Abstract:
An optical scanning device includes a beam generating unit configured to generate a beam of light, a scanning unit with a plurality of scanning planes configured to deflect the beam of light generated by the beam generating unit and cause the beam of light to scan an image carrier in a main-scanning direction, a micro mirror configured to deflect the beam of light generated by the beam generating unit and thereby enable the deflected beam of light to fall on the scanning unit, a position detecting unit configured to detect a position of the beam of light deflected by the micro mirror, an input voltage control unit configured to control an input voltage value for inclining the micro mirror and thereby causing the beam of light to deflect, and an input voltage determining unit configured to control the input voltage value controlled by the input voltage control unit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an optical scanning device which is configured to adjust a micro mirror incorporated in the device. 
         [0003]    2. Description of the Related Art 
         [0004]    The optical scanning device incorporated in an electrophotographic image forming apparatus can use a polygonal mirror that deflects a beam of light and causes the beam to repeatedly scan a photosensitive drum to form an electrostatic latent image on the photosensitive drum. The photosensitive drum is an image carrier that can rotate at a constant speed. A scanning operation in a longitudinal direction of the photosensitive drum is referred to as main scanning. The electrostatic latent image of a line formed in a main scanning operation is referred to as main scanning line. 
         [0005]    There is a problem that the clearance (pitch) between main scanning lines is not uniform when the lines are formed on a photosensitive drum with a beam of light. This is referred to as pitch unevenness (or irregularity) 
         [0006]    One of the reasons is that an actual rotational speed of the photosensitive drum is not constant. This is referred to as “rotation unevenness.” Another reason is that the tilt angle of each plane of a polygonal mirror is not uniform due to manufacturing errors. This is referred to as “plane tilt.”From these reasons, the scanning position of a light beam on a photosensitive drum tends to deviate. 
         [0007]    As discussed in Japanese Patent Application Laid-Open No. 2004-37757, to solve the above-described problem, a conventional system uses a micro mirror that is disposed in optical path and can slant in a sub-scanning direction (i.e., a direction normal to the main-scanning direction). This system can correct the pitch unevenness of main scanning lines by adjusting the inclination of the micro mirror. 
         [0008]    As discussed in Japanese Patent Application Laid-Open No. 2001-270153, a multi-beam image forming apparatus includes a galvanometer mirror that can correct a positional deviation between light beams. This apparatus can operate in a specific mode for adjusting an angle of inclination of the galvanometer mirror to eliminate a deviation if caused in both the main-scanning direction and the sub-scanning direction due to effects of drift. 
         [0009]    The correction amount required for correcting the plane tilt of a polygonal mirror is different for each plane. Therefore, the number of angles of inclination of a micro mirror to be set for adjustment is equal to the number of planes of the polygonal mirror. 
         [0010]      FIG. 8  illustrates an exemplary polygonal mirror  205  that has six reflection planes defined by a plane tilt profile described below. The first plane  205 A has a tilt amount of −3 μm. The second plane  205 B has a tilt amount of −1 μm. The third plane  205 C has a tilt amount of 0 μm. The fourth plane  205 D has a tilt amount of 2 μm. The fifth plane  205 E has a tilt amount of −1 μm. The sixth plane  205 F has a tilt amount of −2 μm. 
         [0011]    In the expression of the plane tilt, the tilt amount indicates a deviation caused by the plane tilt of the polygonal mirror  205 . For example, the expression “tilt amount of 2 μm” indicates that a light spot formed on a photosensitive drum deviates from a reference position (i.e., ideal position) in the sub-scanning direction by an amount of +2 μm. 
         [0012]      FIG. 9A  is a graph illustrating a time sequential deviation in the sub-scanning position relative to the reference position (i.e., ideal position), according to the polygonal mirror  205  having the plane tilt profile illustrated in  FIG. 8 . T represents the period of time required for a main scanning operation. One complete rotation (i.e., rotation of 360°) of the polygonal mirror  205  requires the time of 6 T (=6 planes×T) in total. 
         [0013]    When the scanning operation uses the first plane  205 A, the sub-scanning position (i.e., the position of the light spot formed on a photosensitive drum) deviates from the reference position by an amount of −3 μm, due to the plane tilt of the polygonal mirror  205 . After time elapse of T, when the scanning operation uses the second plane  205 B, the sub-scanning position deviates from the reference position by an amount of −1 μm. After time elapse of T, when the scanning operation uses the third plane  205 C, the sub-scanning position coincides with the reference position. 
         [0014]    Furthermore, after time elapse of T, when the scanning operation uses the fourth plane  205 D, the sub-scanning position deviates from the reference position by an amount of 2 μm. After time elapse of T, when the scanning operation uses the fifth plane  205 E, the sub-scanning position deviates from the reference position by an amount of −1 μm. After time elapse of T, when the scanning operation uses the sixth plane  205 F, the sub-scanning position deviates from the reference position by an amount of −2 μm. In this manner, the sub-scanning position deviates time-sequentially during one complete rotation of the polygonal mirror  205  that requires the time of 6 T in total. 
         [0015]    To correct the sub-scanning positional deviation (i.e., effects of plane tilt) illustrated in  FIG. 9A , it is useful to slant (incline) the micro mirror at a predetermined angle of inclination at which the effects of the plane tilt of the polygonal mirror can be cancelled. Namely, as illustrated in  FIG. 9B , the inclination of the micro mirror can be adjusted so that a beam of light, after it falls on the polygonal mirror, generates a deviation opposed to the sub-scanning positional deviation illustrated in  FIG. 9A .  FIG. 9C  illustrates an ideal state where the deviation in the sub-scanning position is completely suppressed. 
         [0016]    However, the above-described correction system is subjected to the following problems. The micro mirror is not free from effects of environmental changes (e.g., ambient temperature change as well as temperature change of the micro mirror itself). The angle of inclination of a micro mirror changes according to a temperature change as illustrated in FIG.  9 B′. Namely, if any change occurs in the environmental conditions, the micro mirror cannot hold its angle of inclination at a desired value (i.e., an angle to be set in response to an applied input voltage) when the input voltage corresponding to the correction illustrated in  FIG. 9B  is applied to the micro mirror. FIG.  9 C′ illustrates a state where the deviation in the sub-scanning position is not completely removed. 
         [0017]    The above-described problem (i.e., the angle of inclination of a micro mirror deviates from an ideal angle corresponding to a specific input voltage due to environmental change) is described below in more detail. When the scanning operation uses the first plane  205 A of the polygonal mirror  205 , even if an input voltage corresponding to the tilt amount of +3 μm is applied to the micro mirror, an actual sub-scanning positional deviation becomes +2.3 μm due to the environmental change. Similarly, when the scanning operation uses the second plane  205 B, an actual sub-scanning positional deviation becomes 0.8 μm if the input voltage corresponding to the tilt amount of 1 μm is applied to the micro mirror. 
         [0018]    When the scanning operation uses the third plane  205 C, an actual sub-scanning positional deviation becomes 0 μm (because the third plane  205 C requires no correction). When the scanning operation uses the fourth plane  205 D, an actual sub-scanning positional deviation becomes −1.5 μm. When the scanning operation uses the fifth plane  205 E, an actual sub-scanning positional deviation becomes 0.8 μm. When the scanning operation uses the sixth plane  205 F, an actual sub-scanning positional deviation becomes 1.5 μm. 
         [0019]    Namely, according to the example of FIG.  9 B′, if any change occurs in the environmental conditions, the angle of inclination of a micro mirror decreases compared to an angle of inclination to be obtained when an input voltage is applied before the environmental conditions change, even if the micro mirror slants (inclines) in response to the input voltage so as to eliminate a plane tilt amount of the polygonal mirror. 
         [0020]    Therefore, as illustrated in FIG.  9 C′, when the scanning operation uses the first plane  205 A of the polygonal mirror  205 , an error of −0.7 μm is generated in the sub-scanning positional deviation. When the scanning operation uses the second plane  205 B, an error of −0.2 μm is generated in the sub-scanning positional deviation. When the scanning operation uses the third plane  205 C, no error is generated in the sub-scanning positional deviation (because the third plane  205 C requires no correction). 
         [0021]    When the scanning operation uses the fourth plane  205 D, an error of 0.5 μm is generated in the sub-scanning positional deviation. When the scanning operation uses the fifth plane  205 E, an error of −0.2 μm is generated in the sub-scanning positional deviation. When the scanning operation uses the sixth plane  205 F, an error of −0.5 μm is generated in the sub-scanning positional deviation. In this manner, the conventional correction system cannot sufficiently correct the sub-scanning positional deviation. 
       SUMMARY OF THE INVENTION 
       [0022]    Exemplary embodiments, features and aspects of the present invention are directed to an optical scanning device capable of sufficiently correcting a sub-scanning positional deviation even when environmental conditions change. 
         [0023]    According to an aspect of the present invention, an optical scanning device includes a beam generating unit configured to generate a beam of light, a scanning unit with a plurality of scanning planes configured to deflect the beam of light generated by the beam generating unit and cause the beam of light to scan an image carrier in a main-scanning direction, a micro mirror configured to deflect the beam of light generated by the beam generating unit and thereby enable the deflected beam of light to fall on the scanning unit, a position detecting unit configured to detect a position of the beam of light deflected by the micro mirror, an input voltage control unit configured to control an input voltage value for inclining the micro mirror and thereby causing the beam of light to deflect, and an input voltage determining unit configured to control the input voltage value controlled by the input voltage control unit according to a detection result obtained by the position detecting unit. 
         [0024]    The exemplary embodiments of the present invention can provide an optical scanning device that can sufficiently correct the sub-scanning positional deviation regardless of change in environmental conditions. 
         [0025]    If there are two or more angles of inclination to be used for image formation, a tilt adjustment can be performed for only a representative angle of inclination. The time required for adjusting the angle of inclination of a micro mirror can be reduced. 
         [0026]    Further features and aspects of the present invention will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments and features of the invention and, together with the description, serve to explain at least some of the principles of the invention. 
           [0028]      FIG. 1  illustrates an example optical scanning device used in an image forming apparatus, according to an aspect of an exemplary embodiment of the present invention. 
           [0029]      FIG. 2A  is a plan view of an exemplary micro mirror, according to an aspect of the present invention. 
           [0030]      FIG. 2B  is a cross-sectional view of the micro mirror taken along a line  2 B- 2 B of  FIG. 2A , according to an aspect of the present invention. 
           [0031]      FIG. 2C  is a perspective view of the micro mirror illustrated in  FIG. 2A , according to an aspect of the present invention. 
           [0032]      FIG. 3  is a graph illustrating exemplary characteristics of a micro mirror, according to an aspect of the present invention. 
           [0033]      FIG. 4  illustrates an exemplary position sensing device (PSD) and its operation, according to an aspect of the present invention. 
           [0034]      FIG. 5  is a flowchart illustrating an exemplary printing operation from start to end, according to an aspect of the present invention. 
           [0035]      FIG. 6  is a flowchart illustrating an exemplary micro mirror adjusting operation, according to an aspect of the present invention. 
           [0036]      FIG. 7  is a block diagram illustrating an exemplary processing circuit that can adjust the angle of inclination of a micro mirror, according to an aspect of the present invention. 
           [0037]      FIG. 8  illustrates an exemplary plane tilt profile of a polygonal mirror. 
           [0038]      FIG. 9A  is a graph illustrating a time sequential deviation in a sub-scanning position according to the polygonal mirror having the plane tilt profile illustrated in  FIG. 8 . 
           [0039]      FIG. 9B  is a graph illustrating a time sequential waveform of a correction using a micro mirror for correcting the sub-scanning positional deviation illustrated in  FIG. 9A . 
           [0040]      FIG. 9C  is a graph illustrating a time sequential deviation in the sub-scanning position after the sub-scanning positional deviation illustrated in  FIG. 9A  is corrected using the correction waveform of  FIG. 9B . 
           [0041]    FIG.  9 B′ is a graph illustrating a time sequential waveform of a correction using a micro mirror for correcting the sub-scanning positional deviation illustrated in  FIG. 9A , in a case where an environmental change occurs. 
           [0042]    FIG.  9 C′ is a graph illustrating a time sequential deviation in the sub-scanning position after the sub-scanning positional deviation illustrated in  FIG. 9A  is corrected using the correction waveform of FIG.  9 B′. 
           [0043]      FIG. 10  illustrates an example optical scanning device according to a second exemplary embodiment of the present invention. 
           [0044]      FIG. 11  illustrates an inclination table of a micro mirror according to a fourth exemplary embodiment of the present invention. 
           [0045]      FIG. 12  illustrates an optical scanning device according to a third exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0046]    The following description of exemplary embodiments is illustrative in nature and is in no way intended to limit the invention, its application, or uses. It is noted that throughout the specification, similar reference numerals and letters refer to similar items in the following figures, and thus, once an item is described in one figure, it may not be discussed for following figures. 
         [0047]    It is noted that the exemplary optical scanning devices according to the present invention are employable in an electrophotographic image forming apparatus. 
       First Exemplary Embodiment 
       [0048]      FIG. 1  illustrates an optical scanning device of an image forming apparatus according to a first exemplary embodiment of the present invention. An optical scanning device  200  includes a light source  202 , a collimator lens  203 , a cylindrical lens  204 , a rotary polygonal mirror  205 , an fθ lens  206 , a lens dose equivalent (LDE) lens  207 , a micro mirror  209 , a beam shaping slit  210 , a position detection sensor  211 , a processing circuit  212 , and a light detecting sensor (i.e., beam detection (BD) sensor)  213 . 
         [0049]    A beam of light from the light source  202 , such as a semiconductor laser, passes through the collimator lens  203 , the cylindrical lens  204 , and the beam shaping slit  210  successively, and falls on the rotary polygonal mirror  205  via the micro mirror  209 . 
         [0050]    The beam of light reflects on the polygonal mirror  205  and passes through the fθ lens  206  and the LDE lens  207 . The beam of light scans a photosensitive drum  208  in an arrow direction B (i.e., main-scanning direction) which is the longitudinal direction of the photosensitive drum  208 . The photosensitive drum  208  rotates in an arrow direction A (i.e., sub-scanning direction). The optical scanning device repeats the above-described operations to successively form electrostatic latent images in the rotational direction (arrow direction A) of the photosensitive drum  208 . 
         [0051]    The processing circuit  212  receives sub-scanning positional information (i.e., detection result of a position of the main scanning line) obtained by the position detection sensor  211  and a scanning start signal obtained by the light detecting sensor  213 . The position detection sensor  211  is positioned near the photosensitive drum  208 . The processing circuit  212  generates a drive signal based on the sub-scanning positional information and the scanning start signal and outputs the drive signal to the micro mirror  209 . The detection signal of the light detecting sensor  213  can be used as a sync signal for synchronizing the rotation of the polygonal mirror  205  with the data writing to the photosensitive drum  208  with a beam of light. 
         [0052]      FIGS. 2A to 2C  illustrate a detailed configuration of the micro mirror  209 .  FIG. 2A  is a plan view of the micro mirror  209 .  FIG. 2B  is a cross-sectional view of the micro mirror  209  taken along a line  2 B- 2 B of  FIG. 2A .  FIG. 2C  is a perspective view of the micro mirror  209 . 
         [0053]    As illustrated in  FIGS. 2A to 2C , the micro mirror  209  includes a mirror  301 , a torsion bar  302 , a coil  303 , a magnet  304 , and a frame body  305 . The magnet  304  is attached to a reverse surface of the mirror  301 . The torsion bar  302  supports an integrated set of the mirror  301  and the magnet  304  to a frame body  305 . The coil  303 , provided at a position opposed to the magnet  304 , can generate a magnetic field in response to driving current supplied to the coil  303 . The torsion bar  302  allows the mirror  301  to slant (incline) when the generated magnetic field interacts with the magnet  304 . 
         [0054]    The micro mirror  209  can change an inclined angle (i.e., angle of inclination) of the mirror  301  according to an input voltage supplied from the processing circuit  212 . The micro mirror  209  according to the present exemplary embodiment has performances comparable to those of an electrostatic drive resonance micro mirror that can be fabricated using a general semiconductor process. 
         [0055]      FIG. 3  illustrates an exemplary linear relationship between the input voltage and the inclination of the micro mirror  209 . As illustrated in  FIG. 3 , the angle of inclination of the micro mirror  209  increases in proportion to the input voltage. 
         [0056]    The line ( 1 ) represents a gradient-voltage relationship obtained before environmental conditions change, according to which the angle of inclination of the micro mirror  209  is Ta when the input voltage is V1a. The angle of inclination increases to Tb (&gt;Ta) when the input voltage increases to V1b (&gt;V1a). The line ( 2 ) represents a gradient-voltage relationship obtained after environmental conditions change, e.g., when the ambient temperature or the micro mirror temperature changes. The angle of inclination of the micro mirror  209  is Ta when the input voltage is V2a, and the angle of inclination increases to Tb when the input voltage increases to V2b. Thus, a relationship V2b/V1b=V2a/V1a is satisfied. 
         [0057]    Accordingly, if a change rate between “before environmental change ( 1 )” and “after environmental change ( 2 )” in the input voltage value corresponding to the angle of inclination Tb is known, an input voltage value required for obtaining the angle of inclination Ta after the environmental change ( 2 ) can be calculated. 
         [0058]    In the present exemplary embodiment, the position detection sensor  211  includes a position sensing device (PSD)  FIG. 4  illustrates an exemplary position sensing device (PSD) of the position detection sensor  211  according to the present exemplary embodiment. The PSD is configured to output current values Ia and Ib from its output terminals “a” and “b” according to the position of a light beam spot on the PSD. In other words, the current values Ia and Ib are information relating to the position of irradiation. A current-to-voltage conversion circuit converts the current values Ia and Ib into voltage values. The processing circuit  212  identifies the position of irradiation (i.e., the position of a light beam spot) based on the voltage values. 
         [0059]    The torsion bar  302  of the micro mirror  209  is parallel to the main-scanning direction. The micro mirror  209  can deflect a beam of light in the sub-scanning direction (i.e., the direction normal to the main-scanning direction). 
         [0060]    The processing circuit  212  changes the angle of inclination of the micro mirror  209  in synchronism with the deviation in the sub-scanning direction, for example, deviation in the plane tilt of the polygonal mirror  205 . More specifically, when the polygonal mirror  205  has the plane tilt illustrated in  FIG. 9A , the processing circuit  212  controls the angle of inclination of the micro mirror  209  for each plane of the polygonal mirror  205 . Namely, the processing circuit  212  generates the opposite sub-scanning positional deviation illustrated in  FIG. 9B  that can cancel the plane tilt of the micro mirror  209 . 
         [0061]    As described above,  FIG. 9A  illustrates a time sequential deviation in the sub-scanning position caused by the polygonal mirror  205  having the plane tilt profile illustrated in  FIG. 8 . In  FIG. 9A , the ordinate represents a deviation from a sub-scanning reference position and the abscissa represents the time. When T represents the period of time in a main scanning operation, one complete rotation (i.e., rotation of 360°) of the polygonal mirror  205  requires the total time of 6 T (=6 planes×T). 
         [0062]    In the present exemplary embodiment, the reference position indicates a scanning position of a light beam when the polygonal mirror  205  has no plane tilt and the micro mirror  209  does not incline. 
         [0063]    When the scanning operation uses the first plane  205 A, the sub-scanning position (i.e., the position of the light spot formed on a photosensitive drum) deviates from the reference position by an amount of −3 μm, due to the plane tilt of the polygonal mirror  205 . After time elapse of T, when the scanning operation uses the second plane  205 B, the sub-scanning position deviates from the reference position by an amount of −1 μm. After time elapse of T, when the scanning operation uses the third plane  205 C, the sub-scanning position coincides with the reference position. 
         [0064]    Furthermore, after time elapse of T, when the scanning operation uses the fourth plane  205 D, the sub-scanning position deviates from the reference position by an amount of 2 μm. After time elapse of T, when the scanning operation uses the fifth plane  205 E, the sub-scanning position deviates from the reference position by an amount of −1 μm. After time elapse of T, when the scanning operation uses the sixth plane  205 F, the sub-scanning position deviates from the reference position by an amount of −2 μm. In this manner, the sub-scanning position deviates time-sequentially during one complete rotation of the polygonal mirror  205  that requires the total time of 6 T. 
         [0065]    To correct the sub-scanning positional deviation (i.e., effects of plane tilt) illustrated in  FIG. 9A , the processing circuit  212  drives the micro mirror  209  in a similar manner. More specifically, as illustrated in  FIG. 9B , the processing circuit  212  inclines the micro mirror  209  to deflect a beam of light in a direction capable of generating a deviation opposed to the sub-scanning positional deviation illustrated in  FIG. 9A . When the scanning operation uses the first plane  205 A, the processing circuit  212  inclines the micro mirror  209  to a predetermined angle suitable for generating a deviation of +3 μm from the reference position. After time elapse of T, when the scanning operation uses the second plane  205 B, the processing circuit  212  inclines the micro mirror  209  to a predetermined angle for generating a deviation of +1 μm from the reference position. Similarly, the processing circuit  212  inclines the micro mirror  209  to an optimum angle for each of the third to sixth planes. 
         [0066]      FIG. 9C  illustrates a result of the correction illustrated in  FIG. 9B . As illustrated in  FIG. 9C , the sub-scanning positional deviation generated by the plane tilt of the polygonal mirror  205  can be cancelled by controlling the inclination of the micro mirror  209  that can generate the opposite sub-scanning positional deviation. 
         [0067]    However, the micro mirror is not free from effects of environmental changes (e.g., ambient temperature change as well as temperature change of the micro mirror itself). The angle of inclination of a micro mirror changes according to a temperature change as illustrated in FIG.  9 B′. Namely, if any change occurs in the environmental conditions, the micro mirror cannot hold its angle of, inclination at a desired value (i.e., an angle to be set in response to an applied input voltage) when the input voltage corresponding to the correction illustrated in  FIG. 9B  is applied to the micro mirror. 
         [0068]    The above-described problem (i.e., the angle of inclination of a micro mirror deviates from an ideal angle corresponding to a specific input voltage due to environmental change) is described below in more detail. 
         [0069]    When the scanning operation uses the first plane  205 A of the polygonal mirror  205 , even if an input voltage corresponding to the tilt amount of +3 μm is applied to the micro mirror  209 , an actual sub-scanning positional deviation becomes +2.3 μm due to the environmental change. Similarly, when the scanning operation uses the second plane  205 B, an actual sub-scanning positional deviation becomes 0.8 μm if the input voltage corresponding to the tilt amount of 1 μm is applied to the micro mirror  209 . When the scanning operation uses the third plane  205 C, an actual sub-scanning positional deviation becomes 0 μm (because the third plane  205 C requires no correction). 
         [0070]    When the scanning operation uses the fourth plane  205 D, an actual sub-scanning positional deviation becomes −1.5 μm. When the scanning operation uses the fifth plane  205 E, an actual sub-scanning positional deviation becomes 0.8 μm. When the scanning operation uses the sixth plane  205 F, an actual sub-scanning positional deviation becomes 1.5 μm. 
         [0071]    Namely, according to the example of FIG.  9 B′, if any change occurs in the environmental conditions, the angle of inclination of the micro mirror  209  decreases compared to an angle of inclination to be obtained when an input voltage is applied before the environmental conditions change, even if the micro mirror  209  slants (inclines) in response to the input voltage so as to eliminate a plane tilt amount of the polygonal mirror. 
         [0072]    Therefore, as illustrated in FIG.  9 C′, when the scanning operation uses the first plane  205 A of the polygonal mirror  205 , an error of −0.7 μm is generated in the sub-scanning positional deviation. When the scanning operation uses the second plane  205 B, an error of −0.2 μm is generated in the sub-scanning positional deviation. When the scanning operation uses the third plane  205 C, no error is generated in the sub-scanning positional deviation (because the third plane  205 C requires no correction). 
         [0073]    When the scanning operation uses the fourth plane  205 D, an error of 0.5 μm is generated in the sub-scanning positional deviation. When the scanning operation uses the fifth plane  205 E, an error of −0.2 μm is generated in the sub-scanning positional deviation. When the scanning operation uses the sixth plane  205 F, an error of −0.5 μm is generated in the sub-scanning positional deviation. 
         [0074]    In this case, if the correction is continued, it is expected that the sub-scanning positional deviation generated by controlling the inclination of the micro mirror  209  can cancel the sub-scanning positional deviation caused by the plane tilt of the polygonal mirror  205 . 
         [0075]    However, an actual sub-scanning positional deviation generated by the micro mirror  209  is different by an amount of 0.7 μm from the sub-scanning positional deviation caused by the plane tilt of the polygonal mirror  205 . 
         [0076]      FIG. 5  is a flowchart illustrating an exemplary printing operation from start to end.  FIG. 6  is a flowchart illustrating an exemplary micro mirror adjusting operation (step S 02  of  FIG. 5 ). 
         [0077]    To execute the processing of  FIGS. 5 and 6 , a central processing unit (CPU; not illustrated) of the optical scanning device controls various units according to a control program stored in a read only memory (ROM; not illustrated) with a random access memory (RAM; not illustrated) that functions as a work area. 
         [0078]    Now referring to  FIG. 5 , in step S 00 , a user starts a print job. In step S 01 , the CPU activates a polygonal mirror drive motor that drives the polygonal mirror  205 . Furthermore, the CPU turns the light source (laser)  202  on. A beam of light falls on the light detecting sensor  213 . Then, the CPU controls a voltage value applied to the polygonal mirror drive motor so that the time interval of a detection signal (hereinafter, referred to as BD signal) obtained from the light detecting sensor  213  becomes equal to a target value. Thus, the polygonal mirror  209  rotates at a constant speed. In step S 02 , the CPU performs an operation for adjusting the angle of inclination of the micro mirror  209  (as described later with reference to the flowchart of  FIG. 6 ). 
         [0079]    In step S 03 , the CPU turns the light source  202  on according to an image signal and performs an image forming operation for forming an electrostatic latent image on a photosensitive drum  208 . In this operation, the micro mirror  209  performs the above-described plane tilt correction. The micro mirror  209  changes its angle of inclination by an amount capable of canceling a plane tilt amount of each scanning plane of the polygonal mirror  205  when used for image formation. 
         [0080]    In step S 04 , the CPU checks if the image formation of one page has completed. In step S 05 , the CPU determines whether the print job has terminated. If the CPU determines that the print job has terminated (YES in step S 05 ), the processing flow proceeds to step S 07 . In step S 07 , the operation mode shifts to a standby mode. If the CPU determines that the print job has not yet terminated (NO in step S 05 ), the processing flow returns to step S 02  and the CPU performs the adjustment of the micro mirror  209 . 
         [0081]    As described above, the present exemplary embodiment adjusts the angle of inclination of the micro mirror  209  for each page to be printed. However, it may also be useful to perform the angle of inclination adjustment for the micro mirror  209  after completing one print job of plural pages (before starting the next print job). Furthermore, it may also be useful to perform the angle of inclination adjustment on a page-by-page basis only immediately after starting the image formation operation (e.g., from first to tenth pages) because the temperature change is steep. After the temperature change becomes small, it is useful to perform the angle of inclination adjustment on a job-by-job basis. 
         [0082]      FIG. 7  is a block diagram illustrating an exemplary configuration of the processing circuit  212  that performs an operation for adjusting the angle of inclination of the micro mirror  209 . The processing circuit  212  includes a sample-and-hold circuit  501 , a comparison circuit  502 , a voltage generation circuit  503 , a storage circuit  504 , and a micro mirror driving voltage determination circuit  505 . 
         [0083]    The sample-and-hold circuit  501  is configured to sample-and-hold an output voltage (i.e., PSD voltage value) of the above-described position detection sensor (PSD)  211  and output a hold value (i.e., PSD voltage value) to the comparison circuit  502 . The comparison circuit  502  is configured to compare the present PSD voltage value received from the sample-and-hold circuit  501  with a target PSD voltage value received from the storage circuit  504 . 
         [0084]    If the present PSD voltage value is less than the target PSD voltage value, the comparison circuit  502  generates an UP signal that can increase the deflection angle. If the present PSD voltage value is greater than the target PSD voltage value, the comparison circuit  502  generates a DOWN signal that can decrease the deflection angle. 
         [0085]    If the present PSD voltage value is equal to the target PSD voltage value (or if a difference between the present PSD voltage value and the target PSD voltage value is within a predetermined error range), the comparison circuit  502  outputs a coincidence signal to the storage circuit  504 . 
         [0086]    The voltage generation circuit  503  is configured to adjust (increase/reduce) a driving voltage of the micro mirror  209  according to the UP signal or the DOWN signal input from the comparison circuit  502 , and is configured to output a micro mirror adjustment voltage value to the storage circuit  504 . 
         [0087]    The storage circuit  504  is configured to store a target value of the PSD voltage value (i.e., target PSD voltage value) and output the stored target PSD voltage value to the comparison circuit  502 . Furthermore, the storage circuit  504  is configured to rewrite the presently stored PSD target value to the micro mirror adjustment voltage value received from the voltage generation circuit  503  if the coincidence signal is input from the comparison circuit  502 . Moreover, the storage circuit  504  is configured to output the rewritten voltage value as a micro mirror reference voltage value to the micro mirror driving voltage determination circuit  505 . 
         [0088]    The micro mirror driving voltage determination circuit  505  is configured to output a micro mirror driving voltage value to the voltage generation circuit  503 . 
         [0089]    Now referring to  FIG. 6 , in step S 10  the processing circuit  212  starts the angle of inclination adjustment for the micro mirror  209 . In step S 11 , the voltage generation circuit  503  supplies an adjustment voltage value to the micro mirror  209 . The micro mirror  209  inclines by an amount corresponding to the adjustment voltage value. More specifically, the adjustment voltage value is the micro mirror driving voltage value determined by the micro mirror driving voltage determination circuit  505 . 
         [0090]    In step S 12 , the light source  202  turns on at the timing the scanning light falls on the position detection sensor  211 . When a beam of light falls on the position detection sensor  211 , the position detection sensor  211  generates a PSD voltage value representing the position of irradiation. The sample-and-hold circuit  501  holds the detected PSD voltage value even after the beam of light has passed the position detection sensor  211 . 
         [0091]    In step S 13 , the comparison circuit  502  receives the present PSD voltage value held by the sample-and-hold circuit  501  and the target PSD voltage value stored in the storage circuit  504 . The comparison circuit  502  determines whether the present PSD voltage value is equal to the target PSD voltage value (or determines whether a difference between the present PSD voltage value and the target PSD voltage value is within a predetermined error range). 
         [0092]    The present PSD voltage value held by the sample-and-hold circuit  501  is a PSD voltage value obtained in an actual driving operation of the micro mirror  209 . The target PSD voltage value stored in the storage circuit  504  is a PSD voltage value obtained when the micro mirror  209  inclines by a predetermined angle. 
         [0093]    Namely, if the present PSD voltage value is equal to the target PSD voltage value (or if the difference between the present PSD voltage value and the target PSD voltage value is within the predetermined error range), the comparison circuit  502  determines that the micro mirror  209  has a desired angle of inclination corresponding to the applied driving voltage value. Thus, the comparison circuit  502  outputs the coincidence signal to the storage circuit  504 . 
         [0094]    In step S 15 , the micro mirror driving voltage determination circuit  505  generates a micro mirror inclination table (e.g., the line ( 1 ) in  FIG. 3  that represents the gradient-voltage relationship obtained before the environmental conditions change) based on the micro mirror reference voltage value received from the storage circuit  504 . In this case, the storage circuit  504  stores the same contents. As a result, the micro mirror inclination table remains the same. And in step S 21 , the processing circuit  212  terminates the adjustment for the micro mirror  209 . 
         [0095]    If the present PSD voltage value is not equal to the target PSD voltage value (or if the difference between the present PSD voltage value and the target PSD voltage value is out of the predetermined error range) (NO in step S 13 ), the processing flow proceeds to step S 14  to determine whether the present PSD voltage value is greater than the target PSD voltage value. 
         [0096]    If it is determined that the present PSD voltage value is not greater than the target PSD voltage value (NO in step S 14 ), the irradiation spot of a light beam is lower than the target position. The processing flow proceeds to step S 16 . In step S 16 , the comparison circuit  502  outputs the UP signal to the voltage generation circuit  503 . The micro mirror driving voltage generated from the voltage generation circuit  503  increases by a predetermined amount corresponding to the UP signal. The micro mirror  209  increases its angle of inclination. 
         [0097]    If it is determined that the present PSD voltage value is greater than the target PSD voltage value (YES in step S 14 ), the irradiation spot of a light beam is higher than the target position. The processing flow proceeds to step S 17 . In step S 17 , the comparison circuit  502  outputs the DOWN signal to the voltage generation circuit  503 . The micro mirror driving voltage generated from the voltage generation circuit  503  decreases by a predetermined amount corresponding to the DOWN signal. The micro mirror  209  decreases its angle of inclination. 
         [0098]    After the processing of step S 16  or step S 17  is terminated, the processing flow proceeds to step S 18 . In step S 18 , the light source  202  turns on at the timing the scanning light falls on the position detection sensor  211 . When a beam of light falls on the position detection sensor  211 , the position detection sensor  211  generates a PSD voltage value representing the position of irradiation. The sample-and-hold circuit  501  holds the detected PSD voltage value. 
         [0099]    In step S 19 , the comparison circuit  502  receives the present PSD voltage value held by the sample-and-hold circuit  501  and the target PSD voltage value stored in the storage circuit  504 . The comparison circuit  502  determines whether the present PSD voltage value is equal to the target PSD voltage value (or determines whether a difference between the present PSD voltage value and the target PSD voltage value is within a predetermined error range). 
         [0100]    If the present PSD voltage value is equal to the target PSD voltage value (or if the difference between the present PSD voltage value and the target PSD voltage value is within the predetermined error range) (YES in step S 19 ), the comparison circuit  502  outputs the coincidence signal to the storage circuit  504 . 
         [0101]    In step S 20 , the micro mirror driving voltage determination circuit  505  re-generates a micro mirror inclination table based on the micro mirror reference voltage value received from the storage circuit  504 . In this case, the storage circuit  504  stores changed contents. As a result, the micro mirror inclination table is updated. 
         [0102]    If the present PSD voltage value is not equal to the target PSD voltage value (or if the difference between the present PSD voltage value and the target PSD voltage value is out of the predetermined error range) (NO in step S 19 ), the processing flow returns to step S 14  to determine whether the present PSD voltage value is greater than the target PSD voltage value. 
         [0103]    The processing circuit  212  repeatedly executes the processing of steps S 14  to S 19  until the present PSD voltage value agrees with the target PSD voltage value (or the difference between the present PSD voltage value and the target PSD voltage value decreases to the predetermined error range). If the determination in step S 19  is positive, the processing flow proceeds to step S 20 . 
         [0104]    In step S 20 , the micro mirror driving voltage determination circuit  505  re-generates a micro mirror inclination table based on the micro mirror reference voltage value received from the storage circuit  504 . In this case, the storage circuit  504  stores changed contents. As a result, the micro mirror inclination table is updated. In step S 21 , the processing circuit  212  terminates the adjustment for the micro mirror  209 . 
         [0105]    To avoid effects of the plane tilt of polygonal mirror  205 , it is useful to perform the adjustment using a plane having no plane tilt. If there is any tilt on each plane, the adjustment is performed considering a tilt amount. Even in such a case, it is useful to perform the adjustment using the same plane. 
       Second Exemplary Embodiment 
       [0106]    The exemplary device configuration illustrated in  FIG. 1  includes the position sensing device (PSD) incorporated in the position detection sensor  211  for the adjustment of the micro mirror  209 . However, the period of time during which the scanning light falls on the PSD may be insufficient for the PSD that requires a relatively long time to perform the position detecting processing. 
         [0107]    In view of the foregoing, as illustrated in  FIG. 10 , the second exemplary embodiment provides a position sensor (PSD)  211 A at a position adjacent to the micro mirror  209  compared to the position sensor  211  illustrated in  FIG. 1  (which is positioned near the photosensitive drum  208 ). The device configuration of the second exemplary embodiment can increase the time abeam of light falls on the position sensor  211 A. Thus, the second exemplary embodiment can compensate a response delay of the position sensor  211 A and accordingly improve the detection accuracy. 
         [0108]    More specifically, reflection light from the micro mirror  209  has an angle greater than an angle of incident light that falls on the polygonal mirror  205 . Thus, part of the reflection light from the micro mirror  209  advances straight beyond the polygonal mirror  205  and falls on the position sensor  211 A. The position sensor  211 A directly receive the reflection light from the micro mirror  209  (not the scanning light reflected on a scanning plane of the polygonal mirror  205 ). 
         [0109]    Thus, the processing circuit  212  (see  FIG. 1 ) can perform an adjustment of the micro mirror  209  without considering the plane tilt of the polygonal mirror  205 . Furthermore, the position sensor  211 A can detect the position of the reflection light from the micro mirror  209  without requiring a higher response speed. In this case, no beam of light falls on the photosensitive drum  208 . Therefore, the processing circuit  212  can perform a tilt adjustment during an image formation operation. 
         [0110]    Furthermore, the adjustment accuracy can be improved if the angle of inclination of the micro mirror  209  during the gradient adjustment is set to be greater than a plurality of angles of inclination used in the image formation. 
       Third Exemplary Embodiment 
       [0111]    As illustrated in  FIG. 12 , the third exemplary embodiment provides a half mirror  220  (i.e., light splitting unit) between the micro mirror  209  and the polygonal mirror  205 . A position sensor (PSD)  211 B is disposed at a position where a beam of light reaches before the polygonal mirror  205  deflects the beam. The position sensor  211 B receives the reflection light from the micro mirror  209  (not the scanning light reflected on a scanning plane of the polygonal mirror  205 ). 
         [0112]    Thus, the position sensor  211 B can detect the position of the reflection light from the micro mirror  209  without requiring a higher response speed. The processing circuit  212  can perform an adjustment of the micro mirror  209  without considering the plane tilt of the polygonal mirror  205 . 
       Fourth Exemplary Embodiment 
       [0113]    In the first exemplary embodiment, if the input voltage to the micro mirror  209  is changed in step S 16  or step S 17  of  FIG. 6 , the micro mirror driving voltage determination circuit  505  re-generates a micro mirror inclination table in step S 20 . The fourth exemplary embodiment is different from the above-described first exemplary embodiment in that the micro mirror driving voltage determination circuit  505  selects an optimum one of inclination tables which are prepared beforehand. 
         [0114]    The fourth exemplary embodiment uses the device configuration illustrated in  FIG. 1 , and the processing circuit  212  has the configuration illustrated in  FIG. 7 . However, the storage circuit  504  according to the fourth exemplary embodiment stores two or more inclination tables each defining a relationship between the input voltage and the angle of inclination of the micro mirror  209 . In the present exemplary embodiment, as illustrated in  FIG. 11 , the storage circuit  504  stores a total of four inclination tables (i.e., table A, table B, table C, and table D). 
         [0115]    In step S 10  of  FIG. 6 , the processing circuit  212  starts the angle of inclination adjustment for the micro mirror  209 . In step S 11 , the voltage generation circuit  503  outputs an adjustment voltage value (e.g., gradient adjustment voltage value VmC on the table C) to the micro mirror  209 . 
         [0116]    In step S 12 , the light source  202  turns on at the timing the scanning light falls on the position detection sensor  211 . When a beam of light falls on the position detection sensor  211 , the position detection sensor  211  generates a PSD voltage value representing the position of irradiation. The sample-and-hold circuit  501  holds the detected PSD voltage value even after the beam of light has passed the position detection sensor  211 . 
         [0117]    In step S 13 , the comparison circuit  502  receives the present PSD voltage value held by the sample-and-hold circuit  501  and the target PSD voltage value stored in the storage circuit  504 . The comparison circuit  502  determines whether the present PSD voltage value is equal to the target PSD voltage value (or determines whether a difference between the present PSD voltage value and the target PSD voltage value is within a predetermined error range). 
         [0118]    If the present PSD voltage value is equal to the target PSD voltage value (or if the difference between the present PSD voltage value and the target PSD voltage value is within the predetermined error range), the comparison circuit  502  outputs the coincidence signal to the storage circuit  504 . 
         [0119]    In step S 15 , the micro mirror driving voltage determination circuit  505  generates a micro mirror inclination table based on the micro mirror reference voltage value received from the storage circuit  504 . In this case, the storage circuit  504  stores the same contents. As a result, the micro mirror inclination table remains the same. In step S 21 , the processing circuit  212  terminates the adjustment for the micro mirror  209 . 
         [0120]    If the present PSD voltage value is not equal to the target PSD voltage value (or if the difference between the present PSD voltage value and the target PSD voltage value is out of the predetermined error range) (NO in step S 13 ), the processing flow proceeds to step S 14  to determine whether the present PSD voltage value is greater than the target PSD voltage value. 
         [0121]    If it is determined that the present PSD voltage value is not greater than the target PSD voltage value (NO in step S 14 ), the irradiation spot of a light beam is lower than the target position. The processing flow proceeds to step S 16 . In step S 16 , the comparison circuit  502  outputs the UP signal to the voltage generation circuit  503 . The micro mirror driving voltage generated from the voltage generation circuit  503  increases from a gradient adjustment voltage value VmC on the table C to a gradient adjustment voltage value VmD on the table D according to the UP signal. 
         [0122]    If it is determined that the present PSD voltage value is greater than the target PSD voltage value (YES in step S 14 ), the irradiation spot of a light beam is higher than the target position. The processing flow proceeds to step S 17 . In step S 17 , the comparison circuit  502  outputs the DOWN signal to the voltage generation circuit  503 . The micro mirror driving voltage generated from the voltage generation circuit  503  decreases from the gradient adjustment voltage value VmC on the table C to a gradient adjustment voltage value VmB on the table B according to the DOWN signal. 
         [0123]    After the processing of step S 16  or step S 17  is terminated, the processing flow proceeds to step S 18 . In step S 18 , the light source  202  turns on at the timing the scanning light falls on the position detection sensor  211 . When a beam of light falls on the position detection sensor  211 , the position detection sensor  211  generates a PSD voltage value representing the position of irradiation. The sample-and-hold circuit  501  holds the detected PSD voltage value. 
         [0124]    In step S 19 , the comparison circuit  502  receives the present PSD voltage value held by the sample-and-hold circuit  501  and the target PSD voltage value stored in the storage circuit  504 . The comparison circuit  502  determines whether the present PSD voltage value is equal to the target PSD voltage value (or determines whether a difference between the present PSD voltage value and the target PSD voltage value is within a predetermined error range). 
         [0125]    If the present PSD voltage value is equal to the target PSD voltage value (or if the difference between the present PSD voltage value and the target PSD voltage value is within the predetermined error range) (YES in step S 19 ), the comparison circuit  502  outputs the coincidence signal to the storage circuit  504 . 
         [0126]    In step S 20 , the micro mirror driving voltage determination circuit  505  selects a desirable (specific) inclination table from plural inclination tables (stored beforehand) based on the micro mirror reference voltage value received from the storage circuit  504 . In this case, the storage circuit  504  stores changed contents. As a result, the micro mirror inclination table is updated. 
         [0127]    If the present PSD voltage value is not equal to the target PSD voltage value (or if the difference between the present PSD voltage value and the target PSD voltage value is out of the predetermined error range) (NO in step S 19 ), the processing flow returns to step S 14  to determine whether the present PSD voltage value is greater than the target PSD voltage value. 
         [0128]    The processing circuit  212  repeatedly executes the processing of steps S 14  to S 19  until the present PSD voltage value agrees with the target PSD voltage value (or the difference between the present PSD voltage value and the target PSD voltage value decreases to the predetermined error range). If the determination in step S 19  is positive, the processing flow proceeds to step S 20 . 
         [0129]    In step S 20 , the micro mirror driving voltage determination circuit  505  re-selects a desirable (specific) inclination table from the plural inclination tables (stored beforehand) based on the micro mirror reference voltage value received from the storage circuit  504 . In this case, the storage circuit  504  stores changed contents. As a result, the micro mirror inclination table is updated. In step S 21 , the processing circuit  212  terminates the adjustment for the micro mirror  209 . 
       Other Exemplary Embodiments 
       [0130]    Before executing the micro mirror adjustment in step S 02 , it is useful to decelerate the polygonal mirror drive motor to slow a beam of light passing on the position sensor  211 . This embodiment can compensate response delay of the position sensor  211 . In this case, after completing the micro mirror adjustment in step S 02 , it is useful to accelerate the polygonal mirror drive motor for the image formation. 
         [0131]    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 modifications, equivalent structures, and functions. 
         [0132]    This application claims priority from Japanese Patent Application No. 2006-342824 filed Dec. 20, 2006, which is hereby incorporated by reference herein in its entirety.