Electrophotography – Image formation – Transfer
Reexamination Certificate
1999-04-16
2001-11-20
Pendegrass, Joan (Department: 2852)
Electrophotography
Image formation
Transfer
C347S116000
Reexamination Certificate
active
06321060
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a printing apparatus for printing color images through superposed transfer of different color images by means of a plurality of electrostatic recording units having an electrophotographic recording print function for use in printers, copiers, etc., and more particularly to a printing apparatus for detecting and correcting relative positional offsets of color images formed by a plurality of removable electrostatic recording units.
2. Description of the Related Art
Up until now, in color printing apparatuses using electrophotographic recording, electrostatic recording units for four colors, black (K), cyan (C), magenta (M) and yellow (Y) have been arranged in tandem in the direction of conveyance of record paper. The electrostatic recording units for four colors form latent images through the optical scanning of photosensitive drums based on image data, develop these latent images with color toners from developing vessels, and thereafter transfer the yellow (Y), magenta (M), cyan (C) and black (K) toner images in the mentioned order and in a superposed manner onto the record paper being conveyed at a certain velocity, the resultant images being finally subjected to a thermal fixation through a fixing device. In case of running short of color toners, the entirety or part of the electrostatic recording units for yellow (Y), magenta (M), cyan (C) and black (K) must be replaced. To this end, the electrostatic recording units are provided with a structure ensuring easy mounting and dismounting with the apparatus cover opened. In the color printing apparatus comprising the electrostatic recording units for Y, M, C and K arranged in tandem in the record paper conveyance direction, on the other hand, the requirement for enhancing the quality of the color printing is to reduce positional offsets of toner images transferred by the electrostatic recording units onto the record paper in motion to thereby enhance the accuracy of color matching. Providing that the resolutions on the record paper in the horizontal scanning direction (the direction orthogonal to the conveyance direction) and in the vertical scanning direction (the direction in which the record paper is conveyed) are respectively 600 dpi, the pixel pitch results in about 42 &mgr;m, and hence the positional offset must be suppressed to 42 &mgr;m or below.
In the case of the conventional tandem type color printing apparatus, however, it was difficult to realize the color matching ensuring the positional offsets less than 42 &mgr;m through the mechanical machining accuracy or assembling accuracy since the removable electrostatic recording units for Y, M, C and K present larger positional offsets than the fixed electrostatic recording units. In Japanese Patent Laid-open Pub. No. Hei8-85236 for instance, to solve this problem, test pattern resist marks are transferred onto the transfer belt at four points on rectangular corners and read by a CCD to detect the amount of positional offsets of detection coordinates of the resist marks relative to previously set absolute reference coordinates, so that upon the output of image data to a laser scanner, the output coordinate positions are corrected on the basis of the amount of offsets detected. However, such conventional positional offset detection and positional offset correction necessitate the detection of the positional offsets of the resist marks relative to the absolute coordinates, on all the electrostatic recording units for yellow (Y), magenta (M), cyan (C) and black (K). In addition, the use of the CCD for the detection of the resist marks leads to extended processing time for positional offset detection and to increased amount of the hardware, resulting in raised production costs. Although it is also conceivable to correct the positional offsets through the mechanical adjustability of the light emitting array, it would be difficult to realize the adjustment capable of suppressing the positional offset to 42 &mgr;m or below. For this reason, the color printing apparatus using the light emitting array has often caused positional offsets as large as 300 &mgr;m for instance, making it impossible to realize the satisfactory printing quality through the superposition of the color components.
SUMMARY OF THE INVENTION
According to the present invention, there is provided an apparatus which includes electrostatic recording units each having an light emitting array and ensures a high precision correction of positional offsets correction through a simple detection of the positional offsets.
The printing apparatus in accordance with the present invention comprises a conveyance mechanism including a belt moving at a certain velocity for conveying record paper in an adhered manner; and a plurality of electrostatic recording units arranged in the direction of conveyance of the record paper for forming a latent image in conformity with image data through optical scanning of a photosensitive drum, to develop the latent image with different color toner components to thereafter transfer the thus developed image onto the record paper on the belt. Such a tandem type printing apparatus of the present invention further comprises a positional offset detection unit and a positional offset correction unit. The positional offset detection unit includes sensors for optically detecting a plurality of resist marks of color toner components transferred onto the belt at predetermined pitches by the plurality of electrostatic recording units, the positional offset detection unit detecting positions of the resist marks from phase differences &phgr; obtained through Fourier transform of detection signals of the sensors, to detect positional offsets, relative to a reference resist mark of any one color, of the resist marks of the other colors. The positional offset correction unit relatively corrects the plurality of electrostatic recording units other than the electrostatic recording unit associated with the reference image, on the basis of positional offset detection information detected by the positional offset detection unit, so as to eliminate any positional offsets, relative to the reference image, of the other color images.
The positional offset detection unit transfers a plurality of lateral lines and a plurality of slanted lines as the resist marks onto the belt on a color-to-color basis, to detect positions of the lateral lines and positions of the slanted lines from phases obtained through Fourier transform of detection signals of the sensors. In order to detect a positional offset by means of the positional offset detection unit, a single lateral line or slanted line may be transferred onto the belt and then detected by the sensors. However, in case the toner does not lie uniformly on the line or noises are mixed with sensor detection signals, it becomes impossible to accurately detect the position of the line due to the confusion of the position of the line with the noises. Thus, according to the present invention, a plurality of lateral lines and slanted lines for the detection of the positions are transferred onto the belt at a certain pitch to form s striped patterns repeating at a certain period. Then, Fourier transform is applied to detection signals which are obtained by reading the striped patterns using the sensors. This Fourier transform is a discrete Fourier transform in which a cosine Fourier coefficient a and a sine Fourier coefficient b are calculated from sampled raw data and then a phase &phgr; and an amplitude c are obtained. Since the phase &phgr; corresponds to the position of the lateral line or slanted line relative to the predetermined Fourier transform start position with one period 2 &pgr; in the Fourier transform being a line pitch interval, the phase &phgr; can be transformed into the line position. As a result of this, accurate detection of the lines is ensured without being affected by the noises even though they are mixed with the sensor detection signals.
The positional offset
Houki Youji
Nagatomi Tsutomu
Nakayasu Hirofumi
Armstrong Westerman Hattori McLeland & Naughton LLP
Fujitsu Limited
Pendegrass Joan
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