JP2010008778A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To avoid the lengthening of a printing time caused by the temporary interruption of continuous printing operation caused by the correction of optical writing start timing while avoiding the worsening of color misregistration caused by the correction. <P>SOLUTION: After pattern images for the detection of misregistration of colors which are different from one another are formed on photoreceptors 10Y, M, C and K for respective colors and transferred to an intermediate transfer belt 20 on the basis of the timing detected by an optical sensor unit 136, amounts of position misregistration are calculated for pattern images for the detection of the misregistration of respective colors on the belt surface and an arithmetic expression for calculating a logical misregistration amount is corrected based on comparison between the calculation result and the logical misregistration amount. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention obtains other color images by superimposing and transferring visible images of different colors formed on a plurality of latent image carriers on the surface of the endless moving body or a recording member held on the same surface. The present invention relates to an image forming apparatus.

  As this type of image forming apparatus, toner images of different colors are formed on a plurality of photosensitive members as latent image carriers, and these are superimposed on a recording paper or an intermediate transfer member to obtain a color image. It has been known. In such an image forming apparatus, if the transfer positions of the toner images of the respective colors are relatively shifted, color misregistration occurs in the color image. Such color misregistration is caused by sub-scanning registration misalignment or skew misregistration (hereinafter collectively referred to as positional misalignment) of each color toner image. The sub-scanning registration error is a phenomenon in which the normal transfer position of the toner image is totally displaced in the sub-scanning direction, which is the moving direction of the recording paper or the intermediate transfer member. Further, skew is a phenomenon in which the direction of the optical scanning line in the main scanning direction with respect to the photosensitive member is deviated from the axial direction of the photosensitive member.

  The main cause of these positional shifts is that the latent image writing system components such as the reflection mirror and the lens expand and contract with the temperature change. During a continuous printing operation in which images are continuously formed on a plurality of recording papers, the latent image writing apparatus continues to increase in temperature, so that the amount of color misregistration increases as the continuous operation time increases.

  As an image forming apparatus that reduces such color misregistration, the one described in Patent Document 1 is known. This image forming apparatus performs the following misalignment correction process every time a predetermined number of prints are performed. That is, after the toner images formed on the respective color photoconductors are transferred side by side on the belt, the positional deviation amount of each color toner image is detected based on the timing when the toner images are detected by the optical sensor as the image detecting means. Then, by correcting the latent image writing start timing based on the detection result, the sub-scanning registration shift amount can be reduced. However, during the continuous printing operation, as shown in FIG. 1, every time a predetermined number of prints are performed, the continuous printing operation is temporarily interrupted and the above-described misalignment correction processing is performed. It takes a long time to complete.

  On the other hand, Patent Document 2 describes the following image forming apparatus. That is, this image forming apparatus stores in advance in the data storage means a calculation formula for determining the amount of positional deviation from the normal position based on the continuous printing execution time for each color toner image. During the continuous printing operation, the positional deviation amount of each color toner image is calculated based on the calculation formula without interrupting the continuous printing operation, and the latent image writing start timing of each color is calculated based on the calculation result. Correction is made to reduce the amount of sub-scanning registration deviation. According to such a configuration, it is possible to avoid an increase in printing time due to temporarily interrupting the continuous printing operation for the purpose of performing the deviation correction processing.

JP 2003-149905 A JP 2007-182018 A

  However, the present inventors have found through experiments that the image forming apparatus described in Patent Document 2 may deteriorate the color misregistration by correcting the latent image writing start timing based on the calculation formula. I found it. Specifically, the present inventors conducted an experiment to examine the relationship between the continuous print execution time and the amount of positional deviation of the toner image, using a plurality of printer testers of the same model. Then, the relational expression “the increase amount of the positional deviation of the toner image = coefficient B (1−exp (−coefficient A × continuous printing execution time))” may be established between the continuous printing execution time and the positional deviation amount. all right. Regardless of which printer testing machine is used, the “coefficient A” in this relational expression has the same value. That is, it was found that the “coefficient A” is the same value with no individual difference between printers of the same model. On the other hand, it was found that the “coefficient B” shows a relatively large individual difference even between printers of the same model. In spite of such individual differences, if the “coefficient B” common to all individuals is adopted, the error between the positional deviation amount based on the calculation formula and the actual positional deviation amount becomes too large depending on the individual. Therefore, there is a possibility that the color misregistration will be worsened by the correction.

  The present invention has been made in view of the above background, and an object thereof is to provide the following image forming apparatus. That is, a continuous printing operation is performed for the purpose of performing the write condition correction process for correcting the latent image writing conditions such as the latent image writing start timing while avoiding the deterioration of the color misregistration. This is an image forming apparatus that can avoid the lengthening of the printing time due to the temporary interruption.

In order to achieve the above object, the invention of claim 1 is characterized in that a plurality of latent image carriers that carry a latent image on a surface that moves endlessly, and a plurality of images that develop the latent images on these latent image carriers in different colors. An image forming means having a latent image writing means for writing a latent image on the developing means and the latent image carriers, and an endless moving member formed on the surface of each latent image carrier while moving the surface of the endless mover endlessly. A transfer means for transferring a visual image superimposed on a surface of the endless moving body or a recording member held on the surface; and an image forming control means for controlling the image forming means, the image forming control means However, for each color visible image, a theoretical misregistration amount, which is a theoretical positional deviation amount on the surface of the endless moving body, is calculated based on a predetermined arithmetic expression, and the visible image of each color is calculated based on the calculation result. The amount of overlay displacement of In an image forming apparatus that performs a write condition correction process that corrects the latent image writing condition of the latent image writing means and reduces the amount of misalignment, the plurality of latent image carriers have different colors. After forming a visible image for displacement detection and transferring it to the surface of the endless moving body, the visible image for displacement detection of each color on the surface of the endless moving body is based on the timing detected by the image detection means. As the amount of positional deviation from the normal position is calculated as the actual amount of deviation, and the arithmetic expression correction process for correcting the arithmetic expression based on the comparison between the actual deviation amount and the theoretical deviation amount is performed at a predetermined timing. The image forming control means is configured.
According to a second aspect of the present invention, in the image forming apparatus of the first aspect, as the arithmetic expression, for the visible image of each color, a first amount for obtaining the increase amount of the theoretical deviation amount with the increase of the image forming operation time. And the calculation result of the increase amount according to the first equation in the writing condition correction process, using the first equation and the second equation for obtaining the decrease amount of the theoretical deviation amount accompanying the increase in the image formation stop time. Based on the calculation result of the reduction amount according to the second equation and the positional deviation correction amount by the correction of the latent image writing condition, the image formation is performed so as to grasp the overlay deviation amount of the visible images of the respective colors. The control means is configured.
According to a third aspect of the present invention, in the image forming apparatus of the second aspect, as the first expression, an expression “increase amount = coefficient B (1−exp (−coefficient A × image forming operation time))” is used. In addition, as the second equation, the image formation control means is configured to use an equation of “decrease amount = coefficient B (exp (−coefficient A × operation stop time))”. .
According to a fourth aspect of the present invention, in the image forming apparatus according to the third aspect of the present invention, there is provided mark detecting means for detecting a reference mark attached to a predetermined position in the endless moving direction of the surface of the endless moving body and correcting the arithmetic expression. In the processing, for each color shift detection visible image, the actual shift amount based on the detection timing of the shift detection visible image by the image detection unit and the detection timing of the reference mark by the mark detection unit And the image forming control means is configured to correct the coefficient B based on a comparison between the calculation result and the theoretical deviation amount.
According to a fifth aspect of the present invention, in the image forming apparatus according to the third aspect, in the arithmetic expression correction process, the latent image writing means writes a latent image for each color shift detection visible image. The actual shift amount is calculated based on a comparison between a time from when the image detection unit detects a shift detection visible image and a predetermined reference time, and the calculation result and the theoretical shift amount The image forming control means is configured so as to correct the coefficient B based on the comparison.
According to a sixth aspect of the present invention, in the image forming apparatus according to any one of the third to fifth aspects, the write condition correction process is performed as the first write condition correction process. The amount of deviation from the normal position of the deviation detection visible image is calculated based on the timing when the image detection means detects the deviation detection visible image, and the latent image writing condition is determined based on the calculation result. The write condition second correction process to be corrected is also performed, and after the write condition first correction process and the write condition second correction process are properly used according to the situation and the second correction process is performed, The image forming control means is configured to perform processing for resetting the image forming operation time used in the first expression and the operation stop time used in the second expression to zero, respectively. is there.
According to a seventh aspect of the present invention, in the image forming apparatus of the sixth aspect, a command for a continuous print operation for continuously forming images on a plurality of recording members is given, and the number of output sheets in the continuous print operation The image forming control means is configured to execute the continuous printing operation after performing the second writing condition correction process and then perform the arithmetic expression correction process after the second write condition correction process is performed It is characterized by that.

In these inventions, in the writing condition correction process, by calculating the positional deviation amount of the visible image of each color using an arithmetic expression, without forming a visible image for detecting the positional deviation of each color, Know the amount of misalignment of each color. As described above, by calculating the shift amount based on the arithmetic expression, the shift amount of each color is grasped without temporarily interrupting the continuous print operation even during the continuous print operation. Then, if the latent image writing conditions are appropriately corrected based on the grasped shift amounts of the respective colors, it is possible to perform all of the writing condition correction processing in parallel with the continuous printing operation. Therefore, it is possible to avoid an increase in printing time due to a temporary interruption of the continuous printing operation for the writing condition correction process.
In these inventions, the arithmetic expression correction process is performed at a predetermined timing, for example, when an initial operation is performed under the user. Then, in this arithmetic expression correction process, a visible image for detecting a shift of each color is actually formed on the surface of the endless moving body, and the actual position shift of each color is based on the timing at which they are detected by the image detecting means. Detect the amount. Based on the comparison between the actual positional deviation amount and the positional deviation amount calculated based on the arithmetic expression, the arithmetic expression is corrected to an appropriate one suitable for each individual. Thereby, in each individual, the amount of color misregistration can be reliably reduced by performing the writing condition correction process by appropriately calculating the position deviation amount by the arithmetic expression in the writing condition correction process.

Hereinafter, an embodiment in which the present invention is applied to an electrophotographic printer as an image forming apparatus will be described.
FIG. 2 is a schematic configuration diagram illustrating the printer according to the embodiment. The printer includes a housing 1 and a paper feed cassette 2 that can be pulled out from the housing 1. Process units 3Y, 3C, 3M for forming toner images (visible images) of each color of yellow (Y), cyan (C), magenta (S), and black (K) are formed in the central portion of the casing 1. It has 3K. Hereinafter, the subscripts Y, C, M, and K of the respective symbols indicate members for yellow, cyan, magenta, and black, respectively.

  FIG. 3 is an enlarged configuration diagram showing a process unit for Y. The other color process units have the same configuration. In the figure, a process unit 3Y includes a drum-shaped photoconductor 10Y as a latent image carrier that rotates in the direction of arrow A in the figure. The photoconductor 10Y includes, for example, an aluminum cylindrical substrate having a diameter of 40 [mm] and an OPC (organic photo semiconductor) photosensitive layer covering the surface. Around the photoconductor 10Y, there are a charging device 11Y that uniformly charges the photoconductor 10Y, a developing device 12Y that is a developing unit that develops a latent image formed on the photoconductor 10Y, and a cleaning device 13Y that cleans residual toner on the photoconductor 10Y. Etc. Since the process units of other colors have the same configuration, the description is omitted.

  In FIG. 3 described above, an optical writing unit 4 as a latent image writing means for writing a latent image on the photosensitive member of each color is disposed below the process units 3Y, 3C, 3M, and 3K for each color. Has been. Further, above the process units 3Y, 3C, 3M, and 3K for each color, there is a transfer means for transferring the toner images formed on the photoreceptors 10Y, 10C, 10M, and 10K onto the intermediate transfer belt 20 that is an endless moving body. A transfer unit 5 is provided. Further above the transfer unit 5 are toner bottles 7Y, 7C, 7M, and 7K that contain toners of yellow (Y), cyan (C), magenta (M), and black (K). A fixing unit 6 is disposed.

  The toner bottles 7 </ b> Y, 7 </ b> C, 7 </ b> M, and 7 </ b> K are exposed to the outside when the paper discharge tray 8 formed on the upper surface of the housing 1 is opened, and are removed from the housing 1.

  The optical writing unit 4 irradiates the photoconductors 10Y, 10C, 10M, and 10K while deflecting writing light (laser light) L emitted from a laser diode as a light source in the main scanning direction by a polygon mirror or the like. Then, optical scanning is performed on these photoconductors. Thereby, electrostatic latent images for Y, C, M, and K are formed on the photoreceptors 10Y, 10C, 10M, and 10K. The electrostatic latent images for Y, C, M, and K are developed by developing devices 12Y, 12C, 12M, and 12K to become Y, C, M, and K toner images.

  The transfer unit 5 includes an endless intermediate transfer belt 20 that is a surface endless moving body, a driving roller 21, a tension roller 22, and a driven roller 23 that stretch the belt. The intermediate transfer belt 5 also includes primary transfer rollers 24Y, 24C, 24M, and 24K that are disposed inside the loop, a belt cleaning device 26 that is disposed outside the loop, a secondary transfer roller 25, and the like.

  The intermediate transfer belt 20 is wound around a driving roller 21, a tension roller 22, and a driven roller 23. Then, the lower stretched surface is brought into contact with the photoconductors 10Y, 10C, 10M, and 10K to form a primary transfer nip for Y, C, M, and K, and is driven to rotate by a driving unit (not shown). As the roller 21 rotates, it is moved endlessly in the counterclockwise direction in the figure.

  A primary transfer bias is applied to the primary transfer rollers 24Y, 24C, 24M, and 24K that rotate while contacting the back surface of the intermediate transfer belt 20 on the back side of the primary transfer nip for Y, C, M, and K. Yes. By this application, a primary transfer electric field is formed between the photoconductors 10Y, 10C, 10M, and 10K and the primary transfer rollers 24Y, 24C, 24M, and 24K. The Y, C, M, and K toner images formed on the photoreceptors 10Y, 10C, 10M, and 10K are sequentially superimposed and sequentially transferred onto the intermediate transfer belt 20 at each primary transfer nip. As a result, a four-color superimposed toner image is formed on the intermediate transfer belt 20. The four-color superimposed toner image is secondarily transferred onto the recording paper P at a secondary transfer nip described later, and then sent to the fixing unit 6 to be fixed on the recording paper P as a recording member.

  In the process units 3Y, 3C, 3M, and 3K, the photosensitive members 10Y, 10C, 10M, and 10K that are rotationally driven in the clockwise direction in the drawing by a driving unit (not shown) are uniformly supplied by the charging devices 11Y, 11C, 11M, and 11K. Charged. Then, an electrostatic latent image for Y, C, M, and K is carried by scanning the writing light L based on the image information emitted from the optical writing unit 4. These electrostatic latent images are developed with Y, C, M, and K toners carried on the developing rollers 15Y, 15C, 15M, and 15K of the developing devices 12Y, 12C, 12M, and 12K, and Y, C, M, and A K toner image is formed. Thereafter, the images are sequentially superimposed and transferred onto the intermediate transfer belt 20 at the primary transfer nips for Y, C, M, and K. The image forming operation of each color at this time is shifted in timing from the upstream side in the moving direction of the intermediate transfer belt 20 toward the downstream side so that the toner image is transferred to the same position on the intermediate transfer belt 20. Executed.

  A small amount of untransferred toner that has not been transferred onto the intermediate transfer belt 20 adheres to the photoreceptors 10Y, 10C, 10M, and 10K after passing through the primary transfer nip. This is removed from the surface of the photoreceptor by the cleaning blade 13a of the cleaning devices 13Y, 13C, 13M, and 13K.

  A predetermined amount of toner filled in the toner bottles 7Y, 7C, 7M, and 7K is supplied to the developing devices 12Y, 12C, 12M, and 12K of the process units 3Y, 3C, 3M, and 3K by a conveyance path (not shown) according to necessity. To be replenished.

  On the outer side of the loop of the intermediate transfer belt 20, the belt is positioned at a position relatively close to the primary transfer nip for K on the most downstream side and downstream of the primary transfer nip for K in the belt movement direction. A secondary transfer roller 25 that is in contact with each other to form a secondary transfer nip is disposed. A secondary transfer bias is applied to the secondary transfer roller 25. As a result, a secondary transfer electric field is formed between the grounded drive roller 21 and the secondary transfer roller 25.

  The recording paper P in the paper feed cassette 2 is conveyed into the housing 1 by a paper feed roller 27 disposed in the vicinity of the paper feed cassette 2 and sent out to the secondary transfer nip by a registration roller pair 28 at a predetermined timing. It is. In the secondary transfer nip, the four-color toner image is superimposed on the four-color toner image on the intermediate transfer belt 20, and the four-color toner image is collectively secondary-transferred onto the surface by the influence of the nip pressure and the secondary transfer electric field. As a result, the four-color toner image is combined with the white color of the recording paper P to form a full-color image.

  The recording paper P on which the full-color image has been formed is subjected to image fixing processing by passing through the fixing unit 6, and is discharged from the inside of the apparatus onto the discharge tray 8 outside the apparatus by the discharge roller 29. Similar to the photoconductor 10, the transfer residual toner remaining on the intermediate transfer belt 20 is cleaned by a belt cleaning device 26 that contacts the intermediate transfer belt 20.

  FIG. 4 is an enlarged configuration diagram showing the photoreceptors 10Y, 10C, 10M, and 10K for each color together with the optical writing unit 4. In the figure, the optical writing unit 4 includes a polygon mirror unit in which two polygon mirrors 41a and 41b having a regular polygonal column shape arranged in multiple stages so as to rotate coaxially with each other are integrally arranged in multiple stages. ing. These polygon mirrors 41a and 41b have reflection mirrors on their side surfaces, and are rotated at high speed around the central axis of the regular polygonal column by the polygon motor PM. As a result, when writing light (laser light) from a laser diode (light source) (not shown) is incident on the side surface, the laser light is deflected and scanned. Further, the optical writing unit 4 changes the equiangular motion of the laser scanning into the constant velocity linear motion by the polygon mirror unit having the soundproof glasses 42a and 42b for the soundproof effect of the polygon motor PM and the polygon mirrors 41a and 41b. fθ lenses 43a, 43b, mirrors 44a, 44b, 44c, 44d, 46a, 46b, 46c, 46d, 47a, 47b, 47c, 47d, and polygon mirrors for guiding laser light to the photoreceptors 10Y, 10C, 10M, 10K Long lens units 50a, 50b, 50c, and 50d as members to be adjusted to correct surface tilt of the glass, and dust-proof glasses 48a, 48b, 48c, and 48d that prevent dust and the like from falling into the housing. Reference numerals La, Lb, Lc, and Ld in the figure indicate the optical paths of the writing light applied to the photoconductors 10Y, 10C, 10M, and 10K, respectively.

  The optical writing unit 4 includes an adjusting device that adjusts the bending and inclination of the scanning line. The inclination of the scanning line is adjusted by changing the postures of the long lens units 50a, 50b, 50c, and 50d that are optical elements. A mechanism for adjusting the inclination of the scanning line is provided in the long lens units 50a, 50b, and 50c corresponding to the photoreceptors 10Y, 10C, and 10M of yellow (Y), cyan (C), and magenta (M). However, the long lens unit 50d corresponding to black (K) is not provided. This is because the bending and inclination of the Y, C, and M scanning lines are adjusted based on the bending and inclination of the K scanning line. Hereinafter, the long lens unit 50a corresponding to the yellow (Y) photoreceptor 10Y will be described as an example. However, in the following description, the color code is omitted.

  5 and 6 are perspective views showing the long lens unit 50, respectively. The long lens unit 50 includes a long lens 51 that corrects the surface tilt of the polygon mirrors 41a and 41b, a bracket 52 that holds the long lens 51, a bending adjustment leaf spring 53, a long lens 51, and a bracket. Fixing plate springs 54 and 55 for fixing 52, a driving motor 56 for automatically adjusting the scanning line inclination, a driving motor holder 57, a screw receiving portion 58, a housing fixing member 59, and a unit supporting plate It comprises springs 60, 61, 62, smooth surface members 63, 64 as friction coefficient reducing means, a bending adjustment screw 65, and the like.

  In adjusting the inclination of the scanning line, the rotation angle of the drive motor 56 is controlled based on the skew amount calculated by the positional deviation correction control described later. As a result, the lifting screw attached to the rotating shaft of the drive motor 56 moves up and down, and the motor side end of the long lens unit 50 moves in the direction of the arrow in the figure. Specifically, when the lifting screw is raised, the motor side end of the long lens unit 50 is raised against the urging force of the unit supporting leaf spring 61. Thereby, the long lens unit 50 rotates around the support base 66 and changes its posture. On the other hand, when the lifting screw is lowered, the motor side end of the long lens unit 50 is lowered by the urging force of the unit supporting leaf spring 61. As a result, the long lens unit 50 rotates in the opposite direction to the support base 66 as a fulcrum and changes its posture.

  When the posture of the long lens unit 50 changes in this way, the position where the laser light L enters the incident surface of the long lens 51 changes. When the incident position of the laser light L with respect to the incident surface of the long lens 51 changes in a direction (vertical direction) perpendicular to the longitudinal direction of the long lens 51 and the direction of the optical path, the long lens 51 It has the characteristic that the angle (emitting angle) with respect to the vertical direction of the laser beam emitted from the emitting surface changes. Due to this characteristic, when the posture of the long lens unit 50 is changed by the elevating screw, the emission angle of the laser light emitted from the emission surface of the long lens 51 is changed accordingly. As a result, the photosensitive member by this laser light is changed. The slope of the upper scan line changes.

  FIG. 7 is an enlarged perspective view showing a part of the intermediate transfer belt 20 together with an optical sensor unit 136 as image detecting means. As shown in the drawing, the optical sensor unit 13 is opposed to a portion where the intermediate transfer belt 20 is wound around the tension roller 22 with a predetermined gap therebetween. In this printer, each device is driven by a control unit (not shown) including a CPU (Central Processing Unit) as a calculation means, a RAM (Random Access Memory) as a data storage means, and a ROM (Read Only Memory) as a data storage means. I have control. This control unit is configured to perform an actual misalignment correction process as the second write condition correction process immediately after a power switch (not shown) is turned on or whenever a predetermined number of prints are performed. In the actual misregistration correction process, a misregistration detection image including a plurality of toner images called chevron patches PV is formed on one end and the other end in the width direction of the intermediate transfer belt 20, respectively. The optical sensor unit 136 described above includes a first optical sensor 137 that faces one end of the intermediate transfer belt 20 in the width direction, and a second optical sensor 138 that faces the other end in the width direction. The first optical sensor 137 passes the light emitted from the light emitting means through the condenser lens, reflects the light on the surface of the intermediate transfer belt 20, receives the reflected light by the light receiving means, and determines the voltage according to the amount of light received. Is output. When the toner image in the chevron patch PV formed at one end of the intermediate transfer belt 20 passes directly below the first optical sensor 137, the amount of light received by the light receiving means of the first optical sensor 137 changes greatly. Accordingly, the control unit can detect each toner image in the chevron patch PV formed at one end portion in the width direction of the intermediate transfer belt 20. Similarly, each toner image in the chevron patch PV formed at the other end of the intermediate transfer belt 20 can be detected based on the output from the second optical sensor 137. As described above, the first optical sensor 137 and the second optical sensor 138 function as image detection means in combination with the control unit. As the light emitting means, an LED or the like having an amount of light that can generate reflected light necessary for detecting a toner image is used. As the light receiving means, a CCD in which a large number of light receiving elements are arranged in a straight line is used.

  The control unit detects each toner image in the chevron patch PV formed at both ends of the intermediate transfer belt 20 in the width direction, so that the position in the main scanning direction and the sub-scanning direction (belt moving direction) in each toner image. , A magnification error in the main scanning direction, and a skew from the main scanning direction are detected. The main scanning direction here refers to the direction in which the laser light is phased on the surface of the photosensitive member as it is reflected by the polygon mirror. As shown in FIG. 8, the chevron patch is a posture in which the toner images of Y, C, M, and K are inclined by about 45 [°] from the main scanning direction and at a predetermined pitch in the belt moving direction that is the sub scanning direction. It is a line pattern group arranged. For such Y, C, M toner images in the chevron patch PV, the difference in detection time from the K toner image is read. In this figure, the vertical direction of the paper surface corresponds to the main scanning direction, and after the Y, C, M, and K toner images are arranged in order from the left, the postures are different from those by 90 [°]. , Y toner images are further arranged. Based on the difference between the actual measurement value and the theoretical value for the detection time differences tyk, tck, and tmk with respect to K as the reference color, the shift amount in the sub-scanning direction of each color toner image, that is, the registration shift amount is obtained. Then, on the basis of the registration deviation amount, the optical writing start timing for the photoconductor is corrected every other polygon mirror surface of the optical writing unit (4), that is, one scanning line pitch as one unit, and each color toner. Reduce image registration shift. Further, the inclination (skew) of each color toner image from the main scanning direction is obtained based on the difference in the amount of deviation in the sub-scanning direction between both ends of the belt. Then, based on the result, the surface tilt correction described with reference to FIGS. 5 and 6 is performed to reduce skew of each color toner image. As described above, the processing for correcting the registration error and the skew error by correcting the optical writing start timing and the surface tilt based on the detection timing of each toner image in the chevron patch PV, which is a position error detection image, is performed. This is a shift amount correction process.

  As in the case of this printer, four laser beams for the four photoconductors (1Y, C, M, K) are deflected by a common polygon mirror to perform light scanning in the main scanning direction on each photoconductor. In what is performed, the optical writing start timing for each photoconductor is corrected in units of time corresponding to writing for one line (one scanning line). For example, if a registration shift exceeding 1/2 dot occurs between two photoconductors, the optical writing start timing for either photoconductor is around an integral multiple of the writing time for one line. To be displaced. More specifically, for example, in the case of 3/4 dot overlay deviation, it is 1 time of writing time for one line, and in the case of 7/4 dot overlay deviation, it is twice of writing time for 1 line. Therefore, the optical writing start timing is shifted back and forth with respect to the previous timing. Thereby, the amount of misalignment in the sub-scanning direction is suppressed to ½ dot or less.

  In addition, for registration misregistration, in place of the process of correcting the optical writing start timing as the latent image writing condition, the process of correcting the linear velocity of the photosensitive member of each color as the latent image writing condition is performed. However, the amount of deviation can be reduced. However, when the amount of registration deviation is reduced by the process of correcting the linear velocity of each color photoconductor, each color photoconductor is individually driven so that the linear velocity of each color photoconductor can be individually adjusted. Must be driven by.

  The control unit employs a timing at which a predetermined number of prints are performed as one of the timings at which the actual misalignment correction processing is performed. However, in the state of “with planned printing”, when the execution timing for each predetermined number of prints for the actual deviation amount correction processing has arrived, until the “with planned printing” state changes to the “no scheduled printing” state, Extend the start timing of the actual misalignment correction process. Then, after the state of “scheduled printing” is changed to the state of “no scheduled printing” and the actual misalignment amount correction processing is performed, the count value of the cumulative number of printed sheets is reset to zero. Specifically, the state of “scheduled print” means a state in which at least one print based on the received print command remains. On the other hand, the state of “no scheduled print” means a state in which all prints based on print commands received in the past have been completed and no prints to be executed remain. Here, it is assumed that every predetermined number of prints, which is the execution timing of the actual misalignment correction process, is, for example, every 200 prints. In this case, it is assumed that the cumulative count value of the number of prints for every 200 sheets is 199, and 10 continuous print command signals are received in a state of “no scheduled print”. Then, among the 10 continuous prints, the count value of the cumulative number of prints reaches 200 at the time when the first print is performed, but at this point, 9 prints are still left. Is the state. Therefore, the actual deviation correction process is not performed at that time. When all 10 continuous prints are finished and the status of “scheduled print” changes to the status of “no scheduled print”, the actual misalignment correction processing is performed, and then the cumulative print count value Reset to zero.

  In this way, even if the cumulative number of prints exceeds the predetermined number in the “scheduled print” state, the actual misregistration amount correction process starts until the “scheduled print” state changes to the “no planned print” state. The continuous printing is not interrupted due to the execution of the actual deviation amount correction process. However, during the period in which the start of the actual misregistration amount correction process is extended, there is a possibility that the color misregistration amount is increased and the deterioration in image quality is reduced. Therefore, when the start of the actual deviation amount correction process is extended because the cumulative number of printed sheets exceeds the predetermined number in the state of “scheduled print”, the theoretical deviation amount correction process as the first write condition correction process To suppress color misregistration. In this theoretical misalignment correction process, the misregistration amount from the normal position is calculated for each color toner image based on a predetermined arithmetic expression. Then, based on the positional deviation amount of each color toner image, the Y-C color deviation amount, the Y-M color deviation amount, the Y-K color deviation amount, the C-M color deviation amount, Six different color misregistration amounts between two colors, ie, a color misregistration amount between CK and a color misregistration amount between M and K, are calculated. If the maximum amount of color misregistration between two colors exceeds 1/2 dot among these six patterns, the latent color of any one of the two colors related to the amount of misregistration between the two colors is displayed. The timing of image writing start is shifted forward or backward by one line, and a timing correction schedule is made so that the shift amount between two colors is less than 1/2. Then, the six two-color misregistration amounts are recalculated when it is assumed that the timing correction schedule has been executed, and when the maximum two-color misregistration amount is less than 1/2 dot, The latent image writing start timing is corrected according to the timing correction schedule. Further, when the maximum two-color shift amount among the six two-color shift amounts assuming that the timing correction schedule has been executed is also ½ dot or more, the two-color shift amount is further increased. The timing correction schedule for two colors related to 1 is corrected so that the latent image writing start timing of any one color is shifted forward or backward by one line so that the shift amount between the two colors is less than 1/2. To do. This correction is repeated until the maximum amount of misalignment between two colors among the six misregistration amounts between two colors becomes less than 1/2 dot, and then the latent image writing start timing is corrected according to the timing correction schedule.

  During the continuous printing operation, an increase in the color misregistration amount is suppressed by performing such a theoretical misregistration amount correction process. When the latent image writing timing is corrected according to the timing correction schedule, the positional deviation correction amount of each color is stored, and in the subsequent theoretical deviation amount correction processing, the positional deviation amount based on the arithmetic expression is changed from the positional deviation amount up to that point. A value obtained by subtracting the accumulated positional deviation correction amount is employed as the positional deviation amount.

Next, a characteristic configuration of the printer according to the embodiment will be described.
FIG. 9 is an enlarged perspective view showing the intermediate transfer belt 20 of the printer according to the embodiment. A reference mark 20 a is provided at a predetermined position in the circumferential direction of the intermediate transfer belt 20. The reference mark 20a is provided at a location outside the belt in the belt width direction from the location to be examined by the first optical sensor (137) described above on the belt. Therefore, a chevron patch PV or a test pattern described later is not formed on the reference mark 20a.

  A mark detection sensor 139 made of a reflective photosensor is opposed to one end of the intermediate transfer belt 20 in the width direction with a predetermined gap therebetween. The mark detection sensor 139 detects the reference mark 20a of the intermediate transfer belt 20 at a predetermined position around the intermediate transfer belt 20, and outputs a detection signal to the control unit. The controller determines in advance a difference between the timing at which the reference mark 20a is detected by the mark detection sensor 139 (hereinafter referred to as the mark detection timing) and the timing at which the toner image is detected by the optical sensor unit (136) described above. By comparing with the standard difference, it is possible to grasp the positional deviation amount of the toner image from the normal position.

  The inventors prepared a plurality of printer testing machines having the same configuration as the printer according to the embodiment. Then, using each printer tester, the relationship between the continuous printing time (image forming operation time) at the time of continuous printing and the positional deviation amount of each color was examined. Specifically, as shown in FIG. 10, a plurality of test patterns TP, which are misalignment detection images in which Y, C, M, and K toner images of a predetermined size are repeatedly arranged in that order in the belt moving direction, A continuous output deviation amount measurement process is performed in which the positional deviation of each color toner image in the test pattern TP is measured while continuously outputting to the recording paper. In this continuous output deviation amount measuring process, printing is performed on one sheet of recording paper every time the intermediate transfer belt 20 is rotated once. For each color toner image in the test pattern TP corresponding to each recording paper, the above-described mark detection timing and the timing at which the toner image in the test pattern TP is detected by the optical sensor unit (136) (hereinafter, pattern image detection). The difference from the timing is obtained. This difference reflects the distance between the reference mark 20a on the intermediate transfer belt 20 and the toner image, and if it deviates from a predetermined standard difference, a misregistration amount corresponding to the deviation occurs in the toner image. Will be. Therefore, based on the difference between the difference and a predetermined standard difference stored in advance in the ROM in correspondence with each difference, the amount of positional deviation from the normal position is obtained for each color toner image. The control unit is configured. The obtained results are sequentially stored in the RAM.

  Such a continuous output deviation amount measurement process was performed, and the relationship between the continuous printing time and the positional deviation amount was examined for each color toner image. As the continuous printing time, the driving time of a polygon motor (not shown) for rotating the polygon mirror is employed. The results are shown as a graph in FIG. As shown in the figure, in any of the Y, C, M, and K toner images, the positional deviation amount increases with the continuous printing time until a certain amount of time elapses. Increase reaches saturation. However, the inclination and direction of the graph are different for each color. Specifically, in both Y and C colors, the positional deviation amount increases to the minus side as the continuous printing time increases. The minus position shift is a phenomenon in which the toner image is transferred with a shift from the normal position toward the reference mark 20a. The slope of the Y graph is larger than the slope of the C graph. On the other hand, in each of the two colors M and K, the positional deviation amount increases to the plus side as the continuous printing time increases. The plus side misregistration is a phenomenon in which the toner image is transferred with a shift from the normal position in a direction away from the reference mark 20a. The slope of the K graph is larger than the slope of the M graph. Thus, the graphs showing the relationship between the continuous printing time and the positional deviation amount are different for each of the colors Y, C, M, and K.

  Note that the length of the test pattern TP in the belt moving direction is desirably an integral multiple of the circumferential length of the photosensitive member and the circumferential length of the driving roller 21 that drives the intermediate transfer belt 20. By doing so, it is possible to avoid the detection of the periodic positional deviation amount due to the belt speed fluctuation due to the eccentricity of the photosensitive member and the driving roller 21, and to detect the registration deviation amount and the skew deviation amount with high accuracy. Because it becomes.

  Next, the present inventors conducted an experiment to examine the relationship between the stop time and the amount of positional deviation in each of a plurality of printer testing machines. Specifically, the printing operation is stopped after continuous printing is performed until the increase in the positional shift amount with the lapse of the continuous printing time reaches saturation for all colors. Thereafter, only one test pattern was printed every predetermined time, and the amount of positional deviation of each color was measured. Then, as shown in FIG. 12, it was found that the amount of positional deviation increases with the passage of time during continuous printing, while the amount of positional deviation decreases with the passage of time during stoppage. The reason why the amount of misalignment increases during continuous printing is that the members of the optical system gradually expand due to the temperature rise in the optical writing unit 4. The reason why the amount of positional deviation decreases when continuous printing is stopped and left is because the temperature of the members of the optical system gradually decreases as the stop time increases. Hereinafter, the relationship between the passage of time and the amount of positional deviation during continuous printing is referred to as an operating deviation increase characteristic. Further, the relationship between the passage of time and the amount of positional deviation during stoppage is referred to as a stoppage deviation reduction characteristic.

For each of a plurality of printer testers, the shift deviation characteristics during operation and the shift decrease characteristics during stoppage were measured for each color. Then, in each printer testing machine, it was found that the deviation increase characteristic during operation is expressed by the following equation.
Increase amount of misalignment = coefficient B (1-exp (−coefficient A × printing operation time))
Hereinafter, this formula is referred to as a formula for calculating an increase in displacement during operation.

Further, it was found that the deviation reduction characteristic during stop is expressed by the following equation.
Amount of decrease in positional deviation = coefficient B (exp (-coefficient A × stop time))
Hereinafter, this formula is referred to as a formula for calculating the amount of deviation reduction during stoppage.

  The control unit of the printer according to the embodiment performs the above-described theoretical misalignment correction processing using the operating shift increase calculation formula and the stop shift decrease calculation formula described above for each color. ing. It should be noted that, for each color of Y, C, M, and K, the coefficient A and the coefficient B in the operating shift increase calculation formula and the stop shift decrease calculation formula are different.

  The coefficients A included in the operating shift increase calculation formula and the stop shift decrease calculation formula are the same in each formula. The coefficient B was also the same value in each equation. The coefficient A was the same value for a plurality of printer testing machines. That is, it was found that the coefficient A is a numerical value with no individual difference if the model is the same. On the other hand, the coefficient B was different from each other in each printer testing machine. That is, the coefficient B is known to be different for each individual even with the same standard. The fact that the coefficient B is different for each individual means that the slopes of the graphs of the in-operation deviation increasing characteristic and the stationary deviation decreasing characteristic for each color shown in FIG. 12 are different for each individual. Nevertheless, it is assumed that the above-described theoretical deviation amount correction processing is performed using the operating deviation amount calculation formula and the stopping deviation amount calculation formula in which the coefficients B are the same for all individuals. Then, depending on the individual, the error between the amount of positional deviation obtained based on these calculation formulas and the actual positional deviation amount becomes too large, and there is a risk that the color deviation will be worsened by the theoretical deviation amount correction processing. Come.

  Therefore, the control unit of the printer according to the embodiment performs an arithmetic expression correction process as shown in FIG. 13 to individually correct the coefficient B for each color. In the figure, the control unit first performs a deviation amount measurement process (step a: hereinafter, step is referred to as S). In this deviation amount measurement process, the same processing as the above-described continuous output deviation amount measurement process is performed. However, the shift amount measurement process in Sa of FIG. 13 is different from the above-described shift amount measurement process for continuous output in the following points. That is, in the above-described continuous output deviation amount measurement processing, the deviation amount of each color toner image in each test pattern TP is measured while continuously printing the test pattern TP on a plurality of recording papers. On the other hand, in the shift amount measurement process of FIG. 13, only the test pattern TP for one sheet of recording paper is formed. Further, the test pattern TP is cleaned by the belt cleaning device 26 without being secondarily transferred to the recording paper P.

  When the shift amount measurement process is completed, the control unit calculates the actual shift amount measured in the shift amount measurement process and the above-described shift increase calculation formula during operation for each of Y, C, M, and K colors. Find the difference from the amount of deviation. Then, based on the difference, the respective coefficients B in the in-operation deviation increase calculation formulas for Y, M, C, and K are corrected. As already described, the value of the coefficient B is different for each color. In addition, for the same color, the coefficient B of the in-operation deviation increase calculation formula and the coefficient B in the in-stop deviation reduction calculation formula have the same value, so that for each color, the coefficient of the in-stop deviation reduction calculation formula B is corrected to the same value as the coefficient B of the operating shift increase calculation formula.

  In this printer that performs such an arithmetic expression correction process, a test pattern having a toner image for detecting a deviation of each color is formed on the surface of the intermediate transfer belt 20 as an endless moving body in the arithmetic expression correction process. Actually form. The actual color of each color is detected based on the timing at which the optical sensor unit 136 serving as the image detecting means detects the toner image for detecting the positional deviation and the timing at which the reference mark 20a of the intermediate transfer belt 20 is detected by the mark detection sensor 139. The actual displacement amount, which is the positional displacement amount of, is detected. These actual misregistration amounts are misregistration amounts between the reference mark 20a and the Y, M, C, and K toner images, as shown in FIG. Next, based on a comparison between the actual deviation amount and the theoretical deviation amount, which is the positional deviation amount calculated based on the operational deviation increase amount calculation formula as an arithmetic expression, the operational deviation increase amount calculation formula and the stoppage deviation are calculated. The correction magnification of the coefficient B in the decrease amount calculation formula is calculated. Specifically, as shown in FIG. 15, a correction magnification capable of making the calculation formula before correction substantially coincide with the actual characteristics is obtained from the ratio between the theoretical deviation amount and the actual deviation amount. By multiplying the correction magnification by the coefficient B, the corrected calculation formula can be made to substantially coincide with the graph of actual characteristics. By correcting the coefficient B as described above, the positional deviation amount of each color toner image is appropriately calculated based on the operating deviation amount calculation formula and the stopped deviation amount calculation formula by the theoretical deviation amount correction processing in each individual. Thus, the color misregistration amount can be reliably reduced by performing the theoretical misregistration amount correction processing.

  As described above, in the arithmetic expression correction process, only the test pattern TP for one recording sheet is formed. That is, only a printing operation for one sheet of recording paper is performed. On the other hand, in order to appropriately correct the coefficient B described above, it is desirable to measure the amount of positional deviation after performing continuous printing for a relatively long time. Therefore, in the simple process of performing the arithmetic expression correction process during the initial operation of the user, the printing operation time prior to the arithmetic expression correction process is too short, and the coefficient B cannot be corrected with high accuracy.

  Therefore, as shown in the flowchart of FIG. 16, the control unit of the printer performs arithmetic expression correction processing immediately after a predetermined number or more of continuous prints are performed. In the control flow shown in this flowchart, the flag A is a parameter indicating the execution history of the arithmetic expression correction process. When the arithmetic expression correction process is executed once, the flag A is turned from OFF to ON. That is, when the flag A is ON, it indicates that the arithmetic expression correction process has been executed in the past. The flag B is a parameter indicating the execution decision of the arithmetic expression correction process. When the execution of the arithmetic expression correction process is determined, the flag B is turned on from the OFF state.

  When a single print command is issued to print only on one sheet of recording paper (N in S2), the control flow of FIG. 14 is advanced from S2 to S7, and the steps from S3 to S6 in the middle are omitted. Is done. Since the step S5 for setting the flag B to ON is also omitted, the arithmetic expression correction process is not executed in the subsequent series of control flows. On the other hand, when a continuous printing command is issued (Y in S2), the number of prints in the continuous printing is equal to or greater than a predetermined threshold (Y in S3), and the flag A is ON (Y in S4). The flag B is set to ON (S5). Thereby, the arithmetic expression correction process is executed later. However, even if a continuous print command is issued, if the number of prints is not greater than or equal to the threshold value (N in S3), or if the flag A is not ON (N in S4), the step S5 is omitted, so Processing is not performed.

  Regardless of whether the print is single shot or continuous print, when the print job is started, the print job is executed while performing the theoretical misalignment correction processing (S8 to S9). At this time, prior to the start of the print job, the control unit stops the count value of the elapsed time from the end of the previous print job to the current time, that is, the stop time from the end of the previous print job to the current time. Get as time count value. The previous print job referred to here does not include the previous job in the continuous print job. Regardless of single print or continuous print, the print job immediately before the stop state before the start of the current print job is the previous print job. When the stop time count value is acquired, the print operation time count value at the end of the previous print job is read from the RAM, and then the print operation time count value for each of Y, C, M, and K is calculated as an increase in operating deviation Substitute into the formula and perform the calculation. As a result, the total amount of misalignment when it is assumed that the theoretical misalignment correction processing was not performed during the previous print job is obtained. Next, for Y, C, M, and K, the stoppage time count value is substituted into the above-described stoppage shift reduction amount calculation formula to determine the shift reduction amount. Then, the residual amount of deviation is obtained by subtracting the amount of reduction in deviation from the total amount of deviation. When the deviation reduction amount is larger than the total deviation amount, the residual deviation amount is calculated as zero. When the residual amount of misregistration for each color is calculated in this way, the misregistration residual amount is stored as the initial misregistration value. As shown in FIG. 14, the initial deviation values of these colors indicate how much the distance between the reference mark 20a and the Y, M, C, and K toner images is deviated from the normal distance. It does not indicate the amount of color misregistration that is the amount of misregistration between the colors. Therefore, the control unit, based on the initial value of each color misalignment, as shown in FIG. 17, the amount of color misalignment between Y and C, the amount of color misalignment between Y and M, the amount of color misalignment between Y and K, C Six color misregistration amounts between two colors, that is, a color misregistration amount between -M, a color misregistration amount between CK, and a color misregistration amount between M and K are calculated. Then, based on the amount of color misregistration between the two colors, a timing correction schedule is established by the procedure already described, and then the writing start timing is corrected. Thereafter, for the positional shift amount of each color, the sum of the initial value of the shift, the calculated value of the positional shift amount based on the calculation formula for the shift increase amount during operation, and the positional shift correction amount is obtained as the theoretical shift amount of each color. Thereafter, the print job is started (S7). At this time, driving of the polygon motor is started and counting of the printing operation time is started.

  Even in the case of single-shot printing, when not much time has passed since the previous print job, the initial value of deviation of each color exceeds 1/2 dot, and the writing start timing is corrected. Note that the timing determined at the time of executing the above-described actual deviation amount correction processing is employed as the reference writing start timing.

  When the control unit determines to execute the arithmetic expression correction process and turns on the flag B in the above-described step S5, the control unit executes the above-described actual deviation amount correction process prior to starting the continuous print job (S6). ). As a result, the color misregistration amount of each color is reduced to less than 1/2 dot in advance, and then a continuous print job is started (S7). When all the scheduled print jobs are finished (Y in S10), it is determined whether or not the flag B is ON (S11). When the flag B is ON (S11), the arithmetic expression correction process shown in FIG. 13 is performed (S12), and the in-operation deviation increase amount calculation formula and the stop deviation decrease amount calculation formula of each color are calculated. After correcting the coefficient B, the flags A and B are turned off.

  In addition, the control unit determines whether or not the cumulative number of prints has reached 200 or more after completing all scheduled print jobs, regardless of whether the arithmetic expression correction process is executed, and the number is 200 or more. In such a case (Y in S14), after the actual misregistration amount correction process described above is performed (S15), the count value of the cumulative number of printed sheets is reset to zero, and then the series of control flows is ended. If the number is less than 200 (N in S14), the series of control flow is immediately terminated.

  It should be noted that there is a possibility that the coefficient B does not match the individual during the period from the start of the initial operation under the user until the execution of the arithmetic expression correction process. It is not preferable from the viewpoint of image quality that this state continues indefinitely. Therefore, the correction accuracy of the coefficient B may be gradually increased by gradually increasing the threshold value in S3. Specifically, it is assumed that the arithmetic expression correction process is ideally performed after about 100 continuous prints. However, depending on the user, there are cases where a small number of prints are mainstream and there is almost no opportunity to execute 100 continuous prints. If the threshold is set to 100, the arithmetic expression correction process will not be executed under such a user. Therefore, the initial value of the threshold is set to about 5 sheets. With such a numerical value, it is possible to execute the arithmetic expression correction process within a relatively short time from the start of the initial operation. When the arithmetic expression correction process is executed, the threshold value is corrected to a slightly larger value, for example, about 20 sheets. In this way, the correction for gradually increasing the threshold value and the execution of the arithmetic expression correction process are repeated, and finally, the arithmetic expression correction process after continuous printing of about 100 sheets is executed to obtain the coefficient B The correction accuracy is maximized.

Next, modifications of the printer according to the embodiment will be described. Unless otherwise specified below, the configuration of the printer according to each modification is the same as that of the embodiment.
[First Modification]
In the printer according to the first modification, after correcting the coefficient B in the calculation formula for the displacement increase amount during operation and the calculation amount for the shift decrease during stopping by the arithmetic expression correction process, the actual deviation amount after every 200 prints is thereafter obtained. Cancel the correction process. The actual misalignment correction processing is performed only at the start-up operation when the power is turned on. As a result, when all the scheduled jobs are completed, it becomes possible to immediately wait for the next print command regardless of the cumulative number of prints, so that the actual deviation amount correction process is performed between jobs and the user is made to wait. Such a thing disappears.

[Second Modification]
The printer according to the second modification does not include the mark detection sensor 139. For this reason, it is not possible to measure the positional deviation amount of each color based on the distance between the reference mark 20a and each color toner image of the test pattern TP. Instead, the positional deviation amount from the theoretical absolute position is measured as the positional deviation amount of each color toner image in the test pattern TP. This theoretical absolute position is determined based on the timing at which optical writing on the photosensitive member is started for each color toner image in the test pattern TP. A value obtained by adding a unique time for each color (hereinafter referred to as a theoretical absolute position time) corresponds to the theoretical absolute position with respect to each optical writing start timing. If the time from the optical writing start timing to the actual detection of the corresponding toner image by the optical sensor unit 136 deviates from the above-described theoretical absolute position time, the deviation amount reflects the positional deviation amount. Will be. Therefore, the control unit sequentially calculates the theoretical absolute position time corresponding to each based on the optical writing start timing corresponding to each color toner image in the test pattern TP. Then, the optical writing start timing and the theoretical absolute position time are combined and stored in the RAM. Thereafter, when each color toner image is detected by the optical sensor unit 136, an actual position timing, which is an actual time required from the start of writing to the detection, is obtained based on the timing and the optical writing start timing. For each color, the actual deviation amount is obtained based on the difference between the theoretical absolute position time and the actual position timing.

  In such a configuration, the actual misregistration amount of each color toner image can be measured without attaching the reference mark 20a to the intermediate transfer belt 20 or providing the mark detection sensor 139.

  Up to this point, a description has been given of a printer of a type in which toner images formed on the photoreceptors 10Y, 10C, 10M, and 10K of each color are superimposed and transferred onto the surface of the intermediate transfer belt 20 as an endless moving body. The present invention can also be applied to an image forming apparatus. That is, the image forming apparatus employs a method in which a visible image formed on an image carrier of each color is superimposed and transferred onto a recording member held on the surface of an endless moving body.

  As described above, in the printer according to the embodiment, the control unit serving as the image forming control unit obtains the amount of increase in the theoretical misalignment amount with the increase in the printing operation time as the image forming operation time for each color toner image as an arithmetic expression. For calculating the shift amount during operation as the first equation for calculating the amount of shift during operation, and the calculation formula for the decrease amount during stoppage as the second equation for determining the decrease amount of the theoretical shift amount with the increase of the stop time as the image formation stop time. And are used. Then, in the theoretical deviation amount correction process as the write condition correction process, the calculation result of the deviation increase amount by the calculation formula for the deviation amount during operation, the calculation result of the deviation reduction amount by the calculation formula for the deviation amount during stop, and the latent image Based on the positional deviation correction amount obtained by correcting the optical writing start timing, which is a writing condition, the color misregistration amount, which is the overlay misregistration amount of the toner images of the respective colors, is grasped. In such a configuration, for each color toner image, not only the amount of increase in the positional deviation during the continuous printing operation but also the amount of decrease in the positional deviation during the stop is reflected in the theoretical deviation amount. It can be closer to the amount of displacement. Furthermore, even when a print command is issued within a relatively short time after a print operation is stopped and a new print operation is started, the amount of increase in the position shift in the previous print operation or the decrease in position shift at the time of stop While reflecting the amount, it is possible to obtain a calculation result having a value close to the actual positional deviation amount as the theoretical deviation amount in the current printing operation.

  Further, in the printer according to the embodiment, an equation “increase amount = coefficient B (1−exp (−coefficient A × image forming operation time))” is used as a calculation formula for the increase amount during operation that is the first equation. At the same time, the control unit is configured to use the expression “decrease amount = coefficient B (exp (−coefficient A × operation stop time))” as the second formula for calculating the amount of deviation decrease during stop. Since these formulas have been confirmed by experiments to show a good correlation with the actual deviation increase and deviation reduction, using these formulas, the theoretical deviation is close to the actual positional deviation. The operation result can be obtained.

  In the printer according to the embodiment, a mark detection sensor 139 that detects the reference mark 20a attached to a predetermined position in the endless movement direction of the surface of the intermediate transfer belt 20 that is an endless moving body is provided, and an arithmetic expression correction process is performed. The toner images for detecting misregistration of the respective colors in the test pattern TP are based on the actual misregistration based on the detection timing of the toner image by the optical sensor unit 136 serving as the image detecting means and the detection timing of the reference mark 20a by the mark detecting sensor 139. The control unit is configured to calculate the amount and correct the coefficient B based on the comparison between the calculation result and the theoretical deviation amount. In such a configuration, the amount of positional deviation can be grasped for each color based on the difference between the result of actually detecting the distance between the reference mark 20a and the toner image and the theoretical distance between the two.

  In the printer according to the second modification, the latent image of the toner image for detecting the misregistration of each color in the test pattern TP is corrected by the optical writing unit 4 serving as the latent image writing means in the arithmetic correction process. The actual deviation amount is calculated based on a comparison between the time from when writing is started until the toner image is actually detected by the optical sensor unit 136 and the theoretical absolute position time which is a predetermined reference time. The control unit is configured to correct the coefficient B based on the comparison between and the theoretical deviation amount. In such a configuration, the actual misregistration amount of each color toner image can be measured without attaching the reference mark 20a to the intermediate transfer belt 20 or providing the mark detection sensor 139.

  In the printer according to the embodiment, the control unit forms a chevron patch PV for detecting misalignment on the surface of the intermediate transfer belt 20 in addition to performing the theoretical misalignment amount correcting process that is the first correcting process of the writing condition. The amount of deviation from the normal position is calculated for each color toner image in the chevron patch PV based on the timing at which it is detected by the optical sensor unit 136, and writing is started as a latent image writing condition based on this calculation result. An actual misalignment correction process is also performed as a write condition second correction process for correcting the timing. After the theoretical misalignment correction processing and the actual misalignment amount correction processing are properly used according to the situation, and after the actual misalignment amount correction processing is performed, the printing operation time and the in-stop misalignment calculation formula are calculated. A process of resetting the stop time used for the decrease calculation formula to zero is performed. In such a configuration, by performing the correction of the writing start timing corresponding to the actual misregistration amount by the actual misregistration amount correction process, the color misregistration can be reduced as compared with the case where only the theoretical misregistration amount correction process is performed. it can. In addition, after execution of the actual misregistration amount correction process that can surely reduce the color misregistration amount to less than ½ dot, the printing operation time is calculated as the operating misregistration increase amount calculation formula, and the stop used for the misregistration decrease amount calculation formula during stoppage. By resetting the time to zero corresponding to the positional deviation amount of less than ½ dot, it is possible to avoid deterioration in the accuracy of the theoretical deviation amount due to not resetting.

The graph which shows the relationship between the number of continuous output sheets and the amount of color shift. 1 is a schematic configuration diagram illustrating a printer according to an embodiment. FIG. 3 is an enlarged configuration diagram illustrating a process unit for Y of the printer. FIG. 3 is an enlarged configuration diagram illustrating photoconductors of respective colors together with an optical writing unit in the printer. The perspective view which shows the elongate lens unit of the optical writing unit. The perspective view which shows the same elongate lens unit from the reverse direction to FIG. FIG. 3 is an enlarged perspective view showing a part of an intermediate transfer belt of the printer together with an optical sensor unit. FIG. 3 is an enlarged schematic view showing a chevron patch formed on the intermediate transfer belt. FIG. 3 is an enlarged perspective view showing the intermediate transfer belt. FIG. 3 is an enlarged schematic view showing a test pattern formed on the intermediate transfer belt. The graph which shows the relationship between the positional offset amount of each color, and continuous printing time. The graph which shows the characteristic of the shift | offset | difference increase amount during operation | movement in any one color, and the shift | offset | difference decrease amount during a stop. 6 is a flowchart illustrating a processing flow of arithmetic expression correction processing performed by the control unit of the printer. FIG. 4 is an enlarged schematic diagram illustrating a positional deviation amount between a reference mark and each color toner image. The graph which shows the characteristic of the deviation increase amount during operation | movement and the amount of deviation reduction during a stop before coefficient correction | amendment, after coefficient correction, and actually. The flowchart which shows the control flow implemented by the control part. FIG. 4 is an enlarged schematic diagram for explaining a shift amount between two colors.

Explanation of symbols

4: Optical writing unit (latent image writing means)
5: Transfer unit (transfer means)
10Y, C, M, K: photoconductor (latent image carrier)
12Y, C, M, K: Developing device (developing means)
20: Intermediate transfer belt (endless moving body)
20a: Reference mark 136: Optical sensor unit (image detection means)
139: Mark detection sensor (mark detection means)
PV: Chevron patch (visible image for displacement detection)
TP: Test pattern (visible image for displacement detection)

Claims (7)

A plurality of latent image carriers that carry a latent image on a surface that moves endlessly, a plurality of developing units that develop the latent images on the latent image carrier into different colors, and a latent image that writes a latent image on the latent image carrier An image forming means having an image writing means, and a surface of the endless moving body, and a visible image formed on the surface of each latent image carrier, on the surface of the endless moving body, or the surface. A transfer unit that superimposes and transfers the image on the recording member to be held; and an image forming control unit that controls the image forming unit. Calculate the theoretical misregistration amount, which is the theoretical misregistration amount on the surface, based on the predetermined calculation formula, grasp the overlay misregistration amount of the visible image of each color based on the calculation result, and create based on the grasping result. The latent image writing condition of the latent image writing means during image operation is compensated. In the image forming apparatus to carry out the writing condition correction processing for reducing to misalignment amount,
After forming a visible image for detecting a shift of a different color on a plurality of latent image carriers and transferring it to the surface of the endless moving body, a visible image for detecting a shift of each color on the surface of the endless moving body An arithmetic expression for calculating the positional deviation amount from the normal position as an actual deviation quantity based on the timing detected by the image detection means, and correcting the arithmetic expression based on a comparison between the actual deviation quantity and the theoretical deviation quantity. An image forming apparatus, wherein the image forming control unit is configured to perform correction processing at a predetermined timing. The image forming apparatus according to claim 1.
As the calculation formula, for the visible image of each color, the first formula for obtaining the increase amount of the theoretical shift amount with the increase of the image forming operation time, and the decrease of the theoretical shift amount with the increase of the image forming stop time. The calculation result of the increase amount according to the first equation, the calculation result of the decrease amount according to the second equation, and the latent image writing in the writing condition correction process using the second equation for determining the amount An image forming apparatus characterized in that the image forming control means is configured so as to grasp an amount of misalignment of visible images of respective colors based on a position misalignment correction amount by condition correction. The image forming apparatus according to claim 2.
As the first equation, an equation of “increase amount = coefficient B (1-exp (−coefficient A × image forming operation time))” is used,
As the second equation, an expression “amount of decrease = coefficient B (exp (−coefficient A × operation stop time))” is used.
An image forming apparatus comprising the image forming control means. The image forming apparatus according to claim 3.
While providing a mark detection means for detecting a reference mark attached to a predetermined position in the endless moving direction of the surface of the endless moving body,
Based on the detection timing of the shift detection visible image by the image detection unit and the detection timing of the reference mark by the mark detection unit for each color shift detection visible image in the arithmetic expression correction process. An image forming apparatus comprising: the image forming control unit configured to calculate the actual deviation amount and correct the coefficient B based on a comparison between a calculation result and the theoretical deviation amount. The image forming apparatus according to claim 3.
In the arithmetic expression correction process, for each color shift detection visible image, the latent image writing unit starts writing the latent image, and then the image detection unit detects the shift detection visible image. The image formation control is performed so that the actual deviation amount is calculated based on a comparison between the time until the image is calculated and a predetermined reference time, and the coefficient B is corrected based on a comparison between the calculation result and the theoretical deviation amount. An image forming apparatus comprising a means. The image forming apparatus according to any one of claims 3 to 5,
In addition to performing the writing condition correction process as a writing condition first correction process, the deviation detection visible image on the surface of the endless moving body is detected based on the timing detected by the image detection means. A write condition second correction process for calculating a deviation amount of the visible image from the normal position and correcting the latent image writing condition based on the calculation result is also performed. After the second correction process is properly used depending on the situation, the image forming operation time used for the first expression and the operation stop time used for the second expression are used. An image forming apparatus, wherein the image forming control unit is configured to perform a process of resetting each to zero. The image forming apparatus according to claim 6.
When a command for a continuous print operation for continuously forming images on a plurality of recording members is given and the number of output sheets in the continuous print operation is equal to or greater than a predetermined number, the second write condition correction process is performed. An image forming apparatus comprising: the image forming control unit configured to execute the continuous printing operation after the execution and then perform the arithmetic expression correction process.

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