Electrophotography – Image formation – Transfer
Reexamination Certificate
1999-01-21
2001-03-06
Royer, William J. (Department: 2852)
Electrophotography
Image formation
Transfer
C347S116000
Reexamination Certificate
active
06198896
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an image formation apparatus for print forming full color images through superposing transfer of different color images by means of a plurality of electrostatic recording units having electrophotographic record printing facility of a printer, a copier, etc., and more particularly to an image formation apparatus capable of detecting and correcting positional offsets of color images between a plurality of removable electrostatic recording units.
2. Description of the Related Arts
Up until now, an electrophotographic recording type color printer is provided with four electrostatic recording units for black (K), cyan (C), magenta (M) and yellow (Y) which are arranged in tandem in the record paper conveyance direction. The electrostatic recording units for four colors optically scan a photosensitive drum on the basis of image data to form latent images, develop the latent images with color toners in developing vessels, transfer the developed images onto record paper being conveyed at a certain speed in a superposed manner in the order of yellow (Y), magenta (M), cyan (C) and black (K), and finally perform thermal fixing through a fixing device or other processes. In case of lack of color toners, the electrostatic recording units for yellow (Y), magenta (M), cyan (C) and black (K) must be subjected to replacement of the entire unit or of a part of the unit. To this end, the electrostatic recording units are each provided with a structure allowing easy removal with the apparatus cover opened.
On the other hand, to enhance the quality of the color printing in the color printer having such a structure in which the YMCK electrostatic recording units are arranged in tandem in the record paper conveyance direction, color matching accuracy must be enhanced by reducing the positional offsets of the toner images transferred onto the moving record paper by means of the electrostatic recording units. For example, if the resolving powers in the horizontal scanning direction (direction orthogonal to the conveyance direction) and in the vertical scanning direction (record paper conveyance direction) on the record paper are both 600 dpi, then the pixel pitch results in about 42 &mgr;m, so that the positional offset must be reduced to 42 &mgr;m or less.
In the case of the conventional tandem type color printer, however, it was difficult to achieve a color matching accuracy of 42 &mgr;m or less in positional offset through improvement of the mechanical machining accuracy or assembling accuracy because the YMCK electrostatic recording units are removably provided and suffer from larger positional offsets as compared with the case of mounting in a fixed manner. To solve this problem in the case of Japanese Patent Laid-open Pub. No. Hei8-85236 for instance, test pattern resist marks are transferred on a transfer belt at four rectangular corners and read by a CCD to detect image offsets in resist mark detection coordinates relative to previously defined absolute reference coordinates of the apparatus so that upon the output of image data to the laser scanner, correction is made on the basis of image offsets whose output coordinates positions have been detected. In such conventional positional offset detection and positional offset correction, however, all of the electrostatic recording units for yellow (Y), magenta (M), cyan (C) and black (K) necessitate detection of positional offsets of the resist marks relative to the absolute coordinates. In addition, use of the CCD for the detection of the resist mark needs more time for the processing of positional offset detection and a larger hardware amount resulting in the increase of production costs. Although it is also conceivable to correct the positional offsets by allowing the light emitting array to be mechanically adjustable, such a mechanical adjustment suppressing the positional offsets to 42 &mgr;m or less is unfeasible. For this reason, the color printer using the light emitting array often suffered from a positional offset as large as 300 &mgr;m for instance, making it difficult to realize a satisfactory printing quality through the superposition of color components.
Thus, the present inventors proposed a method in which right and left resist mark trains are transferred on the belt in the order of black (K), cyan (C), magenta (M) and yellow (Y) so that the position of the marks is read for each color to calculate an image offset as a difference between the distance to the black (K) mark from the other color marks and its nominal distance (U.S. Ser. No. 09/086,956 filed May 29, 1998). However, this method involved a deficiency that a greater mark-to-mark distance may allow the offset attributable to the fluctuation in the belt conveyance speed to be contained in the detected distance, preventing accurate detection of the image offsets.
SUMMARY OF THE INVENTION
According to the present invention there is provided an image formation apparatus capable of a high accuracy automatic image offset correction without being affected by the offset of the belt conveyance speed.
The image formation apparatus of the present invention comprises a moving member in the form of an endless belt with record paper adhered thereon, conveyed at a certain speed and a plurality of image carrying units (electrostatic recording units) arranged in the record paper conveyance direction, for forming latent images in conformity with image data through optical scanning on a photosensitive drum, developing them with different developing vessels and transferring them onto record paper lying on the moving member. In the case of the present invention, such an image formation apparatus of tandem type is provided with a correction mark formation unit, a correction amount calculation unit and a positional offset correction unit. The correction mark formation unit transfers mixed color mark trains onto the moving member by means of the plurality of image carrying units, the mixed color mark trains serving as marks for correcting image offsets between color images, the mixed color mark trains each consisting of a plurality of marks which are superposed on each other in an offset manner. The correction value calculation unit detects a lightness pattern of the mixed color mark transferred onto the moving member, to calculate correction values for image offsets between color images from the phase of the lightness pattern. The correction unit automatically corrects offsets between color images on the basis of the correction values acquired by the correction value calculation unit. The correction mark formation unit forms, as the mixed color mark trains, a mixed color slanted line mark train consisting of a plurality of transverse line marks substantially orthogonal to the direction in which the moving member moves, and a mixed color slanted line mark train consisting of a plurality of slanted line marks which traverse substantially diagonally the direction in which the moving member moves. Herein the mixed color transverse line mark train is used for the calculation of an image offset correction value &Dgr;Y in the vertical scanning direction which is coincident with the direction in which the moving member moves. The mixed color slanted line mark train is used for the calculation of an image offset correction value &Dgr;Z in the skew direction relative to the horizontal scanning direction substantially orthogonal to the direction in which the moving member moves. Furthermore, both the mixed color transverse line mark train and the mixed color slanted line mark train are used for the calculation of an image offset correction value &Dgr;X in the horizontal scanning direction substantially orthogonal to the direction in which the moving member moves.
The correction mark formation unit forms the mixed color transverse line mark train by superposing a first transverse line of one of two colors and a second transverse line of the other of the two colors on each other so as to have the same line width
Nagatomi Tsutomu
Nakayasu Hirofumi
Armstrong Westerman Hattori McLeland & Naughton LLP
Fujisu Limited
Royer William J.
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