Patent Publication Number: US-11392054-B2

Title: Optical scanning apparatus having plural phase control units for a respective plurality of rotating polygonal mirrors and image forming apparatus therewith

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an optical scanning apparatus for use in an electrophotographic image forming apparatus and the like, and an image forming apparatus. 
     Description of the Related Art 
     Some optical scanning apparatuses used in image forming apparatuses include, for controlling rotation of a polygon mirror (rotating polygonal mirror), both a rotation control circuit for performing rotation control for rotating a polygon mirror at a constant speed and a phase control circuit for controlling a rotation phase of the polygon mirror. For example, in the case where an optical scanning apparatus includes a plurality of polygon mirrors respectively corresponding to a plurality of image carriers, both of such a rotation control circuit and a phase control circuit are provided for each of the polygon mirrors. In this case, it is generally so configured that each phase control circuit independently performs the phase control of the polygon mirror thereof using as a reference a phase reference signal which is common to all of the polygon mirrors (for example, see Japanese Patent Laid-Open No. 2003-127456). 
     However, if the rotation control of the plurality of polygon mirrors is performed by such phase controls that independently control the polygon mirrors, it would possibly take a long time before the rotation phases of the polygon mirrors come to a target phase. As a result of this, the image forming apparatus would possibly take a long time before outputting a first page, that is, would possibly take a long First Page Out Time (FPOT). 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the aforementioned problems and provides a technique for shortening the time necessary for the phase control of a plurality of polygon mirror in an optical scanning apparatus. 
     According to one aspect of the present invention, there is provided an optical scanning apparatus, comprising: a first rotating polygonal mirror to which a light beam is irradiated, and that deflects the light beam such that the light beam scans a surface of a first image carrier; a second rotating polygonal mirror to which a light beam is irradiated, and that deflects the light beam such that the light beam scans a surface of a second image carrier; a first detection unit provided on a scanning path of the light beam deflected by the first rotating polygonal mirror, and that outputs a first detection signal when the light beam is made incident on the first detection unit; a second detection unit provided on a scanning path of the light beam deflected by the second rotating polygonal mirror, and that outputs a second detection signal when the light beam is made incident on the second detection unit; a first phase control unit that performs phase control of the first rotating polygonal mirror based on the first detection signal output from the first detection unit and a first target phase; and a second phase control unit that performs phase control of the second rotating polygonal mirror based on the second detection signal output from the second detection unit and a second target phase, which is set with respect to the first target phase. 
     According to another aspect of the present invention, there is provided an image forming apparatus, comprising: a first image carrier for use in formation of an image of a first color; a second image carrier for use in formation of an image of a second color; and an optical scanning apparatus comprising: a first rotating polygonal mirror to which a light beam is irradiated, and that deflects the light beam such that the light beam scans a surface of the first image carrier; a second rotating polygonal mirror to which a light beam is irradiated, and that deflects the light beam such that the light beam scans a surface of the second image carrier; a first detection unit provided on a scanning path of the light beam deflected by the first rotating polygonal mirror, and that outputs a first detection signal when the light beam is made incident on the first detection unit; a second detection unit provided on a scanning path of the light beam deflected by the second rotating polygonal mirror, and that outputs a second detection signal when the light beam is made incident on the second detection unit; a first phase control unit that performs phase control of the first rotating polygonal mirror based on the first detection signal output from the first detection unit and a first target phase; and a second phase control unit that performs phase control of the second rotating polygonal mirror based on the second detection signal output from the second detection unit and a second target phase, which is set with respect to the first target phase, and wherein the optical scanning apparatus forms an electrostatic latent image on the first image carrier by exposing the first image carrier with the light beam deflected by the first rotating polygonal mirror, and forms an electrostatic latent image on the second image carrier by exposing the second image carrier with the light beam deflected by the second rotating polygonal mirror. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart illustrating a control procedure performed by an engine controller. 
         FIG. 2  is a cross-sectional view illustrating a configuration example of a laser printer. 
         FIG. 3  is a perspective view illustrating a configuration example of a scanner unit. 
         FIGS. 4A to 4C  illustrate examples of color misregistration correction. 
         FIG. 5  is a block diagram illustrating an example of a control configuration of scanner units. 
         FIGS. 6A and 6B  are timing charts for signals relating to a phase control unit. 
         FIG. 7  is a flowchart illustrating a control procedure performed by an engine controller (Second Embodiment). 
         FIGS. 8A and 8B  are timing charts for signals relating to a phase control unit (Second Embodiment). 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments will be described in detail with reference to the attached drawings. Note, the following embodiments are not intended to limit the scope of the claimed invention. Multiple features are described in the embodiments, but limitation is not made an invention that requires all such features, and multiple such features may be combined as appropriate. Furthermore, in the attached drawings, the same reference numerals are given to the same or similar configurations, and redundant description thereof is omitted. 
     First Embodiment 
     In a first embodiment, an example in which phase control of a polygon mirror of a scanner unit for firstly forming an electrostatic latent image on a photosensitive drum is used as a reference for phase controls of the polygon mirrors of the other scanner units will be described. In the following, the first embodiment will be described with reference to  FIGS. 1 to 6B . 
     &lt;Image Forming Apparatus&gt; 
       FIG. 2  is a cross-sectional view illustrating an example of a hardware configuration of a laser printer  201 , which is an image forming apparatus according to the present embodiment. The laser printer  201  is communicably connected with an external apparatus  202  such as a personal computer (PC) via a general-purpose interface  215 . The laser printer  201  includes a video controller  203 , an engine controller  204 , and scanner units  205  and  206 . 
     The laser printer  201  is a color laser printer which is a four-drum type and is configured to form toner images of four colors using four colors of toner, namely Yellow (Y), Magenta (M), Cyan (C), and Black (BK), and to form a color image by overlapping these toner images. The laser printer  201  includes four image forming units that respectively form toner images of Y, M, C and BK for forming the toner images of four colors. The four image forming units respectively include toner cartridges  207  to  210  storing Y, M, C and BK toner (developing material), photosensitive drums  301  to  304  respectively for the toner colors, charging rollers  305  to  308  respectively for the toner colors, and developing units  309  to  312  respectively for the toner colors. The photosensitive drums  301  to  304  are one example of photosensitive members functioning as image carriers. 
     The scanner unit  205  is used for forming a Y image and an M image, whereas the scanner unit  206  is used for forming a C image and a BK image. Thus, the scanner unit  205  constitutes part of the image forming units for forming the Y image and the M image, and the scanner unit  206  constitute part of the image forming units for forming the C image and the BK image. 
     The scanner unit  205  includes laser light sources (laser diodes (LDs)  401  and  402  in  FIG. 3 ) that generate light beams by laser (laser beams) as light beams for performing exposure of the photosensitive drums  301  and  302 , and a polygon mirror (hereinafter, which may be referred to as a “PM”)  213 . The scanner unit  205  is configured to irradiate the laser beams onto the photosensitive drums  301  and  302 . In the present embodiment, the PM  213  is one example of a first rotating polygonal mirror on which a light beam is irradiated, and configured to deflect the light beam such that the light beam scans a surface of the photosensitive drum  301  (first image carrier). 
     The scanner unit  206  includes laser light sources that generate laser beams as light beams for performing exposure of the photosensitive drums  303  and  304 , and a PM  214 . The scanner unit  206  is configured to radiate the laser beams onto the photosensitive drums  303  and  304 . In the present embodiment, the PM  214  is one example of a second rotating polygonal mirror on which a light beam is irradiated, and configured to deflect the light beam such that the light beam scans a surface of the photosensitive drum  302  (second image carrier). 
     If the external apparatus  202  gives a print start instruction to the laser printer  201  via the general-purpose interface  215 , the engine controller  204  controls the image forming units to perform image formation according to the print start instruction. The following operations are performed by the image forming units. The charging rollers  305  to  308  are configured to uniformly charge the surfaces of the photosensitive drums  301  to  304 , respectively. After that, the scanner units  205  and  206  expose the surface of the photosensitive drums  301  to  304  with the laser beams based on image signals output from the video controller  203 , thereby forming electrostatic latent images on the photosensitive drums  301  to  304 , respectively. 
     Next, the developing units  309  to  312  develop (visualize) the electrostatic latent images thus formed on the corresponding photosensitive drums  301  to  304  with the toner stored in the corresponding toner cartridges  207  to  310 . In this way, toner images of the respective colors are formed on the photosensitive drums  301  to  304 . The toner images thus formed on the photosensitive drums  301  to  304  are transferred onto an intermediate transfer belt  211  such that the toner images are overlapped on one another in order. In this way, a color image is formed on the intermediate transfer belt  211 . 
     The color image thus formed on the intermediate transfer belt  211  is transferred onto a recording sheet by a transfer roller  315 . Specifically, the recording sheet in a cassette  313  is fed into a conveyance path by a paper feeding roller  314 . After that, onto the recording sheet thus conveyed via the conveyance path, the color image is transferred from the intermediate transfer belt  211  by the transfer roller  315 . The recording sheet with the color image transferred thereon is conveyed to a fixing unit  316 . The fixing unit  316  fixes the color image onto the recording sheet. After that, the recording sheet is discharged onto a paper discharge tray  317 . 
     The laser printer  201  includes a registration detecting sensor  212  for use in reading a color misregistration detection pattern being formed on the intermediate transfer belt  211 . The registration detecting sensor  212  includes a light receiving sensor configured to receive reflection light reflected from the color misregistration detection pattern as a result of light being irradiated from the light source to the color misregistration detection pattern. The temporal changes in an intensity of an output signal of the registration detecting sensor  212  (light receiving sensor) when the color misregistration detection pattern formed on the intermediate transfer belt  211  passes through a detection region of the registration detecting sensor  212  are obtained as color misregistration information. The color misregistration information is used for color misregistration correction described later. 
     &lt;Configuration of the Scanner Unit&gt; 
       FIG. 3  is a perspective view illustrating one configuration example of the scanner units  205  and  206  according to the present embodiment. The scanner units  205  and  206  have an identical configuration. In the following, the scanner unit  205  will be described, but the same description is applicable to the scanner unit  206  as well. 
     The scanner unit  205  includes the LD (laser diodes)  401  and  402 , which are semiconductor lasers. The LDs  401  and  402  are used as the light sources for the exposure of the photosensitive drums  301  and  302 , respectively. 
     The laser beam output from the LD  401  is reflected on a reflection surface of the PM  213 , and further reflected on a reflection mirror  403 , thereby being irradiated to the surface of the photosensitive drum  301 . The laser beam output from the LD  402  is irradiated to one reflection surface among reflection surfaces of the PM  213 , the reflection surface being a reflection surface to which the laser beam output from the LD  401  is not irradiated. The laser beam reflected on this reflection surface is further reflected on a reflection mirror  404 , thereby being irradiated to the surface of the photosensitive drum  302 . In the present embodiment, the LDs  401  and  402  each include a plurality of light emitting points (light emitting elements) each configured to output a laser beam (light beam), and is a laser light source capable of outputting a plurality of laser beams at once. 
     A BD (beam detection) sensor  405  is a light sensor used as a main scanning synchronization sensor. The BD sensor  405  is provided on a scanning path of the laser beam output from the LD  402 , and is configured to output, when the laser beam is made incident on this sensor, a BD signal as a detection signal indicating detection of the laser beam. The BD signal is used in PM control units  501  and  511  (see  FIG. 5 , which will be described later) for rotation controls of the PMs  213  and  214  (more specifically, rotation controls of polygon motors (scanner motors) configured to respectively drive the PMs  213  and  214 ). Moreover, the BD signal is also used in the video controller  203  for controlling a timing to start writing in the main scanning direction. 
     &lt;Color Shift Correction&gt; 
       FIGS. 4A to 4C  illustrate examples of color misregistration correction performed in the laser printer  201  according to the present embodiment. This example describes a case where the LD  401  ( 402 ) outputs four laser beams, with which the scanning of the photosensitive drum  301  ( 302 ) is performed by using the reflection surfaces of the PM  213  ( 214 ). The scanning with the four laser beams is parallelly performed in four different lines (four main scanning lines aligned in parallel in the sub scanning direction and corresponding to four pixels aligned in the sub scanning direction) on the photosensitive drum  301  ( 302 ). Note that the following describes the image forming unit for the Y image because all of the image forming units for Y, M, C, and BK images have an identical configuration, and therefore the same description is applicable to the image forming units M, C, and BK images. 
       FIG. 4A  illustrates a state where the exposure of the photosensitive drum  301  is performed according to (in synchronization with) the BD signal obtained from the scanner unit  205  (BD sensor  405 ). Using as a reference a writing start signal in the sub scanning direction which is supplied from the engine controller  204 , the video controller  203  determines a timing to start writing in the sub scanning direction, and generates image signals. Specifically, in a period between receiving the writing start signal in the sub scanning direction and receiving one BD signal, the video controller  203  generates image signals for the four lines to be aligned in the sub scanning direction. After that, using as a reference a reception timing of the BD signal, the laser beams corresponding to the respective lines are irradiated to the photosensitive drum  301  based on the image signals for the four lines, thereby an electrostatic latent image being formed on the photosensitive drum  301 . As such, the timing of starting the writing in the main scanning direction for each line is controlled using the reception timing of the BD signal as a reference. 
       FIG. 4B  illustrates a state where the timing of the start of writing in the sub scanning direction is corrected by changing a rotation phase of the PM  213  by a phase control unit  503  ( FIG. 5 ) described later. The rotation phase of the PM  213  is controlled by the phase control unit  503  based on the BD signal as described later.  FIG. 4B  illustrates a case where the rotation phase of the PM  213  is changed such that the BD signal received from the scanner unit  205  (BD sensor  405 ) will be changed from the BD signal indicated with a broken line to the BD signal indicated with a solid line. As such, by delaying the timing of receiving the BD signal by controlling the rotation phase of the PM  213  (phase control), it is possible to delay the timing of starting the writing in the sub scanning direction. On the contrary, by advancing the timing of receiving the BD signal, it is possible to advance the timing of starting the writing in the sub scanning direction. With such control, where to start the writing in the sub scanning direction can be corrected by the unit of less than one pixel in the sub scanning direction. 
       FIG. 4C  illustrates a state where the timing of starting the writing in the sub scanning direction is delayed by one pixel (by 1 line) for the color misregistration correction. In this example, the video controller  203  corrects the image data such that the image data is shifted by one line to delay the timing of starting the writing in the sub scanning direction. In this way, where to start the writing in the sub scanning direction can be corrected by the unit of one pixel, by adjusting a timing of the light emission of the LD  401  ( 402 ) based on the image data. 
     &lt;Control Configuration of the Scanner Units&gt; 
       FIG. 5  is a block diagram illustrating an example of control configuration of the scanner units  205  and  206  according to the present embodiment. The part indicated by a broken line in  FIG. 5  is a part that is added in a second embodiment. As illustrated in  FIG. 5 , the laser printer  201  includes the PM control units  501  and  511 , a time difference measuring unit  507 , and a reference signal generating unit  520 , as constituent elements for controlling the scanner units  205  and  206 . The PM control unit  501  is configured to control the scanner unit  205  (PM  213 ), whereas the PM control unit  511  is configured to control the scanner unit  206  (PM  214 ). 
     The PM control unit  501  includes a rotation control unit  502  and a phase control unit  503 . The PM control unit  501  is configured to receive the BD signal  504  output from the BD sensor  405  of the scanner unit  205 , and perform rotation control and phase control of the PM  213  based on the BD signal  504  by using a rotation control signal  505  and a phase control signal  506 . In the present embodiment, the BD sensor  405  functions as one example of a first detection unit which is provided on the scanning path of the light beam deflected by the PM  213  (first rotating polygonal mirror) and which is configured to output a first BD signal (first detection signal) when the light beam is made incident on the first detection unit. 
     The rotation control unit  502  is configured to measure a cycle of the BD signal, which is equivalent to the rotation speed of the PM  213  (BD cycle), by using the BD signal  504 , and to generate the rotation control signal  505  that will control the rotation such that the BD cycle will approach a target cycle. The rotation control unit  502  performs speed control for making the rotation speed of the PM  213  be converged to a target of the rotation speed such that, if the BD cycle is longer than the target cycle, the rotation speed is accelerated, and if the BD cycle is shorter than the target cycle, the rotation speed is decelerated. 
     In the example above, the BD cycle is used for measuring the rotation speed of the PM  213 . However, the rotation speed of the PM  213  may be measured by using a signal width of the BD signal. Moreover, the rotation speed of the PM  213  may be measured by using an FG signal obtained from a detection element which is provided to a driving motor for rotating the PM  213  and which is configured to detect a magnetic pattern. 
     The phase control unit  503  is configured to generate the phase control signal  506  for control of the rotation phase of the PM  213  (for phase control), by using the phase reference signal  521  generated by the reference signal generating unit  520 , and the BD signal  504 . An operation of the phase control unit  503  will be described later. Note that, in the present embodiment, the phase control unit  503  functions as one example of a first phase control unit configured to perform the phase control of the PM  213  based on the first BD signal output from the BD sensor  405  and a target phase Tn 0 . 
     The PM control unit  511  has a configuration identical with that of the PM control unit  501 , and includes the rotation control unit  512  and the phase control unit  513 . The PM control unit  511  is configured to receive the BD signal  514  from the BD sensor  406 , and to perform the rotation control of the PM  214  based on the BD signal  514  by using the rotation control signal  515  and the phase control signal  516 . Note that, in the present embodiment, the BD sensor  406  is functions as one example of a second detection unit which is provided on the scanning path of the light beam deflected by the PM  214  (second rotating polygonal mirror), and which is configured to output a second BD signal (second detection signal) when the light beam is made incident on the second detection unit. Moreover, the phase control unit  513  functions as one example of a second phase control unit configured to perform the phase control of the PM  214  based on the second BD signal output from the BD sensor  406  and a target phase Tn 1 , which is set with respect to the target phase Tn 0 . 
     The reference signal generating unit  520  is configured to generate the identical phase reference signals  521  for the phase control units  503  and  513 . The phase reference signal  521  is a signal generated with a reference cycle, which is set by the engine controller  204 . The engine controller  204  sets the reference cycle for the reference signal generating unit  520  by outputting a reference cycle setting value to the reference signal generating unit  520 . 
     The time difference measuring unit  507  is configured to measure a time difference between the BD signal  504  and the phase reference signal  521  as a reception time difference ΔT 0 , and sends the measurement value thus obtained to the engine controller  204 . The engine controller  204  computes (sets) the target phase Tn 0  for the phase control unit  503  and the target phase Tn 1  for the phase control unit  513  by using the measurement value of the reception time difference ΔT 0  received from the time difference measuring unit  507 . The engine controller  204  performs the settings for the phase control units  503  and  513  by outputting the setting value of the target phase Tn 0  and the setting value of the target phase Tn 1  to the phase control units  503  and  513 , respectively. 
     &lt;Operation of Phase Control Units&gt; 
       FIGS. 6A and 6B  are timing charts for the signals related to the phase control units  503  and  513  according to the present embodiment. Because the phase control units  503  and  513  perform the identical operation, the following describes the phase control unit  503 , omitting the description for the phase control unit  513 . 
     The phase control unit  503  generates a target signal  508  for phase control of the BD signal  504 , by shifting the phase of the phase reference signal  521  by the target phase Tn 0 . Furthermore, the phase control unit  503  generates the phase control signal  506  such that the target signal  508  and the BD signal  504  match each other, and performs the control of the rotation phase of the PM  213  (performs phase control) by outputting the phase control signal  506  to the PM  213 . Note that the target phase Tn 0  is set for the phase control unit  503  by the engine controller  204 . The engine controller  204  sets the target phase Tn 0  based on detection data obtained from the registration detecting sensor  212 , and outputs the setting value of the target phase Tn 0  to the phase control unit  503 . 
     When the phase control unit  503  receives a control ON signal from the engine controller  204 , the phase control unit  503  starts counting of the BD signal by a BD signal counter and counting of the target signal  508  by a target signal counter. The BD signal counter is a counter for use in counting of the BD signal  504 , and the target signal counter is a counter for use in counting of the target signal  508 . 
     When both of the count values of the BD signal counter and the target signal counter reach the respective setting values set by the engine controller  204 , the phase control unit  503  measures a time difference (phase difference) between the target signal  508  and the BD signal  504  using a difference counter. If the measured phase difference becomes smaller than a predetermined value (that is, the phase difference between the target signal  508  and the BD signal  504  gets closer to zero), the phase control unit  503  ends the phase control of the PM  213 . Furthermore, the phase control unit  503  reports the engine controller  204  that the phase control of the PM  213  is complete (that is, the PM  213  is in a phase lock state). 
       FIG. 6A  illustrates, as one example, a case where the phase control unit  503  outputs a deceleration signal as the phase control signal  506 . Here, the setting values of the BD signal counter and the target signal counter are assumed to be zero. The measurement of the phase difference between the target signal  508  and the BD signal  504  is performed by using the target signal  508  obtained when the count value of the target signal counter reaches three, and the BD signal  504  obtained when the count value of the BD signal counter reaches three. In  FIG. 6A , the count value of the BD signal  504  reaches three (setting value) before the count value of the target signal  508  reaches three, and thus, a deceleration signal is output as the phase control signal  506  for delaying the phase of the BD signal  504 . In this example, it is assumed that the deceleration signal has a pulse width that is one-quarter of the phase difference. However, this is merely an example, and a ratio of the pulse width of the deceleration signal with respect to the pulse width of the phase difference may be determined based on, for example, properties of the polygon motor in practice. 
       FIG. 6B  illustrates, as one example, a case where the phase control unit  503  outputs an acceleration signal as the phase control signal  506 . In  FIG. 6B , the count value of the BD signal  504  reaches three (setting value) after the count value of the target signal  508  reaches three, and thus, an acceleration signal is output as the phase control signal  506  for advancing the phase of the BD signal  504 . In this example, it is assumed that the acceleration signal has a pulse width that is one-quarter of the phase difference. However, this is merely an example, and a ratio of the pulse width of the acceleration signal with respect to the pulse width of the phase difference may be determined based on, for example, properties of the polygon motor in practice. 
     In the example above, the setting values of the target signal counter and the BD signal counter, which define the timing of measuring the phase difference, are three. However, these setting values may be determined in consideration of, for example, the properties of the polygon motor and the signals output from the rotation control units  502  and  512 , for improving the measurement accuracy. 
     &lt;Control Procedure&gt; 
       FIG. 1  is a flowchart illustrating a control procedure performed by the engine controller  204  according to the present embodiment. The engine controller  204  obtains the detection data from the registration detecting sensor  212  by performing the color misregistration correction control, and, based on the detection data, sets the target phases Tn 0  and Tn 1  respectively for the BD signals  504  and  514  for the color misregistration correction. In this Example, assume that the target phase Tn 0  is set to T 0  and the target phase Tn 1  is set to T 1 . In this case, the phase control units  503  and  513  respectively perform the phase controls of the PMs  213  and  214  such that the phase difference of the BD signal  514  with respect to the BD signal  504  will be (T 1 −T 0 ). This phase difference (T 1 −T 0 ) is equivalent to the target phase difference for the phase controls of the phase control units  503  and  513 . After that, the engine controller  204  performs the processing of the control procedure illustrated in  FIG. 1 , when receiving a print start instruction from the external apparatus  202 . 
     To begin with, in step S 101 , the engine controller  204  activates the rotation control units  502  and  512  and the reference signal generating unit  520 . As a result of this, the rotation controls of the PMs  213  and  214  are started to be executed by the rotation control units  502  and  512  such that the PMs  213  and  214  will rotate at the rotation speeds corresponding to the predetermined target cycle of the BD signals. 
     Next, in step S 102 , the engine controller  204  performs a determination regarding the BD signal  504  (first BD signal) obtained from the scanner unit  205  corresponding to the image forming unit that performs the image formation firstly among the four image forming units that perform the image formation of the four colors in order. Specifically, the engine controller  204  determines whether or not the BD cycle of the BD signal  504  is within a predetermined error range from the target cycle (that is, whether or not the BD cycle has been converged to the target cycle). In the present embodiment, the image formation performed by the image forming unit for Y image (formation of an electrostatic latent image on the photosensitive drum  301 ) is performed first. Thus, the BD signal  504  output from the scanner unit  205  corresponding to the image forming unit for Y image is used for the determination. Note that, the predetermined error range may be set and changed by the engine controller  204 . If the engine controller  204  determines that the BD cycle has been converged to the target cycle, it advances the process to step S 103 . 
     In step S 103 , the engine controller  204  obtains, from the time difference measuring unit  507 , a measurement value of the reception time difference ΔT 0  between the BD signal  504  and the phase reference signal  521 . The reception time difference ΔT 0  is equivalent to the phase difference between the phase reference signal  521  and the BD signal  504 . 
     After that, in step S 104 , the engine controller  204  resets the target phases Tn 0  and Tn 1  as below, based on the measurement value of the reception time difference ΔT 0  thus obtained, and the phase difference (T 1 −T 0 ) between the target phases Tn 0  and Tn 1 .
 
 Tn 0=Δ T 0
 
 Tn 1=Δ T 0+( T 1− T 0)  (1)
 
     In this resetting, the target phase Tn 0  of the BD signal  504  (for the phase control unit  503 ) is set to the value equal to the reception time difference ΔT 0 . Furthermore, the target phase Tn 1  of the BD signal  514  (for the phase control unit  513 ) is set to ΔT 0 +(T 1 −T 0 ) so that the phase difference between the BD signal  504  and the BD signal  514  will be (T 1 −T 0 ), which is the target phase difference. 
     As described above, the engine controller  204  according to the present embodiment sets the target phase Tn 0  (first target phase) by using the reception time difference ΔT 0  measured by the time difference measuring unit  507 . Furthermore, the engine controller  204  sets the target phase Tn 1  (second target phase) such that the phase difference between the target phase Tn 0  and the target phase Tn 1  will be equal to the target phase difference (T 1 −T 0 ) with respect to the target phase Tn 0 . 
     As a result of resetting the target phases Tn 0  and Tn 1  as such, the target signal  508  for the phase control, which is generated in the phase control unit  503 , becomes a signal obtained by phase-shifting the phase reference signal  521  by the target phase Tn 0 . Specifically, as the result of the resetting, the target signal  508  becomes a signal having a phase difference of ΔT 0  with respect to the phase reference signal  521 . That is, the phase difference between the target signal  508  and the BD signal  504  will be zero. This means that the PM  213  is already in the phase lock state when the phase control of the PM  213  by the phase control unit  503  is started. 
     After the resetting of the target phases Tn 0  and Tn 1 , in step S 105 , the engine controller  204  activates the phase control unit  503  by inputting a control ON signal into the phase control unit  503  (first phase control unit). The phase control unit  503  generates the target signal  508  based on the target phase Tn 0  set by the engine controller  204  in step S 104 , and starts the phase control of the PM  213 . Specifically, the phase control unit  503  performs the rotation control of the PM  213  by using the phase control signal  506  such that the phase difference between the BD signal  504  and the target signal  508  corresponding to the target phase Tn 0  will get closer to zero. 
     However, at this timing, the phase difference between the target signal  508  and the BD signal  504  has been already smaller than the predetermined value and the PM  213  is in the phase lock state. Thus, the phase control unit  503  completes the phase control immediately after the activation. As a result, the engine controller  204  is notified from the phase control unit  503 , immediately after the activation of the phase control unit  503 , that the PM  213  is in the phase lock state. 
     If the engine controller  204  receives, from the phase control unit  503 , the information indicating that the PM  213  is in the phase lock state, the engine controller  204  transmits (outputs) the writing start signal in the sub scanning direction to the video controller  203  in step S 106 . As a result of this, the generation and output of the image signals for forming Y image and M image by the video controller  203  are started sequentially, and the formation of the Y image and M image onto the photosensitive drums  301  and  302  are started sequentially. As described above, in the present embodiment, since the phase control of the PM  213  included in the scanner unit  205  corresponding to the image forming unit for performing the first image formation is completed in a short time, it is possible to shorten FPOT. 
     After that, in step S 107  the engine controller  204  performs a determination regarding the BD signal  514  (second BD signal) obtained from the scanner unit  206  corresponding to the image forming units that performs the image formation next (after the image forming units for Y image and M image). Since, in the present embodiment, the image formation by the image forming units for C image and BK image (formation of electrostatic latent images on the photosensitive drums  303  and  304 ) are performed sequentially after the image forming units for Y image and M image, the BD signal  514  output from the scanner unit  206  corresponding to the image formation by the image forming units for C image and BK image is used for the determination. Specifically, the engine controller  204  determines whether or not the BD cycle of the BD signal  514  is within a predetermined error range from the target cycle (that is, whether or not the BD cycle has been converged to the target cycle), as in the determination regarding the BD signal  504 . If the engine controller  204  determines that the BD cycle has been converged to the target cycle, it advances the process to step S 108 . 
     In step S 108 , the engine controller  204  activates the phase control unit  513  (second phase control unit) by inputting the control ON signal into the phase control unit  513 . The phase control unit  513  generates the target signal  518  based on the target phase Tn 1 , which is set by the engine controller  204  in step S 104 , and starts the phase control of the PM  214 . Note that the target signal  518  is a signal obtained by phase-shifting the phase reference signal  521  by the target phase Tn 1 . Specifically, the phase control unit  513  performs the phase control of the PM  214  by using the phase control signal  516  such that the phase difference between the BD signal  514  and the target signal  518  corresponding to the target phase Tn 1  gets closer to zero. The phase control unit  513  completes the phase control of the PM  214  if the phase difference becomes smaller than the predetermined value, and reports that the PM  214  is in the phase lock state. 
     If the engine controller  204  receives, from the phase control unit  513 , the information indicating that the PM  214  is in the phase lock state, and if a predetermined time has passed from the start timing of the formation of the Y image and M image, the engine controller  204  starts the formation of the C image and the BK image. As such, some time lag is necessary from the activation of the phase control unit  513  to the completion of the phase control of the PM  214  (bringing the PM  214  into the phase lock state). However, the phase control of the PM  214  by the phase control unit  513  is performed based on the phase difference with respect to the rotation phase of the PM  213  (the phase difference of the BD signal  514  with respect to the BD signal  504 ). This makes it possible to shorten the time necessary for the phase control, thereby making it possible to complete the phase control of the PM  214  before the predetermined time has passed from the start timing of the formation of the Y image and M image. As a result, it is possible to realize FPOT as short as that of a case where the phase control of the polygon mirror is not performed. 
     Even though the present embodiment describes the configuration example in which the two scanner units  205  and  206  are used, any number of scanner units as long as two or more scanner units may be provided. For example, it may be so configured that one scanner unit is provided for each one of the photosensitive drums. Moreover, the present embodiment describes the case where all of the polygon mirrors are started and driven at the same time, a driving sequence in which the polygon mirrors corresponding to the image forming units starting the image formation earlier are started and driven earlier may be employed. 
     As described above, in the optical scanning apparatus according to the present embodiment, the phase control unit  503  corresponding to the scanner unit  205  performs the phase control of the PM  213  based on the BD signal  504  output from the BD sensor  405  and the target phase Tn 0 . Moreover, the phase control unit  513  corresponding to the scanner unit  206  performs the phase control of the PM  214  based on the BD signal  514  output from the BD sensor  406  and the target phase Tn 1 , which is set with respect to the target phase Tn 0 . As such, using as a reference the phase control of the PM  213  of the scanner unit  205 , which forms the electrostatic latent image on the photosensitive drum firstly, the phase control of the PM  214  of the scanner unit  206 , which is another scanner unit than the scanner unit  205 , is performed. 
     Specifically, the engine controller  204  measures the time difference ΔT 0  between the phase reference signal  521  and the BD signal  504  and generates the target signal  508  based on ΔT 0 , regarding the PM  213  of the scanner unit  205 , which forms the electrostatic latent image on the photosensitive drum  301  firstly. After that, the engine controller  204  generates the target signal  518  for the PM  214  (another PM) such that the target signal  518  has a time difference with respect to the target signal  508 , the time difference being equivalent to the target phase difference. The phase control units  503  and  513  perform the phase controls of the PMs  213  and  214  by using the target signals  508  and  518 , respectively. According to the present embodiment, it is possible to shorten the time necessary for the phase controls of the PM  213  and PM  214 . As a result, it becomes possible to shorten the FPOT of the laser printer  201 . 
     Second Embodiment 
     Second embodiment describes an example in which the phase control of each polygon mirror is performed in consideration of a time period from the start of the formation of the electrostatic latent image by the scanner unit that firstly forms the electrostatic latent image on the photosensitive drum, to the start of the formation of the electrostatic latent image by another scanner unit. In the following, the second embodiment will be described with reference to  FIGS. 5, 7, 8A, and 8B . The second embodiment will mainly describe parts different from the first embodiment, omitting the description on the parts common to the first embodiment. 
     &lt;Control Configuration of the Scanner Units&gt; 
     The scanner units  205  and  206  according to the present embodiment have a control configuration basically similar to that of the first embodiment as illustrated in  FIG. 5 , except that the scanner units  205  and  206  according to the present embodiment further has a part indicated with the broken line in  FIG. 5 . Specifically, the laser printer  201  further includes a time difference measuring unit  517 . Moreover, the BD signal  514  output from the BD sensor  406  (scanner unit  206 ) is also input into the time difference measuring unit  517  and the engine controller  204 , and the phase reference signal  521  output from the reference signal generating unit  520  is also input into the time difference measuring unit  517 . 
     The time difference measuring unit  517  has a function similar to the time difference measuring unit  507 . The time difference measuring unit  517  measures the time difference between the BD signal  514  and the phase reference signal  521  as a reception time difference ΔT 1 , and sends a measurement value of the reception time difference ΔT 1  to the engine controller  204 . The engine controller  204  calculates (sets) the target phase Tn 0  for the phase control unit  503  and the target phase Tn 1  for the phase control unit  513 , by using the measurement values of the reception time differences ΔT 0  and ΔT 1  respectively received from the time difference measuring units  507  and  517 . The engine controller  204  performs settings for the phase control units  503  and  513  by outputting the setting values of the target phases T 0  and T 1  to the phase control units  503  and  513 , respectively. 
     &lt;Control Procedure&gt; 
       FIG. 7  is a flowchart illustrating a control procedure performed by the engine controller  204  according to the present embodiment. As in the first embodiment, the engine controller  204  obtains the detection data from the registration detecting sensor  212  by performing the color misregistration correction control, and sets the target phase Tn 0  for the BD signal  504  and the target phase Tn 1  for the BD signal  514  for the color misregistration correction. As in the first embodiment, assume that the target phase Tn 0  is set to T 0 , and the target phase Tn 1  is set to T 1 . If the engine controller  204  is instructed by the external apparatus  202  to start printing, the engine controller  204  performs the process according to the control procedure illustrated in  FIG. 7 . 
     To begin with, in step S 201 , the engine controller  204  activates the rotation control units  502  and  512 , and the reference signal generating unit  520 . As a result of this, the rotation controls of the PMs  213  and  214  are started to be executed by the rotation control units  502  and  512  such that the PMs  213  and  214  will rotate at the rotation speed corresponding to the predetermined target cycle of the BD signals. 
     Next, in step S 202 , the engine controller  204  performs a determination regarding the BD signal  504  (first BD signal) and the BD signal  514  (second BD signal) obtained respectively from the scanner units  205  and  206 . Specifically, the engine controller  204  determines, for each of the BD signals  504  and  514 , whether or not the BD cycle is within a predetermined error range from the target cycle (that is, whether or not the BD cycle has been converged to the target cycle), as in steps S 102  and S 107  in the first embodiment. If the engine controller  204  determines, for the BD cycles of both the BD signals  504  and  514 , that the BD cycle has been converged to the target cycle, it advances the process to step S 203 . 
     In step S 203 , the engine controller  204  obtains the measurement value of the reception time difference ΔT 0  between the BD signal  504  and the phase reference signal  521 , from the time difference measuring unit  507 . Furthermore, the engine controller  204  obtains the measurement value of the reception time difference ΔT 1  between the BD signal  514  and the phase reference signal  521 , from the time difference measuring unit  517 . Here, the reception time difference ΔT 0  is equivalent to the phase difference of the BD signal  504  with respect to the phase reference signal  521 , and the reception time difference ΔT 1  is equivalent to the phase difference of the BD signal  514  with respect to the phase reference signal  521 . 
     In step S 204 , the engine controller  204  determines whether or not the phase control of the PM  214  can be completed within a time period (delay time) from the timing of starting the formation of the Y image which is to be formed firstly, to the timing of starting the formation of the C image which is to be formed firstly by the scanner unit  206 . The engine controller  204  stores, in advance, a maximum value ΔTmax of the phase difference for the BD signal  514 , where the maximum value ΔTmax is such a value that the phase control unit  513  can complete the phase control of the PM  214  (that is, the PM  214  can be brought to the phase lock state) within the delay time from the timing of starting the formation of the Y image to the timing of starting the formation of the C image. 
     The maximum value ΔTmax stored by the engine controller  204  is equivalent to a maximum value of the phase difference between the BD signal  514  and the target signal  518  corresponding to the target phase Tn 1 , where the maximum value is such a value that the phase control by the phase control unit  513  can be completed within the delay time mentioned above. Note that ΔTmax is predetermined based on, for example, the time from the timing of starting the formation of the Y image to the timing of starting the formation of the C image, and inertia of the polygon motor (scanner motor). 
       FIGS. 8A and 8B  are timing charts of the signals relating to the phase control units  503  and  513  according to the present embodiment.  FIG. 8A  illustrates examples of the phase reference signal  521 , the target signal  508  (first target signal), the BD signal  504  (first BD signal), the target signal  518  (second target signal), the BD signal  514  (second BD signal) before the resetting of the target phases Tn 0  and Tn 1  is performed. 
     Moreover,  FIG. 8B  illustrates examples of these signals after the resetting of the target phases Tn 0  and Tn 1  is performed as in the first embodiment (the resetting in step S 104  as illustrated in  FIG. 1 ). In this case, the target signal  508  is generated as a signal having the phase difference of ΔT 0  with respect to the phase reference signal  521 , so as to match the BD signal  504 . On the other hand, the target signal  518  is generated as a signal having the phase difference of (ΔT 0 +(T 1 −T 0 )) with respect to the phase reference signal  521 . 
     Here, a phase difference ΔTdif between the target signal  518  and the BD signal  514  is calculated according to the following equation. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     If this phase difference ΔTdif is less than or equal to ΔTmax, the resetting of the target phases Tn 0  and Tn 1  as described above makes it possible that the phase control unit  513  completes the phase control of the PM  214  before the timing of starting the formation of the C image by the scanner unit  206 . On the other hand, if the phase difference ΔTdif is greater than ΔTmax, the resetting of the target phases Tn 0  and Tn 1  as described above cannot make it possible that the phase control unit  513  completes the phase control of the PM  214  before the timing of starting the formation of the C image by the scanner unit  206 . 
     Accordingly, in step S 204 , the engine controller  204  compares ΔTmax and ΔTdif, and determines whether or not ΔTdif≤ΔTmax. As a result, the engine controller  204  advances the process to step S 205  if ΔTdif is less than or equal to ΔTmax, and the engine controller  204  advances the process to step S 206  if ΔTdif is greater than ΔTmax. 
     (In the Case Where ΔTdif≤ΔTmax) 
     In step S 205 , the engine controller  204  resets the target phases Tn 0  and Tn 1  as below, based on the measurement value of the reception time difference ΔT 0  and the phase difference (T 1 −T 0 ) between the target phases Tn 0  and Tn 1 , as in step S 104  in the first embodiment.
 
 Tn 0=Δ T 0
 
 Tn 1=Δ T 0+( T 1− T 0)  (3)
 
     Next in step S 207 , the engine controller  204  activates the phase control unit  503  (first phase control unit) and the phase control unit  513  (second phase control unit), thereby causing the phase control unit  503  and the phase control unit  513  to start the phase control of the PMs  213  and  214 . After that, in step S 208 , the engine controller  204  determines whether or not the phase control by the phase control unit  503  has been completed. If the engine controller  204  receives, from the phase control unit  503 , the information indicating that the PM  213  is in the phase lock state, the engine controller  204  determines that the phase control has been completed, and advances the process to step S 209 . In step S 209 , the engine controller  204  starts the image formation of each color sequentially from that for the Y image (in the order of Y, M, C, and BK), by transmitting (outputting) writing start signal in the sub scanning direction to the video controller  203 . 
     Here, as in the first embodiment, the PM  213  has been in the phase lock state right after the activation of the phase control unit  503  (immediately after the start of the phase control of the PM  213 ), and thus the phase control by the phase control unit  503  is completed soon. This allows the formation of the Y image and the M image by the scanner unit  205  to be started. Moreover, this makes it possible that the phase control unit  513  completes the phase control of the PM  214  within the delay time from the timing of starting the formation of the Y image to the timing of starting the formation of the C image performed by the scanner unit  206 . Thus, the timing of starting the formation of C image will not be delayed due to the phase control performed by the phase control unit  513 , and the FPOT will not be prolonged. As such, it becomes possible to complete the phase control of the PM  214  without prolonging the FPOT. 
     (In the Case Where ΔTdif&gt;ΔTmax) 
     On the other hand, if ΔTdif&gt;ΔTmax, the resetting in step S 205  would result in that the phase control of the PM  214  by the phase control unit  513  is not completed within the delay time from the timing of starting the formation of the Y image to the timing of starting the formation of the C image. In this case, an additional time of (ΔTdif−ΔTmax) would be needed to complete the phase control by the phase control unit  513 . In such a case, the engine controller  204  performs the process of step S 206 , instead of that of step S 205  in the present embodiment. 
     In step S 206 , the engine controller  204  resets the target phases Tn 0  and Tn 1  as below, based on ΔTdif as well as the measurement value of the reception time difference ΔT 0 , and the phase difference (T 1 −T 0 ) between the target phases Tn 0  and Tn 1 .
 
 Tn 0=Δ T 0+(Δ T dif−Δ T max)/2
 
 Tn 1=Δ T 0+(Δ T dif−Δ T max)/2+( T 1− T 0)  (4)
 
     As such, the engine controller  204  sets the target phase Tn 0  by using a value obtained by adding, to the time difference ΔT 0  measured by the time difference measuring unit  507 , the adjustment value (ΔTdif−ΔTmax)/2 which is based on the delay time mentioned above. Furthermore, the engine controller  204  sets the target phase Tn 1  such that the phase difference between the target phase Tn 0  and the target phase Tn 1  will be equal to the target phase difference (T 1 −T 0 ) with respect to the target phase Tn 0 . This adjustment value is set based on the ΔTmax and the time difference ΔT 1  measured by the time difference measuring unit  517  (ΔTdif obtained from ΔT 1 ), as shown in equation (4). 
     As a result of the resetting in step S 206 , the time from the timing of starting the formation of the Y image to the timing of starting the formation of the C image becomes equal to the time from the completion of the phase control of the PM  213  by the phase control unit  503  to the completion of the phase control of the PM  214  by the phase control unit  513 . In this case, the phase difference between the BD signal  514  having the phase difference ΔT 1  with respect to the phase reference signal  521 , and the target signal  518  generated by the phase control unit  513  will become smaller than ΔTdif illustrated in  FIG. 8B . As a result, the time additionally required to complete the phase control by the phase control unit  513  will change from (ΔTdif−ΔTmax) to (ΔTdif−ΔTmax)/2. By reducing the time required to complete the phase control by the phase control unit  513  as such, the phase control by the phase control unit  513  will be completed earlier, thereby making it possible to shorten the FPOT. 
     After the resetting in step S 206  is completed, the engine controller  204  performs the processes of steps S 207  to S 209 . In step S 207 , the engine controller  204  activates the phase control unit  503  (first phase control unit) and the phase control unit  513  (second phase control unit), thereby causing the phase control of the PMs  213  and  214  to be started. 
     After that, in step S 208 , the engine controller  204  determines whether or not the phase control by the phase control unit  503  has been completed. After the time period of (ΔTdif−ΔTmax)/2 has passed, the PM  213  is brought to be in the phase lock state by the phase control by the phase control unit  503 . As a result of this, the engine controller  204  receives, from the phase control unit  503 , the information indicating that the PM  213  is in the phase lock state, and advances the process to step S 209 . In step S 209 , the engine controller  204  starts the image formation of each color sequentially from that for the Y image (in the order of Y, M, C, and BK) by transmitting (outputting) the writing start signal in the sub scanning direction to the video controller  203 . 
     Even though the present embodiment describes the configuration example in which the two scanner units  205  and  206  are used, any number of scanner units as long as two or more scanner units may be provided. For example, it may be so configured that one scanner unit is provided for each one of the photosensitive drums. 
     As described above, in the present embodiment, the engine controller  204  sets the target phases Tn 0  and Tn 1  such that the phase control by the phase control unit  513  will be completed within the delay time between the timing of starting the exposure of the photosensitive drum  301  with the light beam deflected by the PM  213  (the timing of starting the formation of Y image), and the timing of starting the exposure of the photosensitive drum  303  with the light beam deflected by the PM  214  (the timing of starting the formation of C image). In this way, the phase control is performed by using the target phases Tn 0  and Tn 1 , which are set based on the delay time from the timing of starting the exposure by the scanner unit  205  for firstly forming an electrostatic latent image on the photosensitive drum to the timing of starting the exposure by another scanner unit  206 . 
     With such a configuration that the phase control by the phase control unit  513  is completed earlier by performing the phase control in this way, it becomes possible to shorten the FPOT. Moreover, even in the case where the phase control by the phase control unit  513  is not completed before the timing of starting the exposure by the scanner unit  206 , it is still possible to shorten the FPOT. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2019-173404, filed Sep. 24, 2019, which is hereby incorporated by reference herein in its entirety.