IMAGE FORMING APPARATUS


Application Number: 15751560 Application Date: 13.07.2016
Publication Number: 20180236779 Publication Date: 23.08.2018
Publication Kind : A1
Prior PCT appl.: Application Number:PCTJP2016070627 ; Publication Number: Click to see the data
IPC:
B41J 2/47
G02B 26/12
G03G 15/04
G03G 21/14
B41J 2/44
G03G 15/01
CPC:
B41J 2/44
B41J 2/47
G02B 26/12
G03G 15/01
G03G 15/04
G03G 21/14
Applicants: KYOCERA Document Solutions Inc.
Inventors: Aiichiro Otana
Priority Data: 2015-158720 11.08.2015 JP
Title: (EN) IMAGE FORMING APPARATUS
Abstract: front page image

(EN)

An image forming apparatus includes: a housing in which a light source, a laser scanning member, a light detecting portion configured to detect light incident on a predetermined position in a scanning path of the light scanned by the laser scanning member, and a scanning lens configured to cause the light scanned by the laser scanning member to be scanned at an equal speed on an image carrier, are disposed; a temperature gradient detecting portion, configured to detect a temperature gradient of the housing, a heater configured to heat the image carrier or a sheet; and a correction processing portion configured to correct the emission start timing based on the temperature detected by the temperature gradient detecting portion, and a predetermined expression, and change content of the expression based on an operation state of the heater at a start of an image formation process.

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TECHNICAL FIELD

      The present invention relates to an electrophotographic image forming apparatus.

BACKGROUND ART

      In a tandem-type image forming apparatus including a plurality of image forming units, a so-called color shift may occur in which relative positions of toner images of different colors formed on image carriers of the image forming units are shifted in a scanning direction (main scanning direction) of light irradiated from a laser scanning device to the image carriers. For example, in this type of image forming apparatus, the color shift may occur due to expansion and contraction of optical members that are caused by the temperature change in the apparatus. As a result, a color shift correction for correcting the color shift may be executed in the image forming apparatus. In particular, a color shift correction using different correction tables for a temperature increase and a temperature decrease in the image forming apparatus may be executed (see, for example, PTL 1).

CITATION LIST

Patent Literature

      [PTL 1] Japanese Patent No. 5103349

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

      However, there is a problem that the storage capacity of a storage portion, such as a ROM, mounted on the image forming apparatus increases since correction tables corresponding to situations, such as a temperature increase and a temperature decrease, require a large amount of data.
      The present invention has been made in view of such conventional circumstances, and it is an object of the present invention to provide an image forming apparatus that can correct positional shifts in the main scanning direction of the scanning light with high accuracy, without using individual correction tables corresponding to situations.

Solution to the Problems

      An image forming apparatus according to an aspect of the present invention includes a light source, a laser scanning member, a light detecting portion, a light source control portion, a scanning lens, a housing, a temperature gradient detecting portion, a heater, and a correction processing portion. The laser scanning member scans light emitted from the light source. The light detecting portion detects light incident on a predetermined position in a scanning path of the light scanned by the laser scanning member. The light source control portion, each time the light detecting portion detects light, causes the light source to emit light corresponding to image data of a line, at a predetermined emission start timing. The scanning lens causes the light scanned by the laser scanning member to be scanned at an equal speed on an image carrier. The laser scanning member, the light detecting portion, and the scanning lens are disposed in the housing. The temperature gradient detecting portion detects a temperature gradient of the housing. The heater heats the image carrier or a sheet before image transferring. The correction processing portion corrects the emission start timing based on the temperature gradient detected by the temperature detecting portion, the ambient temperature detected by the ambient temperature detecting portion, and a predetermined expression, and change content of the expression based on an operation state of the heater at a start of an image formation process.

Advantageous Effects of the Invention

      According to the present invention, it is possible to provide an image forming apparatus that can correct positional shifts in the main scanning direction of the scanning light with high accuracy, without using individual correction tables corresponding to situations.

BRIEF DESCRIPTION OF THE DRAWINGS

       FIG. 1 is a diagram showing a configuration of an image forming apparatus according to an embodiment of the present invention.
       FIG. 2 is a diagram showing a configuration of a laser scanning device according to the embodiment of the present invention.
       FIG. 3 is a block diagram showing a system configuration of the image forming apparatus according to the embodiment of the present invention.
       FIG. 4 is a diagram showing a setting method of an expression used in the image forming apparatus according to the embodiment of the present invention.
       FIG. 5 is a diagram showing a setting method of an expression used in the image forming apparatus according to the embodiment of the present invention.
       FIG. 6 is a diagram showing a setting method of an expression used in the image forming apparatus according to the embodiment of the present invention.
       FIG. 7 is a flowchart showing an example of a procedure of an image formation control process executed in the image forming apparatus according to the embodiment of the present invention.
       FIG. 8 is a diagram showing an execution result of the image formation control process in the image forming apparatus according to the embodiment of the present invention.
       FIG. 9 is a diagram showing an execution result of the image formation control process in the image forming apparatus according to the embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

      The following describes an embodiment of the present invention with reference to the accompanying drawings for the understanding of the invention. It should be noted that the following embodiment is an example of a specific embodiment of the present invention and should not limit the technical scope of the present invention.
      As shown in FIG. 1, an image forming apparatus 10 includes a plurality of image forming units 1– 4, an intermediate transfer belt 5, a laser scanning device 6, a secondary transfer roller 7, a fixing device 8, a sheet discharge tray 9, toner containers 11– 14, a sheet feed cassette 21, a conveyance path 22, and an ambient temperature detecting portion 23. The image forming apparatus 10 is a printer that forms a color or monochrome image on a sheet supplied along the conveyance path 22 from the sheet feed cassette 21, and discharges the sheet with the image to the sheet discharge tray 9. It is noted that in the following description, an explanation may be given with reference to a left-right direction D 1, an up-down direction D 2, and a front-rear direction D 3 defined in the drawings.
      In the present embodiment, the image forming apparatus 10 includes two laser scanning devices 6 in correspondence with the image forming units 1– 4. On the other hand, as other embodiments, a configuration where four laser scanning devices are provided in correspondence with the image forming units1– 4, and a configuration where one laser scanning device is provided in correspondence with the image forming units 1– 4, are considered. In addition, the image forming apparatus according to the present invention is not limited to a printer, but may be a facsimile, a copier, or a multifunction peripheral, for example.
      The image forming units 1– 4 are arranged in alignment along the intermediate transfer belt 5, and constitute a so-called tandem-type image forming portion. Specifically, the image forming units 1– 4 form toner images corresponding to colors of Y (yellow), C (cyan), M (magenta), and K (black). Each of the image forming units 1– 4 is an electrophotographic image forming unit that includes a photoconductor drum 31 (an example of the image carrier), a charging portion 32, a developing portion 33, and a primary transfer roller 34.
      In each of the image forming units 1– 4, after the photoconductor drum 31 is charged by the charging portion 32, an electrostatic latent image corresponding to image data is formed on the photoconductor drum 31 by light irradiated from the laser scanning device 6. Subsequently, toner images formed on the photoconductor drums 31 are sequentially transferred to the intermediate transfer belt 5 by the primary transfer rollers 34. This allows a color or monochrome toner image to be formed on the intermediate transfer belt 5. Thereafter, the toner image on the intermediate transfer belt 5 is transferred to a sheet by the secondary transfer roller 7, and the toner image is fused by the fixing device 8 to as to be fixed to the sheet.
      Meanwhile, in a case where an a-Si (amorphous silicon)-based material is used in the photoconductor drum 31, a so-called “image deletion” may occur in which the electrostatic latent image is disturbed by water molecules that have adhered to the surface of the photoconductor drum 31 due to increase in humidity. As a result, each of the image forming units 1– 4 includes a heater 35 (see FIG. 3) configured to heat the photoconductor drum 31. The heater 35is, for example, a planar heater in which resistance elements are arranged on a substrate, and is provided in the vicinity of the photoconductor drum 31 so as to heat the photoconductor drum 31. In the image forming apparatus 10, a control portion 100 that is described below, has a heater function to heat the photoconductor drum 31 by using the heater 35 such that the water molecules of the photoconductor drum 31 are evaporated and removed before an image formation process is started. For example, the heater 35 is operated in a predetermined situation such as when the image forming apparatus 10 is resumed from a power saving mode, or when the image forming apparatus 10 is powered on. It is noted that the control portion 100 can switch between enabling and disabling the heater function in response to a user operation.
      The ambient temperature detecting portion 23 is a thermistor or the like that is provided inside or outside of the image forming apparatus 10, and is used to detect an ambient temperature of a place where the image forming apparatus 10 is installed. For example, the ambient temperature detecting portion 23is provided in the vicinity of an air intake port that allows inside and outside of the image forming apparatus 10 to communicate with each other, and detects the temperature of the outside air as the ambient temperature.
      Next, the laser scanning devices 6 are described. The laser scanning devices 6 form electrostatic latent images corresponding to image data on the surfaces of the photoconductor drums 31 by scanning laser beams on the surfaces of the photoconductor drums 31. Specifically, as shown in FIG. 1 andFIG. 2, each of the laser scanning devices 6 includes light sources 61, a polygon mirror 62, a motor 63, fθ lenses 64, reflection mirrors 65, light detecting portions 66, and a housing 60 in which these components are disposed. In addition, a first temperature detecting portion 67 is provided in one of the two laser scanning devices 6, and a second temperature detecting portion 68 is provided in the other of the two laser scanning devices 6. It is noted that although a cover member is attached to an upper surface of the housing 60 of each of the laser scanning devices 6, in FIG. 2, the cover member is omitted.
      The light sources 61 are, for example, laser diodes that irradiate laser beams. The polygon mirrors 62 are rotary polygon mirrors that each have six reflection surfaces for reflecting two laser beams emitted from the two light sources 61, and are axially supported by the motors 63 in a rotatable manner. It is noted that the polygon mirrors 62 are an example of the laser scanning member. The motors 63 are an example of the driving portion that rotationally drives the polygon mirrors 62. Upon being rotationally driven by the motor 63, each polygon mirror 62 scans, in different directions, the two laser beams emitted from the two light sources 61. In the following, scanning directions of the laser beams scanned by the polygon mirrors 62 are referred to as a main scanning direction (a direction parallel to the front-rear direction D 3 in FIG. 2), and a direction orthogonal to the main scanning direction above the surface of each photoconductor drum 31 is referred to as a sub scanning direction.
      The fθ lenses 64 are scanning lenses that cause the laser beams scanned by the polygon mirrors 62 in the main scanning direction, to be focused on the surfaces of the photoconductor drums 31 that are irradiation objects, and to be scanned at an equal speed. The four fθ lenses 64 shown in FIG. 2 are, from left to right of FIG. 2, scanning lenses that correspond to the four image forming units 1– 4 for yellow, cyan, magenta, and black, respectively.
      The reflection mirrors 65 are reflection members that are elongated in the main scanning direction in which the laser beams are scanned by the polygon mirrors 62. The reflection mirrors 65 are supported by the housings 60 at opposite ends thereof in the longitudinal direction. The reflection mirrors 65sequentially reflect laser beams that have passed through the fθ lenses 64, and guide the laser beams to the surfaces of the photoconductor drums 31.
      The light detecting portions 66 are provided in correspondence with the image forming units 1– 4 and disposed at predetermined positions on the scanning paths of the laser beams scanned by the polygon mirrors 62 in the main scanning direction so as to detect incident laser beams. For example, the light detecting portions 66 are provided in the vicinities of corners of the housing 60. In the image forming apparatus 10, the control portion 100 described below controls emission start timing of emitting a laser beam corresponding to image data of each line, namely, a writing timing of writing an image of each line, based on a detection timing at which the light detecting portion 66 detects a laser beam. It is noted that as another embodiment, one light detecting portion 66 may be provided in correspondence with one of the image forming units 1– 4. In addition, as a further embodiment, one light detecting portion 66may be provided in correspondence with the image forming units 1 and 2, and one light detecting portion 66 may be provided in correspondence with the image forming units 3 and 4.
      The first temperature detecting portion 67 is included in, among the two laser scanning devices 6, an laser scanning device 6 that is on the right side and corresponds to black and magenta. The first temperature detecting portion 67 is a thermistor or the like that is used to detect the temperature of the fθ lenses 64. In addition, the second temperature detecting portion 68 is included in, among the two laser scanning devices 6, an laser scanning device 6 that is on the left side and corresponds to cyan and yellow. The second temperature detecting portion 68 is a thermistor or the like that is used to detect the temperature of the light detecting portions 66. For example, as shown in FIG. 2, the first temperature detecting portion 67 is disposed in the vicinity of the center of the housing 60, and the second temperature detecting portion 68 is disposed in the vicinity of a corner of the housing 60. That is, the first temperature detecting portion 67 and the second temperature detecting portion 68 are disposed to be separate from each other by a predetermined distance in the housings 60, and temperatures detected by the first temperature detecting portion 67 and the second temperature detecting portion 68 can be used as an index of the temperature gradient of the housings 60. In the present embodiment, the first temperature detecting portion 67 and the second temperature detecting portion 68 are an example of the temperature gradient detecting portion configured to detect the temperature gradient of the housings 60.
      Meanwhile, in the image forming apparatus 10, a so-called color shift may occur in which relative positions of toner images of respective colors formed on the photoconductor drums 31 of the image forming units 1– 4 are shifted in the main scanning direction due to expansion and contraction of the optical members, such as the fθ lenses 64, that are caused by the temperature change in the apparatus. On the other hand, in the image forming apparatus 10, a color shift correction is executed in which the color shift is corrected based on the temperatures detected by the first temperature detecting portion 67 and the second temperature detecting portion 68. In particular, as described below, in the image forming apparatus 10, positional shifts of scanning light beams in the main scanning direction are corrected without using individual correction tables corresponding to situations. In addition, the temperature gradient of the housings 60 may change differently depending on the ambient temperature of a place where the image forming apparatus 10 is installed, or the surrounding temperature of the laser scanning devices 6 in the image forming apparatus 10. In that case, the amount of color shift of the laser beams in the main scanning direction that occurs in the image formation process, may change. As a result, in the image forming apparatus 10, the color shift of the laser beams in the main scanning direction is corrected by taking into consideration the change of color shift amount that is caused by, for example, the ambient temperature of a place where the image forming apparatus 10 is installed, or the surrounding temperature of the laser scanning devices 6 in the image forming apparatus 10.
      Specifically, as shown in FIG. 3, the image forming apparatus 10 includes the control portion 100 that controls the image forming operation in the image forming apparatus 10. It is noted that the control portion 100 may be a main control portion that comprehensively controls the whole image forming apparatus 10, or may be provided independently of the main control portion. The control portion 100 is connected with an LD driving circuit 61A, a motor driving circuit 63A, the light detecting portions 66, the first temperature detecting portion 67, and the second temperature detecting portion 68. The LD driving circuit 61A is configured to cause the light sources 61 to emit light, and the motor driving circuit 63A is configured to control the rotation of the motors63.
      The control portion 100 includes a CPU, a ROM, a RAM, and an EEPROM™. In addition, the control portion 100 includes a light source control portion101 and a correction processing portion 102. Specifically, the control portion 100 functions as the light source control portion 101 and the correction processing portion 102 by causing the CPU to execute processes in accordance with control programs stored in the ROM or the like. It is noted that the light source control portion 101 and the correction processing portion 102 may be configured as electric circuits.
      Each time the light detecting portion 66 of any of the image forming units 1– 4 detects a laser beam, the light source control portion 101 causes the light source 61 corresponding to the image forming unit to emit light corresponding to image data of a line at a predetermined emission start timing. For example, when the light detecting portion 66 of the image forming unit 1 detects a laser beam, the light source control portion 101 causes the light source 61corresponding to the image forming unit 1 to emit a laser beam corresponding to image data of one line after a predetermined time elapses since the detection. That is, in each of the image forming units 1– 4, a timing to write an image on the photoconductor drum 31 in the main scanning direction is determined by a timing at which a laser beam is detected by the light detecting portion 66.
      The correction processing portion 102 corrects the emission start timing based on the temperatures detected by the first temperature detecting portion67 and the second temperature detecting portion 68, the ambient temperature detected by the ambient temperature detecting portion 23, and a predetermined expression. Specifically, the correction processing portion 102 calculates a shift amount of the emission start timing based on a change of the temperature detected by the first temperature detecting portion 67, a difference between the temperatures detected by the first temperature detecting portion67 and the second temperature detecting portion 68 (temperature gradient), and the ambient temperature detected by the ambient temperature detecting portion 23, and corrects the emission start timing based on the calculated shift amount. It is noted that the difference between the temperatures detected by the first temperature detecting portion 67 and the second temperature detecting portion 68 is used as an index of the temperature gradient of the housings60.
      In addition, the correction processing portion 102 changes the content of the expression based on the operation state of the heater 35 at the start of the image formation process. Specifically, in a case where the heater 35 had been operated before the start of the image formation process, the ambient temperature of the laser scanning devices 6 is higher than a case where the heater 35 had not been operated. As a result, in a case where the heater 35had been operated before the start of the image formation process, the correction processing portion 102 changes the content of the expression so that a correction amount of the emission start timing becomes smaller than a case where the heater 35 had not been operated.
      In the present embodiment, a shift amount of a toner image formed by the image forming unit 3 for magenta with reference to a predetermined toner image formed by the image forming unit 4 for black is calculated as the shift amount of the emission start timing. Here, it is considered that the housings 60are similarly deformed due to temperature changes in the two laser scanning devices 6 of the image forming apparatus 10. As a result, it can be assumed that a shift amount of toner images of magenta and black is equal to a shift amount of a toner image formed by the image forming unit 1 for yellow with reference to a toner image formed by the image forming unit 2 for cyan. Accordingly, the correction processing portion 102 realizes a color shift correction by changing the emission start timings of the image forming units 1 and 3 with reference to the image forming units 2 and 4, based on the calculated shift amount of the emission start timing.
      The expression is represented by the following equation (1), wherein T 0 denotes a temperature detected by the first temperature detecting portion 67 at a predetermined reference time, T 1 denotes a current temperature detected by the first temperature detecting portion 67, T 2 denotes a current temperature detected by the second temperature detecting portion 68, T 3 denotes a current temperature detected by the ambient temperature detecting portion 23, d 1denotes a calculated color shift amount that is the shift amount of the emission start timing, and K 1, K 2 and A 1 denote predetermined coefficients.


d1= A1/ T3*( K1*( T1− T2)+ K2*( T1− T0))  (1)

      It is noted that the expression may be the following equation (2). It is noted that a coefficient A 2 is a predetermined coefficient, and may be the same value as the coefficient A 1 or a different value from the coefficient A 1.


d1=1/ T3*( A1* K1*( T1− T2)+ A2* K2*( T1− T0))  (2)

      The reference time is a time at which a color shift correction process called a color registration is executed by the control portion 100 when, for example, the image forming apparatus 10 is powered on, or the image forming apparatus 10 is resumed from a power saving mode. In the color shift correction process, for example, a predetermined toner image is actually formed on the intermediate transfer belt 5 by using the image forming units 1– 4, and a color shift amount is detected by using a density sensor or the like, and then the emission start timing of each of the image forming units 1– 4 is changed based on the color shift amount. That is, the temperature T 0 is stored in the RAM of the control portion 100 or the like, as a set value that indicates a temperature that was detected by the first temperature detecting portion 67 when a color shift had not been generated, and is deleted when, for example, the image forming apparatus 10 is powered off or transitions to the power saving mode.
      In addition, the coefficients K 1 and K 2 included in the expression are constants that are set in advance based on an experiment or a simulation. Here, an example of a method for setting the coefficients K 1 and K 2 is described with reference to FIG. 4 to FIG. 6FIG. 4 to FIG. 6 show a detected color shift amount d 2 and the temperatures T 1 and T 2 detected by the first temperature detecting portion 67 and the second temperature detecting portion 68, wherein the detected color shift amount d 2 indicates a color shift amount that was actually measured when printing had been executed intermittently after printing had been executed continuously for 18 minutes by the image forming apparatus 10.
      In addition, FIG. 6 shows a calculated color shift amount d 1 and a post-correction color shift amount d 3, the calculated color shift amount d 1 being calculated by substituting the temperatures T 1 and T 2 detected by the first temperature detecting portion 67 and the second temperature detecting portion68 for the equation (1), the post-correction color shift amount d 3 indicating a difference (d 2−d 1) between the calculated color shift amount d 1 and the detected color shift amount d 2. The post-correction color shift amount d 3 represents a value of a color shift amount that remains after a color shift correction is performed based on the calculated color shift amount d 1.
      The table of FIG. 6 shows data of a spreadsheet program created to reference calculation results obtained from the equation (1). The spreadsheet program is configured to automatically calculate and display the calculated color shift amount d 1 and the post-correction color shift amount d 3 upon input of arbitrary values of the coefficients K 1 and K 2.
      Specifically, FIG. 6 shows a calculation result for a case where the coefficient K 1 was set to “0.55”, and the coefficient K 2 was set to “−0.10”. The values of the coefficients K 1 and K 2 were obtained as a result of searching for the coefficients K 1 and K 2 that make the difference between the calculated color shift amount d 1 and the detected color shift amount d 2 smallest, while changing values of the coefficients K 1 and K 2 by using the data of the spreadsheet program shown in FIG. 6. That is, when the coefficients K 1 and K 2 have these values, the post-correction color shift amount d 3 has its smallest value. In the image forming apparatus 10, the values of the coefficients K 1 and K 2 that make the difference between the calculated color shift amount d 1 and the detected color shift amount d 2 smallest, are set for each model of the image forming apparatus 10 in advance and stored in the ROM of the control portion 100 or the like.
      In particular, in the image forming apparatus 10, the values of the coefficients K 1 and K 2 are set in advance for each of different situations: a situation where the heater 35 had been operated before the start of the image formation process; and a situation where the heater 35 had not been operated before the start of the image formation process. Specifically, coefficients K 11 and K 12 are set as values of the coefficients K 1 and K 2 that correspond to the case where the heater 35 had been operated before the start of the image formation process. In addition, coefficients K 21 and K 22 are set as values of the coefficients K 1 and K 2 that correspond to the case where the heater 35 had not been operated before the start of the image formation process. For example, in a case where the temperature gradient of the housings 60 during execution of the image formation process is gentler when the heater 35 had been operated than when the heater 35 had not been operated, the housings 60 have been deformed by a small amount, and it is required to correct the emission start timing only by a small amount. As a result, in the present embodiment, the coefficient K 11 is smaller than the coefficient K 21, and the coefficient K 12 is smaller than the coefficient K 22.
      Furthermore, the expression includes the ambient temperature T 3, and the correction amount of the emission start timing changes depending on the ambient temperature T 3. Specifically, according to the expression, the higher the ambient temperature T 3 is, the smaller the amount of correction of the emission start timing by the correction processing portion 102 is; and the lower the ambient temperature T 3 is, the larger the amount of correction of the emission start timing by the correction processing portion 102 is. In addition, the coefficients A 1 and A 2 are set in advance to allow the ambient temperature T 3 to be reflected in the calculation of the calculated color shift amount d 1, and the degree of influence of the ambient temperature T 3 on the calculated color shift amount d 1 changes depending on the values of the coefficients A 1 and A 2.
      [Image Formation Control Process]
      Next, an image formation control process that is executed by the control portion 100 in the image forming apparatus 10, is described with reference toFIG. 7. It is noted that the control portion 100 executes the image formation control process when, for example, a print job is received from an external information processing apparatus such as a personal computer.
      <Step S 1>
      In step S 1, the control portion 100 determines whether or not a temperature detected by the first temperature detecting portion 67 while the color registration was executed in the image forming apparatus 10, has been set as the temperature T 0. Here, when it is determined that the temperature T 0 has not been set (S 1; No), the process moves to step S 2, and when it is determined that the temperature T 0 has been set (S 1; Yes), the process moves to step S 3.
      <Step S 2>
      In step S 2, the control portion 100 obtains the temperature T 1 of the scanning lenses 64 by using the first temperature detecting portion 67, and sets the temperature T 1 as the temperature T 0. That is, in this case, the reference time corresponding to the temperature T 0 in the expression is the start time of the image formation process. With this configuration, even before the color registration is executed, the color shift correction is performed by taking into consideration a temperature rise that is caused due to execution of the image formation process, based on the current temperatures, and the color shift is restricted.
      <Step S 3>
      In step S 3, the control portion 100 obtains the current temperature T 1 of the scanning lenses 64, the current temperature T 2 of the light detecting portions 66, and the ambient temperature T 3 by using the first temperature detecting portion 67, the second temperature detecting portion 68, and the ambient temperature detecting portion 23. The temperatures T 1 and T 2 gradually increase during execution of the image formation process in the image forming apparatus 10, and gradually decrease after the image formation.
      <Step S 4>
      In step S 4, the control portion 100 determines whether or not the heater 35 had been operated before the start of the image formation process. Here, when it is determined that the heater 35 had been operated (S 4: Yes), the process moves to step S 41, and when it is determined that the heater 35 had not been operated (S 4: No), the process moves to step S 5. For example, in the image forming apparatus 10, in a case where the heater function has been set to an effective state, the heater 35 is operated before the start of the image formation process, and in a case where the heater function has been set to an ineffective state, the heater 35 is not operated before the start of the image formation process. As a result, in step S 4, it may be determined whether or not the heater 35 had been operated before the start of the image formation process, based on whether the heater function has been set to the effective state or to the ineffective state. In addition, in step S 4, it may be determined whether or not the heater 35 had been operated over a predetermined time period immediately before the start of the image formation process. The predetermined time period is a time period that is set in advance as an index for determining whether or not the temperature has increased by the heating operation of the heater 35 to a temperature at which it is appropriate to use the coefficients K 21 and K 22 in the expression.
      It is noted that in the present embodiment, the heater 35 is taken as an example of a heater that influences change of the ambient temperature of the laser scanning devices 6. On the other hand, the heater 35 may be, for example, a heater that is used in the sheet feed cassette 21 or the sheet feed portion22 to heat a sheet before image transferring. In this case, the heater 35 is disposed, for example, between the laser scanning devices 6 and the sheet feed cassette 21. It is noted that in a case where both a heater 35 for heating the photoconductor drums 31 and a heater 35 for heating a sheet before image transferring are provided, it may be determined in step S 4 whether or not at least one of the heaters 35 had been operated.
      <Step S 41>
      In step S 41, the control portion 100 sets the coefficient K 1 that is used in the expression, to the coefficient K 11, and sets the coefficient K 2 to the coefficient K 12. It is noted that as another embodiment, only one of the coefficient K 1 and the coefficient K 2 may be changed.
      <Step S 5>
      On the other hand, in step S 5, the control portion 100 sets the coefficient K 1 that is used in the expression, to the coefficient K 21, and sets the coefficient K 2 to the coefficient K 22. It is noted that as another embodiment, only one of the coefficient K 1 and the coefficient K 2 may be changed.
      As described above, in the image formation control process, the control portion 100 changes a correction amount of the emission start timing by changing the content of the expression, namely, changing the values of the coefficients K 1 and K 2 depending on the operation state of the heater 35 at the start of the image forming process. It is noted that this process is executed by the correction processing portion 102 of the control portion 100. This allows the calculated color shift amount d 1 to be calculated with a high accuracy from the expression by taking into consideration the ambient temperature of the laser scanning devices 6 that changes depending on the operation state of the heater 35 at the start of the image forming process, and the correction accuracy of the emission start timing is increased.
      <Step S 6>
      In step S 6, the control portion 100 calculates the calculated color shift amount d 1 based on the temperatures T 1 and T 2 obtained from the first temperature detecting portion 67 and the second temperature detecting portion 68, the ambient temperature T 3 obtained from the ambient temperature detecting portion 23, and the expression represented by the equation (1). This makes it possible to obtain an assumed value of a color shift amount that is generated when a color image is formed by the image forming units 1– 4 at the current point in time.
      <Step S 7>
      In step S 7, the control portion 100 corrects the emission start timing in each of the image forming units 1– 4 based on the calculated color shift amount d1 calculated in the step S 6. It is noted that this process is executed by the correction processing portion 102 of the control portion 100. Specifically, in the present embodiment, the correction processing portion 102 corrects the emission start timings of the image forming units 1 and 3 among the image forming units 1– 4, based on the calculated color shift amount d 1. It is noted that the emission start timing is defined by an elapsed time since detection of a laser beam by the light detecting portions 66.
      <Step S 8>
      In step S 8, the control portion 100 executes the image forming process once (for one sheet) in the print job by using the image forming units 1– 4, based on the emission start timing corrected in the step S 7. More specifically, the image forming process includes a process in which, each time the light detecting portion 66 detects a laser beam, the control portion 100 causes the light source 61 to emit, at the emission start timing, light corresponding to image data of a line. It is noted that this process is executed by the light source control portion 101 of the control portion 100.
      <Step S 9>
      In step S 9, the control portion 100 determines whether or not the print job is completed. Here, until the print job is completed (S 9: No), the process is returned to the step S 3, and when the print job is completed (S 9: Yes), the color shift correction process is ended. That is, the correction of the emission start timing performed in the steps S 3 to S 7 is executed each time the image forming process is performed once during the execution of the print job. It is noted that as another embodiment, the processes of the steps S 3 to S 7 may be executed each time the image forming process is performed a predetermined number of times such as several times to several tens of times (several sheets to several tens of sheets).
      As described above, in the image forming apparatus 10, even in a case where the temperature in the image forming apparatus 10 gradually increases during execution of a print job, a color shift is restricted by a color shift correction based on the expression, without executing the color registration by interrupting the image forming process in the middle of the print job. In addition, in a case where a print job is executed while the temperature in the image forming apparatus 10 gradually decreases after execution of a print job, a color shift is restricted since a color shift correction is performed based on the expression, without executing the color registration.
      In the image forming apparatus 10, the calculated color shift amount d 1 is calculated based on the temperatures T 1 and T 2 detected by the first temperature detecting portion 67 and the second temperature detecting portion 68, and the expression represented by the equation (1), and the color shift correction is executed based on the calculated color shift amount d 1. As a result, positional shifts of the scanning light beams in the main scanning direction are corrected with high accuracy, without using individual correction tables corresponding to situations such as a temperature increase and a temperature decrease. In addition, in the image forming apparatus 10, a change of the color shift amount that is generated by the ambient temperature T 3 of a place where the image forming apparatus 10 is installed and the operation state of the heater 35 (the ambient temperature of the laser scanning devices 6), is taken into consideration. As a result, a positional shift of a scanning light beam in the main scanning direction is corrected with higher accuracy.
       FIG. 8 shows an amount of color shift of a magenta image from a black image in a case where the color shift correction process had not been executed by the correction processing portion 102 in the image forming apparatus 10. On the other hand, FIG. 9 shows an amount of color shift of a magenta image from a black image in a case where the color shift correction process had been executed by the correction processing portion 102 in the image forming apparatus 10. It is noted that the color shift amounts shown in FIG. 8 and FIG. 9 are average values of color shift amounts at a left end, a center, and a right end in the main scanning direction. As shown in FIG. 8, in the case where the color shift correction process had not been executed, a color shift of approximately three pixels occurred at maximum. On the other hand, as shown in FIG. 9, in the case where the color shift correction process had been executed, the color shift was restricted to approximately less than one pixel.
      In addition, in the present embodiment, the first temperature detecting portion 67 is provided in one of the two laser scanning devices 6, and the second temperature detecting portion 68 is provided in the other. That is, each of the laser scanning devices 6 is provided with one temperature detecting portion. With this configuration, it is possible to mount a common circuit board on the laser scanning devices 6. It is noted that the circuit board is common to a circuit board on which the light sources 61 are mounted, or common to a circuit board on which the driving circuit of the motor 63 is mounted.
      As another embodiment, in the two laser scanning devices 6, a plurality of first temperature detecting portions 67 may be provided in correspondence with a plurality of fθ lenses 64, and a plurality of second temperature detecting portions 68 may be provided in correspondence with a plurality of light detecting portions 66. In this case, for example, the coefficients K 1 and K 2 in the equation (1) may be provided individually in correspondence with the image forming units 1– 4, and the correction processing portion 102 may calculate the color shift amounts of the image forming units 1– 4 individually based on the first temperature detecting portions 67 and the second temperature detecting portions 68 that respectively correspond to the image forming units 1– 4. In addition, the correction processing portion 102 may realize the color shift correction by changing the emission start timing for each of the image forming units 1– 4. With this configuration, the color shift correction with a higher accuracy is realized than a case where one first temperature detecting portion 67and one second temperature detecting portion 68 are provided.
      Meanwhile, the positions of the first temperature detecting portion 67 and the second temperature detecting portion 68 described in the present embodiment are not limited to the positions shown in FIG. 2 as far as they are in the vicinities of the fθ lenses 64 and the light detecting portions 66. For example, the first temperature detecting portion 67 may be located at another position as far as the position is suited for detection of the temperature of the fθ lenses 64. For example, the first temperature detecting portion 67 may be located at a position that is opposite to the polygon mirrors 62 when viewed from the fθ lenses 64 and does not interfere with the laser beams. This allows the first temperature detecting portion 67 to detect the temperature of the fθ lenses 64 at a position where it is hardly influenced by the heat of the polygon mirrors 62. As a further example, the first temperature detecting portion 67may be disposed at a position that is between the polygon mirror 62 and the fθ lens 64 and does not interfere with the laser beams.
      In addition, the present embodiment describes a case where the second temperature detecting portion 68 detects the temperature of the light detecting portions 66. On the other hand, the laser scanning device 6 may be provided with a reflection mirror for detection that reflects light incident on a predetermined position in the scanning path of light scanned by the polygon mirror 62, toward the light detecting portion 66. In this case, the second temperature detecting portion 68 may be disposed at a position where it can detect the temperature of the reflection mirror for detection, and the temperature of the reflection mirror for detection may be used in place of the temperature of the light detecting portion 66.
      Furthermore, the temperature detected by the first temperature detecting portion 67 is not limited to the temperature of the fθ lenses 64. Specifically, the first temperature detecting portion 67 and the second temperature detecting portion 68 may be disposed at any positions as far as the temperature gradient of the housings 60 of the laser scanning devices 6 can be detected. For example, the first temperature detecting portion 67 and the second temperature detecting portion 68 may be disposed at separate positions such as a vicinity of a center and a vicinity of an outer circumference of the laser scanning devices 6. Furthermore, with regard to each of the first temperature detecting portion 67 and the second temperature detecting portion 68, the number thereof is not limited to one, but may be two or more. That is, the first temperature detecting portion 67 or the second temperature detecting portion 68 may be disposed at three or more separate positions in the laser scanning device 6.
      According to the present embodiment, the correction processing portion 102 changes the content of correction depending on both the ambient temperature T 3 and the operation state of the heater 35. On the other hand, as another example, the correction processing portion 102 may change the content of correction depending on either the ambient temperature T 3 or the operation state of the heater 35. For example, the steps S 4, S 41, and S 5may be omitted, and the coefficient K 1 and the coefficient K 2 may be set to be constant. In addition, in step S 6, the calculated color shift amount d 1 may be calculated based on the following equation (3) in which the ambient temperature T 3 is not taken into consideration.


d1= K1*( T1− T2)+ K2*( T1− T0)  (3)

      Furthermore, the ambient temperature of the laser scanning devices 6 changes depending on the length of heating time period during which the heater35 had been operated before the start of the image formation process. Thus, as another embodiment, the length of heating time period may be reflected on the result of the expression. For example, in step S 6, the calculated color shift amount d 1 may be calculated based on the following equation (4) or equation (5). It is noted that t 1 denotes the length of heating time period during which the heater 35 had been operated before the start of the image formation process, and B 1 and B 2 denote predetermined coefficients.


d1= B1/ t1*( K1*( T1− T2)+ K2*( T1− T0))  (4)
d1=1/ t1*( B1* K1*( T1− T2)+ B2* K2*( T1− T0))  (5)

Category: I