Abstract:
In an optical scanning apparatus, an optical element has power for predominantly correcting a position of a scanning line by scanning in a sub-scanning direction; a scanning line curvature adjusting unit adjusts a curvature of the scanning line by deforming the optical element in the sub-scanning direction; a scanning line inclination adjusting unit adjusts an inclination of the scanning line by rotating the scanning line about an axis that is at right angles to a main scanning direction and the sub-scanning direction; and one automatic actuating unit each provided in the scanning line curvature adjusting unit, and the scanning line inclination adjusting unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present document incorporates by reference the entire contents of Japanese priority document, 2005-078248 filed in Japan on Mar. 17, 2005. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a technology for adjusting curvature and inclination of the scanning lens in an optical scanning apparatus and an image forming apparatus. 
   2. Description of the Related Art 
   An image forming apparatus employed in a laser beam printer, digital copier, or laser facsimile machine includes a regular polygon mirror that performs deflective scanning of a light beam emitted by a light source, and an optical element (scanning lens) that images the light beam scanned by the polygon mirror on a photosensitive member. Additionally, the image forming apparatus includes a photodetector that determines the starting point of recording. The photodetector is provided on the side where scanning starts, outside an effective exposure area. 
   In such a color image forming apparatus, a laser beam scanning line curves/inclines depending on curvature characteristics of the optical element, skew of the optical housing, heat distortion of a motor driving the polygon mirror, heat distortion within the main unit due to heat for another unit, skew in the photosensitive member, etc. Due to the curvature/inclination in the scanning line, three or four scanning lines fail to merge, resulting in color shift. 
   To counter this problem, Japanese Patent Laid-Open Publication No. H11-287966 discloses a technology in which a plate-shaped glass is provided inside the optical housing along the scanning direction. The curvature/inclination of the scanning line can be adjusted by placing the glass in the housing tilted in cross-sectional view or by changing the thickness of the glass. 
   However, even though the amount of curvature of the scanning line can be adjusted by this conventional technology, the inclination of the scanning line cannot be adjusted. Therefore, the inability to adjust the inclination of the scanning line leads to degradation of image quality due to unevenness of color, color shift, and the like. Further, inserting the glass in the housing degrades the optical characteristics by causing wavefront aberration, etc. 
   Japanese Patent Laid-Open Publication No. 2001-100135 discloses a technology in which the inclination of the scanning line is adjusted by inclining a folded mirror in the optical housing perpendicular to the optical axis. 
   However, in this conventional technology, even though the inclination of the scanning line can be adjusted, adjustment of the inclination results in variation of the curvature of the scanning line, necessitating further readjustments and making it difficult to meet the correction values. Further, after correction of the curvature/inclination of the scanning line, the magnification of each toner image varies (the light path length from each toner image varies because of the spinning of the folded mirror). 
   To correct the varying magnification of the different toner images, a technology is disclosed in Japanese Patent Laid-Open Publication No. 2002-182145, in which a mechanism for adjusting the curvature/inclination of the scanning line is provided in a long lens corresponding to each semiconductor laser. 
   However, the conventional technology disclosed in Japanese Patent Laid-Open Publication No. 2002-182145 described above present the following problem. 
   Though the mechanism provided for adjusting the curvature/inclination of the scanning line takes care of the problem faced in the earlier conventional technology, the mechanism itself takes up a large space and is difficult to be placed inside the optical housing. In addition, the mechanism pushes up the cost significantly. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to at least solve the problems in the conventional technology. 
   According to an aspect of the present invention, an optical scanning apparatus includes an optical housing having a first surface and a second surface facing one another, where the first surface bears an optical element having power for predominantly correcting a position of a scanning line by scanning in a sub-scanning direction; a scanning line curvature adjusting unit that adjusts a curvature of the scanning line by deforming the optical element in the sub-scanning direction; a scanning line inclination adjusting unit that adjusts an inclination of the scanning line by rotating the scanning line about an axis that is at right angles to a main scanning direction and the sub-scanning direction; and a first actuating unit provided in the scanning line curvature adjusting unit, and a second actuating unit provided in the scanning line inclination adjusting unit, the first actuating unit and the second actuating unit being auto-driven; where the optical element is directly borne on the first surface, and the scanning line curvature adjusting unit and the scanning line inclination adjusting unit are provided on the second surface. 
   According to another aspect of the present invention, in an image forming apparatus of a single-drum type or a tandem color type, an optical scanning apparatus includes an optical housing having a first surface and a second surface facing one another, where the first surface bears an optical element having power for predominantly correcting a position of a scanning line by scanning in a sub-scanning direction; a scanning line curvature adjusting unit that adjusts a curvature of the scanning line by deforming the optical element in the sub-scanning direction; a scanning line inclination adjusting unit that adjusts an inclination of the scanning line by rotating the scanning line about an axis that is at right angles to a main scanning direction and the sub-scanning direction; and a first actuating unit provided in the scanning line curvature adjusting unit, and a second actuating unit provided in the scanning line inclination adjusting unit, the first actuating unit and the second actuating unit being auto-driven; where the optical element is directly borne on the first surface, and the scanning line curvature adjusting unit and the scanning line inclination adjusting unit are provided on the second surface. 
   The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic of a digital color writing system according to the present invention; 
       FIG. 2  is a schematic of a digital color image forming apparatus; 
       FIG. 3A  and  FIG. 3B  are oblique perspectives of a scanning line curvature/inclination adjusting mechanism; 
       FIG. 4  is a cross-sectional view of the scanning line curvature/inclination adjusting mechanism; 
       FIG. 5A  and  FIG. 5B  are elevation views of the scanning line curvature/inclination adjusting mechanism; 
       FIG. 6  is a side view of the scanning line curvature/inclination adjusting mechanism; 
       FIG. 7  is an oblique perspective of a bracket; 
       FIG. 8  is a schematic of an action that occurs when a long lens is secured to the bracket; 
       FIG. 9  is a schematic of a scanning line inclination performed by a scanning line curvature adjuster; 
       FIG. 10A ,  FIG. 10B , and  FIG. 10C  are schematics of a mechanism by which the scanning line inclination is performed by the scanning line curvature adjuster; and 
       FIG. 11  is a schematic of a scanning line curvature adjustment amount. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Exemplary embodiments of the present invention are explained next with reference to the accompanying drawings. 
     FIG. 1  is a schematic of a digital color writing system according to the present invention. The digital color writer system includes polygon mirrors  1   a  and  1   b,  sound-damping glasses  2   a  and  2   b , fθ lenses  3   a  and  3   b , first mirrors  4   a  through  4   d , second mirrors  6   a  through  6   d , third mirrors  7   a  through  7   d , long lenses  5   a  through  5   d , dust protection glasses  8   a  through  8   d , photosensitive members  9   a  through  9   d  on which images are formed, an optical housing  15  that includes  13   a  (side walls) and  13   b  (optical element bearing face), a top cover  11  that hermetically seals the optical housing  15 , a bottom cover  12 , and a polygon mirror cover  10 . Laser beams  14   a  through  14   d , respectively, are optical paths corresponding to each of the photosensitive members  9   a  through  9   d . The polygon mirrors  1   a  and  1   b,  having reflective mirrors on regular polygonal side faces, spin at high speed, and perform deflective scanning of a laser beam. The sound-damping glasses  2   a  and  2   b  dampen the noise of a motor driving the polygon mirrors  1   a  and  1   b.  The fθ lenses  3   a  and  3   b  change motion of the laser beam scanned on the photosensitive members  9   a  through  9   d  by the polygon mirrors  1   a  and  1   b  from equiangular motion to uniform linear motion. The first mirrors  4   a  through  4   d , the second mirrors  6   a  through  6   d , and the third mirrors  7   a  through  7   d  guide the laser beam towards the photosensitive members  9   a  through  9   d . The long lenses  5   a  through  5   d  compensate for the slant of the sides of the polygon mirrors  1   a  and  1   b.  The dust protection glasses  8   a  through  8   d  protect the optical housing  15  against dust. 
     FIG. 2  is a schematic of a digital color image forming apparatus  20  according to the present invention. The digital color image forming apparatus  20  includes the optical housing  15 , the laser beams  14   a  through  14   d  that actually perform scanning, the photosensitive members  9   a  through  9   d  on which images are formed, an intermediate transfer belt  18 , and a paper feeding cassette  19 . An arrow A in  FIG. 2  indicates the direction in which the intermediate transfer belt  18  is driven. 
   A mechanism for adjusting the curvature/inclination of a scanning line is explained next, with reference to  FIG. 3A ,  FIG. 3B ,  FIG. 4 ,  FIG. 5A ,  FIG. 5B , and  FIG. 6 .  FIG. 3A  and  FIG. 3B  are oblique views,  FIG. 4  is a cross-sectional view,  FIG. 5A  and  FIG. 5B  are elevation views, and  FIG. 6  is a side view of the scanning line curvature/inclination adjusting mechanism. The scanning line curvature/inclination adjusting mechanism includes a long lens  5  that corrects the slant of the sides of the polygon mirrors, a bracket  21  for adjusting the curvature/inclination of the scanning line, plate springs  22  through  24  for securing the long lens  5  and the bracket  21  together, a scanning line curvature adjuster  25 , an automatic inclination adjuster driving motor  26 , a driving motor holder  27 , an adjuster  28 , an optical housing receiving surface  29 , and plate springs  30  through  33  that secure the long lens  5 . 
   The automatic inclination adjuster driving motor  26  and the scanning line curvature adjuster  25  are each housed in respective driving motor holders so that their actuating units are more rigid than their respective driving motor holders. 
   The optical element (the long lens  5 ) that has the power of correcting the scanning line in the sub-scanning direction, and the bracket  21 , which is composed of a material more rigid than the long lens  5 , are secured together by the plate springs  22  through  24 . 
   A long lens receiving surface is provided on the bracket  21  (see  FIG. 7 ). The long lens  5  is secured so fast to the bracket  21  by a middle plate spring  22  as to cause the long lens  5  to flex (see  FIG. 8 ). 
   Assuming the flexed state of the long lens  5  as the initial state, if an adjusting screw of the scanning line curvature adjuster  25  is driven into the long lens  5  towards an optical housing surface  13  on which the long lens  5  is placed, the long lens  5  gradually flexes in a direction opposite to the initial state. Curvature adjustment performed at this stage results in inclination of the scanning line. This mechanism is explained next. 
   When the scanning line curvature adjuster  25  operates, the absolute position of the bracket  21 , which functions as a correcting mechanism, shifts in Z-axis by an amount of operation ‘d’ of the scanning line curvature adjuster  25  because the mid part of the long lens  5  is set in the housing  15  (see  FIG. 10A ). The position of the bracket  21  in Z-axis on the side of the inclination adjuster  26 - 28  does not change because the bracket  21  is pressed by the plate spring  32  towards an inclination adjuster  26 - 28  (see  FIG. 10B ), which includes the automatic inclination adjuster driving motor  26 , the driving motor holder  27 , and the adjuster  28 . Thus, the end of the bracket  21  opposite to the end on the side of the inclination adjuster  26 - 28  moves in a direction opposite to that of the curvature adjustment direction (see  FIG. 10C ). 
   An adjustment amount Y of the automatic inclination adjuster driving motor  26  is calculated by the expression given below.
 
 Y =( L 1 +L 2). X/L 1
 
   where X is an amount of operation of the scanning line curvature adjuster  25 , L 1  is a distance from a setting position of the long lens  5  in the housing  15  up to the point the inclination adjuster has the bracket  21  over it, and L 2  is a distance of the bracket  21  from the setting position of the long lens  5  in the housing  15  up to end of the bracket  21  opposite to that of the inclination adjuster. 
   Thus, by adjusting the curvature of the scanning line as described above, the change in the amount of inclination adjustment can be eliminated. 
   Thus, curvature adjustment of the scanning line can be automatically performed. Consequently, imposition can be automated, colors during scanning are merged, and the duration required for color merging adjustment can be shortened. 
   According to the present invention, degradation of optical characteristics that may be caused by placement error, etc., of the optical element reduce, and curvature and inclination adjustments are performed automatically. 
   Moreover, imposition is automated, colors during scanning are merged, and the duration required for color merging adjustment is shortened. 
   Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.