on which the toner image is primarily transferred by the primary transfer device 104 and the toner image is secondarily transferred to the sheet S by the secondary transfer roller 106. However, the carrier belt 105 is not limited to this, and may be, for example, a sheet conveying belt that conveys the sheet S on which the toner image is directly transferred from the photosensitive drum 101.

  A pattern sensor (pattern detection means) 500 as an optical sensor for detecting a predetermined registration correction pattern 501 formed on the carrier belt 105 is provided in the vicinity of the carrier belt 105 downstream of the image forming unit 10. ing. The pattern sensor 500 will be described later. A belt cleaning device 12 for removing the toner remaining on the carrier belt 105 after the secondary transfer is provided in the vicinity of the carrier belt 105 upstream of the image forming unit.

  A feeding cassette 109 that accommodates the sheet S is disposed below the image forming apparatus 100. Further, a manual feed tray 110 on which the sheet S is placed is disposed on the side of the image forming apparatus 100. The sheet S is fed from the feeding cassette 109 or the manual feed tray 110 by the respective pickup rollers 18. The sheet S is conveyed to the secondary transfer roller 106 by the conveyance roller 114 and the registration roller 111. The fixing device 107 is disposed downstream of the secondary transfer roller 106 in the conveyance direction D of the sheet S. A discharge tray 77 for stacking sheets S on which images are formed is provided on the downstream side of the fixing device 107 in the conveyance direction D of the sheet S.

(Image formation process)
Next, an image forming process of the image forming apparatus 100 will be described. Since the image forming processes in the four image forming units 10 are the same, the image forming process in the yellow image forming unit 10Y will be described. The description of the image forming process in the magenta image forming unit 10M, the cyan image forming unit 10C, and the black image forming unit 10K is omitted. The photosensitive drum 101Y rotates in the direction indicated by the arrow R1. The charging device 102Y uniformly charges the surface of the photosensitive drum 101Y. The optical scanning device 200 emits laser light (hereinafter referred to as a light beam) modulated according to image data of the yellow component, and forms an electrostatic latent image on the surface of the uniformly charged photosensitive drum 101Y. The developing device 103Y develops the electrostatic latent image with yellow toner into a yellow toner image. The primary transfer device 104 </ b> Y primarily transfers the yellow toner image on the photosensitive drum 101 </ b> Y onto the carrier belt 105. The toner remaining on the photosensitive drum 101Y after the primary transfer is removed by the drum cleaning device 4.

  Similarly, the magenta toner image formed by the magenta image forming unit 10M is accurately superimposed on the yellow toner image on the carrier belt 105 and transferred. Thereafter, the cyan toner image and the black toner image are sequentially transferred onto the magenta toner image on the carrier belt 105. As a result, four color toner images are superimposed on the carrier belt 105.

  The sheet S conveyed from the feeding cassette 109 or the manual feed tray 110 is conveyed to the secondary transfer roller 106 by the registration roller 111 in synchronization with the toner image on the carrier belt 105. The four-color toner images superimposed on the carrier belt 105 are secondarily transferred onto the sheet S all at once by the secondary transfer roller 106. Thereafter, the sheet S carrying the toner image is conveyed to the fixing device 107. The fixing device 107 heats and pressurizes the sheet S to fix the toner image on the sheet S, and forms a full color image on the sheet S. The sheet S on which the image is formed is discharged onto the discharge tray 77 by the discharge roller 108.

  In the case of double-sided printing, the sheet S that has passed through the fixing device 107 is guided to the double-sided reversing path 112 and reversed, and conveyed to the double-sided path 113. The sheet S that has passed through the double-sided path 113 is again conveyed to the image forming unit 10 by the conveying roller 114. Similar to the front surface (first surface), an image is formed on the back surface (second surface) of the sheet S. The sheet S on which images are formed on both sides is discharged onto the discharge tray 77 by the discharge roller 108.

  The image forming apparatus 100 can switch the image forming speed between a plurality of image forming speeds according to the paper type (paper quality, thickness, basis weight, surface texture) and image quality of the sheet S. For example, in the case of cardboard and high image quality, the image forming speed is switched to a low speed. In the case of thin paper or speed-priority image quality, the image forming speed is switched to high speed. The image forming speed is switched according to the image forming speed for each page included in the image forming job output from the image processing unit provided in the main body of the image forming apparatus 100.

(Optical scanning device)
Next, an optical scanning device 200 as a light beam emitting device will be described. FIG. 2 is an explanatory diagram of the optical scanning device 200. FIG. 2A is a plan view schematically showing components disposed inside the optical scanning device 200. FIG. FIG. 2B is a side view showing optical paths of light beams 221Y, 221M, 221C, and 221K from the rotary polygon mirror 205 to the photosensitive drum 101. FIG. FIG. 2C is a layout diagram of a laser light emitting element (hereinafter referred to as a light source) 201 as viewed from an arrow IIC in FIG. As shown in FIG. 2C, the optical scanning device 200 includes four light sources 201Y, 201M, 201C, and 201K corresponding to yellow, magenta, cyan, and black, respectively.

  The optical scanning device 200 includes a light source 201, a collimator lens 202, an aperture stop 203, a cylindrical lens 204, a rotary polygon mirror 205, a motor 206, a toric lens 207, a diffractive optical element 208, and reflecting mirrors 209, 130, and 131. The light sources 201 (201Y, 201M, 201C, 201K) emit a light beam 220 according to the image data of each color component. The collimator lens 202 converts the light beam 220 emitted from the light source 201 into a substantially parallel light beam. The aperture stop 203 limits the light beam 220. The cylindrical lens 204 has a predetermined refractive power (degree of refraction) only in the sub-scanning direction, and forms the light beam 220 as an ellipse image long in the main scanning direction on the reflecting surface of the rotary polygon mirror 205. The optical paths of the light beams 220Y and 220K from the light source 201 to the rotary polygon mirror 205 are parallel to each other, and the optical paths of the light beams 220M and 220C from the light source 201 to the rotary polygon mirror 205 are parallel to each other. The light beam 220Y is incident on the same reflecting surface of the rotary polygon mirror 205 from obliquely below and the light beam 220M is obliquely upward. The light beam 220K is incident on the same reflecting surface of the rotating polygonal mirror 205 from obliquely below and the light beam 220C is obliquely upward. The reflecting surfaces on which the light beams 220Y and 220M are incident are different from the reflecting surfaces on which the light beams 220C and 220K are incident.

  The rotating polygonal mirror 205 as the deflecting means is rotated at a constant speed in a direction indicated by an arrow R3 in FIG. The light beams 220Y and 220M formed on the reflecting surface of the rotating polygonal mirror 205 are deflected to the left of the rotating polygonal mirror 205, and the deflected light beams 221Y and 221M are indicated by an arrow X2 in FIG. Is scanned in the main scanning direction indicated by. The light beams 220C and 220K formed on the reflecting surface of the rotating polygonal mirror 205 are deflected to the right of the rotating polygonal mirror 205, and the deflected light beams 221C and 221K are the arrows X1 in FIG. Is scanned in the main scanning direction indicated by. The main scanning direction X2 in which the light beams 221Y and 221M are scanned is opposite to the main scanning direction X1 in which the light beams 221C and 221K are scanned. Scanning a plurality of light beams in opposite directions in this way is hereinafter referred to as counter scanning.

  The position where the light beam is reflected at 90 ° when viewed from the light source 201 is set to be the center of the photosensitive drum 101 in the axial direction. Image formation is performed so that the center of the image coincides with the center of the photosensitive drum 101 in the axial direction. When the light beam 221 is irradiated onto the center of the photosensitive drum 101, the light beam 220 emitted from the light source 201 is incident on the reflecting surface of the rotary polygon mirror 205 at an angle of 45 ° and is incident on the optical axis of the light beam 220. On the other hand, it is reflected in the direction of 90 °. The light beam 221Y reflected by the rotary polygon mirror 205 passes through the toric lens 207a and the diffractive optical element 208a, is reflected by the reflecting mirror 209a, and is irradiated onto the photosensitive drum 101Y. The light beam 221M reflected by the rotary polygon mirror 205 passes through the toric lens 207a and the diffractive optical element 208a, is reflected by the reflecting mirrors 130a and 131a, and is irradiated onto the photosensitive drum 101M. The position of the light beam 221Y imaged on the photosensitive drum 101Y and the position of the light beam 221M imaged on the photosensitive drum 101M are the same in the main scanning direction X2. The light beam 221C reflected by the rotary polygon mirror 205 passes through the toric lens 207b and the diffractive optical element 208b, is reflected by the reflecting mirrors 130b and 131b, and is irradiated onto the photosensitive drum 101C. The light beam 221K reflected by the rotary polygon mirror 205 passes through the toric lens 207b and the diffractive optical element 208b, is reflected by the reflecting mirror 209b, and is irradiated onto the photosensitive drum 101K. The position of the light beam 221C imaged on the photosensitive drum 101C and the position of the light beam 221K imaged on the photosensitive drum 101K are the same in the main scanning direction X1.

  The toric lenses 207 (207a, 207b) are optical elements having fθ characteristics, and are refractive members having different refractive indexes in the main scanning direction and the sub-scanning direction. Both the front and back lens surfaces of the toric lens 207 in the main scanning direction have an aspherical shape. The diffractive optical element 208 (208a, 208b) is an optical element having an fθ characteristic, and is a long diffractive member having different magnifications in the main scanning direction and the sub-scanning direction.

  The light beam 220K emitted from the light source 201K outside the image forming area is reflected by the rotary polygon mirror 205, and the reflected light beam 222K enters a photodetector (hereinafter referred to as a BD) 214. When receiving the light beam 222K, the BD 214 serving as the light detection means receives a synchronization signal (hereinafter referred to as BD) for making the writing position in the main scanning direction of the electrostatic latent image on the photosensitive drum 101K scanned by the light beam 221K constant. Signal). Since the writing positions of the light beam 221K and the light beam 221C are the same, image writing is started after a predetermined time from when the BD signal output from the BD 214 is detected. On the other hand, the light beam 221Y and the light beam 221M also start image writing based on the BD signal of the BD 214. As if the virtual light beam 222Y was incident on the virtual BD 215 and a virtual BD signal was output, the light beam 221Y and the light beam 221M are based on the BD signal of the BD 214, and the light beam 221K, the light beam 221C, and so on. Forms an image by scanning in the reverse direction. Since the reflecting surface of the rotary polygon mirror 205 that deflects the light beam 222K incident on the BD 214 is different from the reflecting surface that deflects the light beams 221Y and 221M at that moment, the virtual BD signal of the virtual BD 215 is the actual BD 214 actual It is different from the BD signal. Therefore, the writing timing of the light beams 221Y and 221M is calculated by adding or subtracting a predetermined time to the writing timing (exit start timing) generated based on the actual BD signal of the actual BD 214. As a result, the positions of the black and cyan images formed by the light beams 221K and 221C are made to coincide with the positions of the yellow and magenta images formed by the light beams 221Y and 221M, thereby preventing color misregistration. To do.

  The light spot of the light beam emitted from the light source 201 and deflected by the rotating polygon mirror 205 is linearly parallel to the rotation axis 1 of the photosensitive drum 101 on the surface of the photosensitive drum 101 uniformly charged by the charging device 102. Move at a constant speed. The potential of the surface of the photosensitive drum 101 is displaced depending on the intensity of the light beam. While the photosensitive drum 101 is rotated in the sub-scanning direction R1 perpendicular to the main scanning directions X1 and X2, the light beam is repeatedly scanned on the photosensitive drum 101 in the main scanning directions X1 and X2, thereby statically moving in the sub-scanning direction R1. An electrostatic latent image is formed.

(Delay of export timing)
With reference to FIG. 3, a delay in the timing of writing the electrostatic latent image on the surface of the photosensitive drum 101 by the light beam will be described. FIG. 3 is an explanatory diagram of the delay times of the BD signal and the video signal. In order to simplify the description, it is assumed that the optical scanning device 200 is assembled with mechanically ideal accuracy. The BD 214 and the buffer IC 301 are provided on the substrate 231 of the optical scanning device 200. The CPU 141 and the buffer IC 303 are provided on the image control board 232 provided in the main body of the image forming apparatus 100. The substrate 231 of the optical scanning device 200 and the image control substrate 232 of the main body of the image forming apparatus 100 are electrically connected by an assembled wire (wire harness) 302. The light source driving unit 142 and the light source 201 as light source driving means for driving the light source 201 are provided on the light source control board 233 of the optical scanning device 200. The image control board 232 of the main body of the image forming apparatus 100 and the light source control board 233 of the optical scanning apparatus 200 are electrically connected by the assembled wire 304.

  First, when the BD 214 receives the light beam 222K (TA), the BD 214 performs light / charge conversion on the light beam 222K and outputs it as a BD signal. Here, a delay is caused by the light / charge conversion of the BD 214. Next, the BD signal is input to the buffer IC 301. Generally, the rise time or fall time of a BD signal is about several tens of nanoseconds (ns). Here, a delay is caused by the time until the BD signal reaches the H level or L level detection threshold value of the buffer IC 301, so that a delay occurs in the output of the BD signal from the buffer IC 301. Next, the BD signal is delayed until it reaches the image control board 232 provided with the CPU 141 through the assembled wire 302. Further, a delay occurs because the signal passes through the buffer IC 303 of the image control board 232 for waveform shaping of the BD signal. The CPU 141 detects a BD signal from the buffer IC 303 (TB). Therefore, a delay time Td1 occurs from TA when the light beam 222K scans the BD 214 to TB when the CPU 141 detects the BD signal.

  When the CPU 141 detects the BD signal (TB), the CPU 141 determines an output timing (writing timing) of a light source driving signal (hereinafter referred to as a video signal) for driving the light source according to the image data. There is also a slight delay in the internal circuit of the CPU 141. The CPU 141 outputs a video signal based on the BD signal (TC). The video signal output from the CPU 141 is transmitted to the light source driving unit 142 of the light source control board 233 through the assembled wire 304. Depending on the length of the assembled wire 304, a delay for transmission of the video signal occurs. When receiving the video signal, the light source driving unit 142 applies a driving current 305 to the light source 201 in accordance with the video signal. A delay occurs in the application of the drive current 305. Furthermore, a light emission delay occurs until the light source 201 to which the drive current 305 is applied emits the light beam 220. The light beam 220 is deflected by the rotating polygon mirror 205. The light beam 221 deflected by the rotating polygon mirror 205 writes an electrostatic latent image on the surface of the photosensitive drum 101 (TD). Therefore, the delay time Td2 occurs from TC when the CPU 141 outputs the video signal to TD when the electrostatic latent image is written on the surface of the photosensitive drum 101.

  Here, in FIG. 3, the CPU 141 handles the BD signal on the same time axis as the video signal. Registration correction in the main scanning direction is performed by adjusting the output timing (writing timing) of the video signal determined by the CPU 141 based on the BD signal based on the color misregistration amount. In the image formation, the delay including the delay time Td1 and the delay time Td2 from TA when the light beam 222K scans the BD 214 to TD when the light beam 221 writes the electrostatic latent image on the surface of the photosensitive drum 101. Time Td is included. Also in the registration correction in the main scanning direction, the video signal output timing (writing timing) is adjusted based on the data including the delay time Td including the delay time Td1 and the delay time Td2.

(Image position shift due to delay time Td)
Next, the image position shift (color shift) due to the delay time Td will be described with reference to FIG. FIG. 4 is an explanatory diagram of an image position shift due to the delay time Td. FIG. 4 shows an image position shift (hereinafter referred to as color shift) when the image forming speed is changed from the first image forming speed to the second image forming speed. Since the optical scanning device 200 is a counter scanning type as described above, the light beams 221C and 221K (hereinafter referred to as CK light beam) are based on the BD signal of the BD 214 (hereinafter referred to as CK-BD signal). Scan in the main scanning direction X1. Light beams 221Y and 221M (hereinafter referred to as YM light beams) are main-scanned based on a BD signal (hereinafter referred to as YM-BD signal) generated based on the CK-BD signal of BD 214 and a predetermined time. Scan in the main scanning direction X2 opposite to the direction X1.

First, registration correction in the main scanning direction is performed at the first image forming speed, and the first speed writing timing is determined. The images I ₁ Y, I ₁ M, I ₁ C and I ₁ K are formed on the carrier belt 105 according to the data of the image center position at the first image forming speed based on the determined first speed writing timing. Yellow, magenta, cyan and black images are shown. Since the images I ₁ Y, I ₁ M, I ₁ C, and I ₁ K are formed after performing registration correction at the first image forming speed, the images I ₁ Y, I ₁ M, I ₁ C, and I ₁ K are formed to coincide with the image center CL without color misregistration. ing. Thereafter, the image forming speed is changed from the first image forming speed to the second image forming speed. Without correcting the registration in the main scanning direction at the second image forming speed, the second speed writing timing is determined based on the first speed writing timing and the speed ratio between the first image forming speed and the second image forming speed. decide. The images I ₂ Y, I ₂ M, I ₂ C, and I ₂ K are formed on the carrier belt 105 according to the data of the image center position at the second image forming speed based on the determined second speed writing timing. Yellow, magenta, cyan and black images are shown. There is almost no color shift between the image I ₂ Y and the image I ₂ M, and there is almost no color shift between the image I ₂ C and the image I ₂ K. However, the image I ₂ C and the image I ₂ K cause a color shift so as to be separated from the image center CL toward the downstream side in the main scanning direction X1. The image I ₂ Y and the image I ₂ M also cause a color shift so as to move away from the image center CL in the main scanning direction X2. When the color shift amount in the main scanning direction (hereinafter referred to as main scan shift amount) is expressed with reference to the yellow image I ₂ Y, the main scan shift amount Δym2 between the image I ₂ Y and the image I ₂ M is almost equal. Zero. The main scanning deviation amount Δyc2 between the image I ₂ Y and the image I ₂ C is substantially the same as the main scanning deviation amount Δyk2 between the image I ₂ Y and the image I ₂ K. As described above, in the opposed scanning optical scanning device 200, after the image forming speed is changed, the write start timing of the changed speed is determined based on only the speed ratio without performing registration correction in the main scanning direction. When an image is formed, a color shift due to the delay time Td occurs.

  As can be seen from FIG. 4, for example, if the scanning directions of the light beams of a plurality of colors are the same direction with respect to the recording medium, the light beams of all the colors are shifted in the same direction, so that it is difficult to recognize the color shift. However, when one light beam scans the recording medium in a direction opposite to the scanning direction of the other light beam as in the counter scanning optical scanning device 200, the color shift due to the error is shifted in the opposite direction. Therefore, the color shift becomes remarkable.

  Hereinafter, a correction method for preventing the occurrence of color misregistration due to the delay time Td will be described with reference to FIG. FIG. 5 is a timing chart showing the relationship between the delay times Td1, Td2, Td and the central image writing timing BPC, BPC1. FIG. 5A shows a case where a light beam (CK light beam, YM light beam) 221 is moved from the TA to the surface of the photosensitive drum 101 when a light beam (hereinafter referred to as a BD light beam) 222K scans the BD 214. An ideal case where there is no delay time Td until TD when writing an electrostatic latent image is shown. When the BD light beam 222K emitted from the light source 201K scans the BD 214, the BD 214 outputs a CK-BD signal, and the CPU 141 detects the CK-BD signal without delay. The CPU 141 outputs a video signal for the CK light beam after elapse of a predetermined center image writing timing BPC from the CK-BD signal. Receiving the video signal, the light source driving unit 142 emits the CK light beam from the light sources 201C and 201K, and writes the latent image of the central image on the surface of the photosensitive drums 101C and 101K without delay. On the other hand, no BD is provided on the side of the YM light beam emitted from the light sources 201Y and 201M, but a predetermined center is obtained from the YM-BD signal generated based on the CK-BD signal of the BD 214 and a predetermined time. A video signal is output after the image writing timing BPC has elapsed. In FIG. 5, the CK-BD signal and the YM-BD signal are shown in the same waveform for convenience. Similarly, the YM light beam writes the latent image of the central image on the surface of the photosensitive drums 101Y and 101M without delay. Therefore, in an ideal case, the time from TA when the BD light beam 222K scans the BD 214 to TD when the light beam 221 writes the central electrostatic latent image on the surface of the photosensitive drum 101 is a predetermined center image writing time. Timing BPC.

  However, actually, the delay time Td described above occurs. FIG. 5B shows a case where a delay time Td occurs from TA when the BD light beam 222K scans the BD 214 to TD when the light beam 221 writes the central electrostatic latent image on the surface of the photosensitive drum 101. Show. A delay time Td1 occurs from TA when the BD light beam 222K scans the BD 214 to when the CPU 141 detects the CK-BD signal. The CPU 141 outputs video signals for the light beams 220Y and 220M after elapse of a predetermined center image writing timing BPC from the CK-BD signal. However, a delay time Td2 occurs between the time when the CPU 141 outputs the video signal and the time TD when the electrostatic latent image is written on the surface of the photosensitive drum 101. Therefore, the delay time Td including the delay times Td1 and Td2 is between TA when the BD light beam 222K scans the BD 214 and TD when the light beam 221 writes the central electrostatic latent image on the surface of the photosensitive drum 101. Occur. The writing position of the central electrostatic latent image is shifted by the delay time Td.

  Therefore, as shown in FIG. 5C, the writing position of the center electrostatic latent image is corrected by subtracting the delay time Td from the predetermined center image writing timing BPC. The central image writing timing BPC1 is calculated by subtracting the delay time Td from the predetermined central image writing timing BPC. By outputting a video signal after the elapse of the central image writing timing BPC1 from when the CPU 141 detects the CK-BD signal, the central electrostatic latent image can be written in the image center CL.

  Next, color misregistration on the sheet S due to the opposite of the main scanning direction X1 of the CK light beam and the main scanning direction X2 of the YM light beam in the counter scanning type optical scanning device 200 will be described. FIG. 6 is a timing chart in the main scanning direction X of the light beam 220 with reference to the sheet. The CK-BD signal is a signal output from the BD 214 when the BD light beam 222K scans the BD 214. The YM-BD signal is a BD signal for a YM light beam generated based on the CK-BD signal and a predetermined time. When the delay time Td1 occurs in the detection of the CK-BD signal by the CPU 141, the same delay time Td1 occurs in the generation of the YM-BD signal. The CK light beam is emitted from the light source 201 in accordance with a video signal output based on the CK-BD signal. The CK light beam is scanned in the main scanning direction X1 from the left end of the sheet toward the right end of the sheet. On the other hand, the YM light beam is output from the light source 201 according to the video signal output based on the YM-BD signal. The YM light beam is scanned from the right end of the sheet to the left end of the sheet in the main scanning direction X2 opposite to the main scanning direction X1 of the CK light beam.

  FIG. 6A shows the BD signal, video signal, and image position on the sheet S in an ideal case without the delay time Td. The image IC and IK are formed on the image center CL on the sheet S by outputting the video signal of the center image of the CK light beam after the elapse of a predetermined center image writing timing BPC from the CK-BD signal. Similarly, the video signal of the central image of the YM light beam is output after the elapse of a predetermined central image writing timing BPC from the YM-BD signal, whereby the images IY and IM are formed on the image center CL on the sheet S. . As described above, the image positions of the four-color images IY, IM, IC, and IK coincide with each other at the image center CL.

On the other hand, FIG. 6B shows the BD signal, video signal, and image position on the sheet S when the delay time Td occurs. Each of the CK light beams is scanned in the same main scanning direction X1, and the delay time Td from the image center CL until the images I ₁ C and I ₁ K are formed is also substantially the same. Accordingly, the positions of the images I ₁ C and I ₁ K formed on the sheet S in the main scanning direction X1 are substantially the same. Also, each of the YM light beams is scanned in the same main scanning direction X2, and the delay time Td from the image center CL until the images I ₁ Y and I ₁ M are formed is also substantially the same. Accordingly, the positions of the images I ₁ Y and I ₁ M formed on the sheet S in the main scanning direction X2 are substantially the same. However, since the YM light beam and the CK light beam are scanned in different main scanning directions X1 and X2, respectively, the images I ₁ Y and I ₁ M are images I ₁ C and I ₁ K with respect to the image center CL. Shifts in the opposite direction. The main scanning deviation amounts Δyc1 and Δyk1 between the images I ₁ Y and I ₁ M and the images I ₁ C and I ₁ K are twice the main scanning deviation amounts from the image center CL, and even with a slight signal delay time. Easily recognized as a color shift.

Therefore, in order to prevent color misregistration due to the delay time Td, the positions of the images I ₁ Y, I ₁ M, I ₁ C and I ₁ K formed on the sheet by correcting the predetermined center image writing timing BPC are determined. Match with the image center CL. As shown in FIG. 6C, the delay time Td is subtracted from the predetermined center image writing timing BPC to obtain the center image writing timing BPC1. By outputting a video signal based on the center image writing timing BPC1, the positions of the four-color images I ₁ Y, I ₁ M, I ₁ C, and I ₁ K can be matched with the image center CL.

  Next, the correction amount ΔBD when the image forming speed is changed from the first image forming speed to the second image forming speed will be described. FIG. 7 is a timing chart showing the relationship between the correction of the central image writing timing BPC based on the speed ratio and the correction amount ΔBD. Here, in order to facilitate understanding, correction of a predetermined center image writing timing BPC for writing a center image in the main scanning direction will be described. However, the correction of the predetermined center image writing timing BPC is the same as the correction of the first image writing timing BPS for making the image writing position in the main scanning direction constant. When the writing timing BPS1 is corrected by the registration correction at the first image forming speed and then changed to the second image forming speed, the corrected writing timing BPS1 is multiplied by the speed ratio to start writing at the second image forming speed. Timing BPS2 is calculated. Similarly, when the image forming speed is changed from the first image forming speed to the second image forming speed, the following description will be made assuming that the predetermined center image writing timing BPC is corrected based on the speed ratio. In addition, the numerical value used for the following description is an illustration, Comprising: This Embodiment is not limited to the following numerical values.

  FIG. 7 shows the BD signal, video signal, and center image writing timing when the image forming speed is changed from the first image forming speed V1 to the second image forming speed V2. Here, the second image forming speed V2 is lower than the first image forming speed and is set to one half of the first image forming speed V1 (V2 = V1 / 2). FIG. 7A is a timing chart at the first image forming speed V1. At the first image forming speed V1, a predetermined (ideal) center image writing timing BPC from an ideal BD signal 41 to an ideal video signal 51 that does not cause the delay time Td is set to 100 μs. The delay time Td1 from the ideal BD signal 41 to the actual BD signal 42 detected by the CPU 141 is 1 μs. The delay time Td2 from the actual video signal 52 until the image is formed at the image center CL is 1 μs. The delay time Td (= Td1 + Td2) is 2 μs (= 1 μs + 1 μS) from the delay time Td1 (= 1 μs) of the BD signal and the delay time Td2 (= 1 μs) of the video signal. By performing registration correction so that an image according to the data at the center of the image is formed at the image center CL on the carrier belt 105 at the first image forming speed V1, the corrected center image writing timing BPC1 is 98 μs. become. The corrected center image writing timing BPC1 is obtained by subtracting the delay time Td (= 2 μs) from the predetermined center image writing timing BPC (= 100 μs) (BPC1 = BPC−Td).

  FIG. 7B is a timing chart at the second image forming speed V2. The speed ratio (V1 / V2) between the first image forming speed V1 and the second image forming speed V2 is 2. The predetermined (ideal) center image writing timing BPC from the ideal BD signal 41 to the ideal video signal 51 that does not cause the delay time Td at the second image forming speed V2 is 200 μs (= 100 μs × 2). Become. This is obtained by multiplying a predetermined (ideal) central image writing timing BPC (= 100 μs × 2) at the first image forming speed V1 by a speed ratio (V1 / V2 = 2). Here, the central image writing timing BPC1 (= 98 μs) at the first image forming speed V1 is also 196 μs when doubled based on the speed ratio (V1 / V2 = 2). However, the central image writing timing BPC2 for forming an image at the image center CL at the second image forming speed V2 is 198 μs obtained by subtracting the delay time Td (= 2 μs) from the predetermined central image writing timing BPC (= 200 μs). is there. That is, the central image writing timing BPC2 is represented by BPC2 = BPC−Td. Therefore, 196 μs obtained by multiplying the central image writing timing BPC1 of the first image forming speed V1 by the speed ratio is 2 μs less than 198 μs of the central image writing timing BPC2 obtained by subtracting the delay time Td from the predetermined central image writing timing BPC. . Therefore, color misregistration due to the shortage of 2 μs occurs. That is, when the image forming speed is changed, a color misregistration occurs simply by multiplying the central image writing timing by the speed ratio. Therefore, it is necessary to obtain a correction amount ΔBD for correcting this 2 μs.

(Calculation method of correction amount ΔBD)
Hereinafter, a method for calculating the correction amount ΔBD will be described. The calculation of the correction amount ΔBD is executed in the main scanning registration image formation speed correction mode. After the image forming apparatus 100 is assembled, the image forming apparatus 100 is operated in the main scanning registration image forming speed correction mode in the factory. The image forming apparatus 100 may operate in the main scanning registration image forming speed correction mode every time a predetermined number of images are formed. Note that the image forming apparatus 100 can operate in the registration correction mode before the start of the image forming operation. The registration correction mode will be described later.

(Main scanning registration image formation speed correction mode)
Hereinafter, the main scanning registration image formation speed correction mode will be described with reference to FIG. 8, FIG. 9, FIG. 10, and FIG. FIG. 8 is a flowchart of the control operation in the main scanning registration image formation speed correction mode. FIG. 9 is a block diagram of the control system 300. FIG. 10 is a diagram showing registration correction patterns 501 (501Y, 501M, 501C, 501K). FIG. 11 is a diagram showing a registration correction pattern 501 and a pattern sensor 500 formed on the carrier belt 105. The CPU 141 executes a control operation in the main scanning registration image forming speed correction mode according to a program stored in the storage unit 140 as a storage unit. The storage unit 140 may include a ROM that stores a program and a RAM that stores detected data and calculated data. The image forming apparatus 100 starts operating in the main scanning registration image forming speed correction mode by an operation unit (not shown) or by the CPU 141.

  When the control operation in the main scanning registration image forming speed correction mode is started, the CPU 141 starts the image forming operation at the preset first image forming speed V1 (S401). The CPU 141 controls a motor 206 that rotates the rotary polygon mirror 205 so that the light beam scans the photosensitive drum 101 at a scanning speed SV1 corresponding to the first image forming speed V1. The CPU 141 forms an electrostatic latent image of the registration correction pattern 501 shown in FIG. 10 on the surface of the photosensitive drum 101 with a light beam. The electrostatic latent image is developed into a toner image with toner by the developing device 103. The toner image is transferred onto the carrier belt 105 by the primary transfer device 104, and a registration correction pattern 501 is formed on the carrier belt 105 (S402). A plurality of yellow, magenta, cyan, and black registration correction patterns 501Y, 501M, 501C, and 501K are formed in the rotation direction (conveying direction) R2 of the carrier belt 105 as the sub-scanning direction as shown in FIG. . Further, the registration correction pattern 501 is arranged in front, center and back of the carrier belt 105 as shown in FIG. A pattern sensor 500 that detects the registration correction pattern 501 is arranged in front, center, and back of the carrier belt 105.

  The registration correction pattern 501 is formed of yellow, magenta, cyan, and black toners, and is formed on the carrier belt 105 in order to detect misregistration amounts of other colors with respect to a predetermined reference color. Pattern. The CPU 141 detects the registration correction pattern 501 by the pattern sensor 500 (S403). The CPU 141 serving as the deviation amount detection means calculates the main scanning deviation amount of each color from the detection timing of the registration correction pattern 501 and stores it in the storage unit 140 (S404). In the present embodiment, yellow Y is used as the reference color. At the first image forming speed V1, the main scanning deviation amounts (distances) of magenta M, cyan C, and black K with respect to yellow Y are Δym1, Δyc1, and Δyk1. In the case of the four-color opposed scanning optical scanning device 200 of this embodiment shown in FIG. 2, Δym1 is approximately 0, and Δyc1 and Δyk1 are approximately the same value.

  Next, the CPU 141 changes the image forming speed from the first image forming speed V1 to the second image forming speed V2. The CPU 141 starts an image forming operation at the second image forming speed V2 (S406). The CPU 141 controls the motor 206 that rotates the rotary polygon mirror 205 so that the light beam scans the photosensitive drum 101 at the scanning speed SV2 corresponding to the second image forming speed V2. The scanning speed SV of the light beam that scans on the photosensitive drum 101 is proportional to the image forming speed V. Accordingly, the ratio (SV2 / SV1) of the scanning speed SV2 corresponding to the second image forming speed V2 to the scanning speed SV1 corresponding to the first image forming speed V1 is equal to the second image forming speed V2 relative to the first image forming speed V1. It is a speed ratio (V2 / V1). Further, the rotational speed of the photosensitive drum 101 and the conveying speed of the carrier belt 105 are also proportional to the image forming speed V, similarly to the scanning speed SV. Then, the CPU 141 forms a registration correction pattern 501 on the carrier belt 105 in the same manner as described above (S407). The pattern shape of the registration correction pattern 501 at the second image forming speed V2 may be the same as that at the first image forming speed V1.

  The CPU 141 detects the registration correction pattern 501 by the pattern sensor 500 (S408). The CPU 141 calculates the main scanning deviation amount of each color from the detection timing of the registration correction pattern 501 and stores it in the storage unit 140 (S409). At the second image forming speed V2, the main scanning deviation amounts (distances) of magenta M, cyan C, and black K with respect to yellow Y are Δym2, Δyc2, and Δyk2. Δym2 is substantially 0, and Δyc2 and Δyk2 have substantially the same value.

  The CPU 141 calculates the correction amount ΔBD from the main scanning deviation amounts Δym1, Δyc1, Δyk1 of the first image forming speed V1 and the main scanning deviation amounts Δym2, Δyc2, Δyk2 of the second image forming speed V2, and stores them in the storage unit 140. Save (S410). The correction amount ΔBD is calculated based on each half value of the main scanning deviation amounts Δym1, Δyc1, Δyk1, Δym2, Δyc2, and Δyk2. The correction amount ΔBD for colors other than yellow Y as the reference color is shown below.

The correction amount ΔBD12 when the image is formed at the second image forming speed V2 is expressed as follows. The second scanning speed SV2 is a scanning speed at which the light beam scans on the photosensitive drum 101 at the second image forming speed V2.

  This main scanning registration image formation speed correction mode may be executed once after factory assembly and the correction amount ΔBD may be stored in the storage unit 140 in advance, or may be executed every time a predetermined number of images are formed, and the correction amount ΔBD is set. You may preserve | save in the memory | storage part 140. FIG. Note that the image forming speed V in the present embodiment is not limited to two speeds. The image forming speed V may have three or more plural speeds. When there are three or more speeds, the main scanning deviation amount is calculated at each speed and stored in the storage unit 140. Then, the correction amount ΔBD for image formation at each speed may be calculated and stored in the storage unit 140.

(Registration correction mode)
Next, the registration correction mode will be described. The image forming apparatus 100 can operate in the registration correction mode before the start of the image forming operation based on a predetermined condition. The predetermined conditions are an elapsed time since the previous execution of the registration correction mode, the number of image formations, and the like. The CPU 141 executes a control operation in the registration correction mode in accordance with a program stored in the storage unit 140. The registration correction mode is executed to correct color misregistration in the main scanning direction and the sub-scanning direction.

  When the registration correction mode is started before the image forming operation is started, the CPU 141 forms a registration correction pattern 501 as shown in FIG. 10 on the carrier belt 105. As shown in FIG. 11, the CPU 141 detects the position of the registration correction pattern 501 using the pattern sensor 500. The CPU 141 determines a registration correction value based on the detection result of the pattern sensor 500. As a registration correction value, the CPU (writing timing determination means) 141 determines the first image writing timing BPS for making the image writing position in the main scanning direction constant for each color. The registration correction mode can be executed at the first image forming speed V1 and the second image forming speed V2, but in the present embodiment, the registration correction mode is executed at the first image forming speed V1 before the start of the image forming operation. The That is, before starting the image forming operation, the CPU 141 determines the writing start timing BPS1 at the first image forming speed V1. The CPU 141 stores the determined writing timing BPS1 in the storage unit 140 as a registration correction value, and stores the first image forming speed V1 in the storage unit 140 as the image forming speed when the writing timing BPS1 is determined. In this embodiment, the image forming apparatus 100 operates in the registration correction mode before the start of the image forming operation. However, the image forming apparatus 100 operates in the registration correction mode every time a predetermined number of images are formed. May be.

(Control operation for image formation)
Next, an image forming control operation will be described. FIG. 12 is a flowchart of the image forming control operation executed by the CPU 141. In the present embodiment, the image forming apparatus 100 can operate at the first image forming speed V1 and the second image forming speed V2. The image forming apparatus 100 operates in the registration correction mode at the first image forming speed V1 before the start of the image forming operation. The CPU 141 executes an image formation control operation in accordance with a program stored in the storage unit 140. When the image forming control operation is started, the CPU 141 determines whether or not the currently set image forming speed is the first image forming speed V1 (S901). If the currently set image forming speed is the first image forming speed V1 (YES in S901), the CPU 141 forms an image based on the writing start timing BPS1 determined in the previous registration correction mode (S902). . On the other hand, when the currently set image forming speed is not the first image forming speed V1 (NO in S901), the CPU 141 calculates the writing timing BPS2 in the main scanning direction of the second image forming speed V2 as follows ( S903). The image is formed at the writing timing BPS1 determined in the registration correction mode executed at the first image forming speed V1, the speed ratio SV1 / SV2 of the first scanning speed SV1 with respect to the second scanning speed SV2, and the second image forming speed V2. In this case, the correction amount ΔBD12 is used. The writing timing BDS2y, BDS2m, BDS2c and BDS2k in the main scanning direction for each color is expressed as follows.

  The CPU 141 forms an image at the second image formation speed V2 using the calculated writing timing BPS2 (S904). Since the writing timing BDS2 is corrected by the correction amount ΔBD12, the color misregistration amount is reduced. The CPU 141 ends the image formation control operation.

  According to the present embodiment, at the first image forming speed V1, image formation is performed at the writing start timing BDS1 determined in the previous registration correction mode. When the image forming speed is switched from the first image forming speed V1 to the second image forming speed V2, image formation is performed at the writing start timing BDS2 calculated using the correction amount ΔBD12 without executing the registration correction mode. Therefore, according to the present embodiment, it is possible to prevent image shift when the image forming speed is switched while reducing the stop time (down time) of image formation.

  In the counter scanning optical scanning device 200, the yellow light beam 221Y and the magenta light beam 221M scan the recording medium in the direction opposite to the scanning direction of the black light beam 221K and the cyan light beam 221C. Therefore, if color misregistration due to the delay time Td occurs, the color misregistration becomes significant because the color misregistration directions are opposite to each other. For this reason, in the case of the conventional counter scanning type optical scanning device, the registration correction mode is executed for each different image forming speed, so that productivity is lowered. However, according to the present embodiment, since the writing timing BDSn of the next sheet can be determined without registration correction based on the correction amount ΔBDpn, it is particularly effective in the counter scanning type optical scanning device 200. The deviation can be corrected.

  In the case of the opposed scanning optical scanning device 200 according to the present embodiment, as shown in FIG. 2, the yellow light beam 221Y and the magenta light beam 221M face the black light beam 221K and the cyan light beam 221C. In the main scanning registration image formation speed mode, the correction amount ΔBDym is substantially 0 (zero) when the yellow light beam 221Y is used as a reference. The correction amount ΔBDyc and the correction amount ΔBDyk are substantially the same. Therefore, it is only necessary to calculate the correction amount ΔBDyc or ΔBDyk. Further, when the magenta light beam 221M is used as a reference, it is sufficient that the correction amount ΔBDmc or ΔBDmk can be calculated. Therefore, the registration correction pattern may be only two colors in the main scanning registration image formation speed correction mode. FIG. 13 is a diagram showing registration correction patterns 501Y or 501M and 501C or 501K for only two colors. As shown in FIG. 13, a yellow or magenta registration correction pattern 501Y or 501M and a cyan or black registration correction pattern 501C or 501K are formed. By calculating any one of ΔBDyc, ΔBDyk, ΔBDmc, or ΔBDmk from the two-color registration correction patterns, the same correction as described above can be performed.

  The same applies to an image forming apparatus using a counter scanning type optical scanning device in which the yellow light beam 221Y and the cyan light beam 221C oppose the magenta light beam 221M and the black light beam 221K. A yellow or cyan registration correction pattern 501Y or 501C and a magenta or black registration correction pattern 501M or 501K are formed. By calculating any one of ΔBDym, ΔBDyk, ΔBDcm, or ΔBDck from the two-color registration correction patterns, the same correction as described above can be performed.

  In the above embodiment, for each image forming apparatus 100, the image forming apparatus 100 is operated in the main scanning registration image forming speed correction mode, the main scanning deviation amount is actually measured, and based on the measured main scanning deviation amount. The correction amount ΔBD was calculated. This is because the correction amount ΔBD is generally different for each image forming apparatus 100 due to variations in components such as the buffer ICs 301 and 303 and the BD 214. However, when the signal transmission path is relatively short and simple, the signal transmission delay variation is small, or the inexpensive image forming apparatus that can tolerate the signal transmission delay variation, the correction amount ΔBD for each image forming apparatus 100 The design correction amount ΔBD may be used without measuring the. The correction amount ΔBD (ΔBDy, ΔBDm, ΔBDc, ΔBDk), which is a nominal value in design, may be stored in the storage unit 140 of the image forming apparatus 100 as a fixed value (standard value).

  In the present embodiment, the image forming apparatus 100 can operate at the first image forming speed V1 and the second image forming speed V2. The image forming apparatus 100 operates in the registration correction mode at the first image forming speed V1 before starting the image forming operation, and determines the writing start timing BPS1 of the first image forming speed V1. When the image forming apparatus 100 forms an image at the second image forming speed V2, the writing timing BPS2 in the main scanning direction is calculated as follows. Correction amounts ΔBD12y, ΔBD12m, ΔBD12c, and ΔBD12k are used as nominal values of yellow, magenta, cyan, and black when an image is formed at the second image forming speed V2. The writing timing BPS1 determined in the registration correction mode executed at the first image forming speed V1 and the speed ratio SV1 / SV2 of the first scanning speed SV1 with respect to the second scanning speed SV2 are used. The writing timing BDS2y, BDS2m, BDS2c and BDS2k in the main scanning direction for each color is expressed as follows.

  In accordance with the flowchart shown in FIG. 12, the writing timing BDS2 of the second image forming speed V2 is calculated using the correction amount ΔBD12 which is the nominal value and the writing timing BDS1 determined in the registration correction mode, and image formation is performed. Therefore, even if the image forming speed of the image forming apparatus 100 is switched, the color misregistration can be reduced by correcting the writing start timing according to this embodiment.

  In this embodiment, a light source that emits a plurality of light beams is used. However, the present invention can also be applied to an image forming apparatus having a light source that emits a single light beam. In this case, not the color misregistration but the image position with respect to the sheet S can be corrected.

DESCRIPTION OF SYMBOLS 100 ... Image forming apparatus 101 ... Photosensitive drum (photoconductor)
104 ... Primary transfer device (transfer means)
105 ... carrier belt 140 ... storage unit (storage means)
141... CPU (writing timing determination means)
201... Light source 205... Rotating polygon mirror (deflection means)
500... Pattern sensor (pattern detection means)

Claims (9)

An image forming apparatus operable at a plurality of image forming speeds,
A photoreceptor,
A light source that emits a light beam;
Deflecting means for deflecting the light beam so that the light beam emitted from the light source scans the surface of the photoreceptor in the main scanning direction;
Transfer means for developing an electrostatic latent image on the surface of the photoconductor formed by the light beam with toner and transferring the developed toner image to a carrier or a recording medium conveyed by the carrier;
Pattern detecting means for detecting a registration correction pattern transferred to the carrier by the transfer means when the image forming apparatus operates at a preset first image forming speed;
Based on the detection result of the pattern detection means, the electrostatic latent image is written onto the photosensitive member by the light beam emitted from the light source when the image forming apparatus operates at the first image forming speed. Writing timing determining means for determining timing;
Storage means for preliminarily storing a correction amount for correcting the writing timing when the image forming apparatus operates at a second image forming speed different from the first image forming speed;
With
When the image forming apparatus operates at the first image forming speed, an image is formed based on the writing timing determined by the writing timing determining unit,
When the image forming apparatus operates at the second image forming speed, the writing timing determined by the writing timing determining means is set to the correction amount stored in the storage means and the second image forming speed. An image forming apparatus that performs correction based on a speed ratio of a first image forming speed and forms an image based on the corrected writing start timing. A deviation amount detection means for detecting a deviation amount of the registration correction pattern in the main scanning direction based on a detection result of the pattern detection means;
The image forming apparatus according to claim 1, wherein the correction amount is calculated based on the shift amount detected by the shift amount detection unit. The photoconductor includes a first photoconductor and a second photoconductor,
The light source emits a first light beam and a second light beam,
In the first main scanning direction in which the first light beam emitted from the light source scans the first photoconductor, the second light beam emitted from the light source passes through the second photoconductor. The image forming apparatus according to claim 1, wherein the image forming apparatus is opposite to a second main scanning direction for scanning.   The correction amount is a first registration correction pattern in which the electrostatic latent image formed on the first photosensitive member by the first light beam is developed with toner and transferred to the carrier by the transfer unit. And an electrostatic latent image formed on the second photosensitive member by the second light beam, developed with toner, and transferred to the carrier by the transfer means. The image forming apparatus according to claim 3, wherein the image forming apparatus is calculated based on a shift amount in the main scanning direction.   The image forming apparatus according to claim 4, wherein the correction amount is calculated based on a half value of the shift amount in the main scanning direction.   The correction amount is a first shift in the main scanning direction between the first registration correction pattern and the second registration correction pattern transferred to the carrier at the first image forming speed. And a second shift amount in the main scanning direction between the first registration correction pattern and the second registration correction pattern transferred to the carrier at the second image forming speed. The image forming apparatus according to claim 4, wherein the image forming apparatus is calculated based on   The image forming apparatus according to claim 6, wherein the correction amount is stored in advance in the storage unit at a factory.   The pattern detection means includes the first registration correction pattern and the second registration correction pattern transferred to the carrier at the first image forming speed before the image forming apparatus forms an image. The image forming apparatus according to claim 4, further comprising:   The image forming apparatus according to claim 1, wherein the correction amount is a design nominal value.

Images