Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
1999-12-06
2002-07-16
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S248000
Reexamination Certificate
active
06421076
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an image forming apparatus, and more particularly relates to an image forming apparatus in which a photoreceptor is scanned with plural light beams to form plural images on it and the plural images are overlapped to form and generate a single image.
2. Description of the Related Art
Apparatus for forming an electrostatic latent image by scanning with a light beam modulated correspondingly to an image to be formed and forming an image on a photoreceptor have been used for apparatus such as printers and copying machines heretofore, and concomitantly with commercialization of digital or color apparatus the image forming apparatus having the above-mentioned structure have been used widely. A color image is realized by forming images of different colors on a photoreceptor successively so that, for example, images of four colors (for example, C, M, Y, and K) different in color each other are overlapped on the single photoreceptor, however this method is disadvantageous in that it takes a time for forming a final color image.
To solve the problem, a so-called tandem type image forming apparatus in which the image forming apparatus has plural photoreceptors, the photoreceptors are scanned and exposed simultaneously with plural light beams respectively to form images different in color each other on the respective photoreceptors, and the color images of different colors are overlapped on a single intermediate transfer medium to form a color image is proposed. Because a tandem type image forming apparatus forms images of respective colors simultaneously, the time required for forming a color image is significantly shortened.
In a tandem type image forming apparatus, it is required to successively delay the writing start timing (modulation start timing) of light beams corresponding to respective colors synchronously with the motion of the intermediate transfer medium to overlap images (toner image) of respective colors formed on the respective photoreceptors without deviation along the vertical scanning direction (the direction perpendicular to the scanning direction of the light beam) on the intermediate transfer medium. The delay magnitude is determined based on the error of the incident position of the light beam on the photoreceptor, the error of a half circumference length of the photoreceptor, and the error of moving speed of the intermediate transfer medium between photoreceptors of respective colors.
However, the image positional deviation magnitude allowable for overlapping of images is generally at most 0.1 mm though it depends on the image data to be generated; on the other hand, the values of the above-mentioned parameters vary when the parts are deformed due to heat or external force. Even though the mechanical accuracy such as dimensional accuracy of the parts are merely improved, it is difficult to maintain the image overlapping accuracy continuously. If the magnitude of the image positional deviation is significantly larger, for example, the hue of a color varies in an area where the hue is to be uniform or the hue becomes partially different in the area to cause non-uniformity, and the non-uniform hue is regarded as the image quality detect.
To solve the above-mentioned problem, a method for correcting the positional deviation of an image in which the position of images of respective colors transferred onto an intermediate transfer medium is measured and the writing delay time is corrected correspondingly to the detected positional deviation magnitude has been employed heretofore.
In the case plural images are overlapped to form and generate a single image, the positional deviation of images to be overlapped occurs in the horizontal scanning direction (the scanning direction of a light beam), it is possible for the horizontal direction to correct the image position in a length unit smaller than the pixel interval along the horizontal scanning direction, and as the result, the positional deviation of the image can be corrected precisely.
The image position along the horizontal scanning direction of the image varies depending on the modulation start timing of a light beam in one scanning with the light beam, because usually the interval from the time when a beam position detection sensor such as SOS sensor detects the light beam to the time when modulation of the light beam is started is controlled by use of the data represented by the number of pulses of a clock signal, by employing the method in which the clock signal having the frequency higher than the frequency corresponding to a pixel interval is used and the data is corrected correspondingly to the positional deviation magnitude in the horizontal scanning direction, the image position is corrected in a length unit smaller than the pixel interval along the horizontal scanning direction.
However, because a pixel interval along the vertical scanning direction (scanning line interval) is the minimum unit for correction of the image position usually for the image positional deviation along the vertical scanning direction, a method in which the delay time of modulation start timing for correcting the image positional deviation along the vertical scanning direction is divided by the time equivalent to one scanning with a light beam to figure out the number of delay scanning lines and, if a fraction is generated, the fraction is subjected to rounding process to generate an integer, and the delay time of the modulation start timing is converted to an integral multiple of the time equivalent to one scanning with the light beam has been employed. Various rounding methods such as rounding up the fraction, rounding down the fraction, and counting fractions of 5 and over as a unit and disregarding the rest are available as described in, for example, Japanese Published Unexamined Patent Application No. Sho 64-38764 as the fraction rounding process of the number of delay scanning lines.
For example, in the case of rounding method of counting fractions of 5 and over as a unit and disregarding the rest, a fraction of the number of delay scanning lines exceeding ½ is rounded up and a fraction smaller than ½ is rounded down, as the result the writing positional deviation (=(the actual value of the number of delay scanning lines before rounding by means of counting fractions of 5 and over as a unit and disregarding the rest−the integral value of the number of delay scanning lines after rounding by means of counting fractions of 5 and over as a unit and disregarding the rest)×scanning line interval) is suppressed to a value equal to or smaller than ½ of the scanning line interval. However, in the case that the number of light beams to be written is 3 or larger, because the numbers of delay scanning lines of respective light beams are not correlative each other (at least the fraction values of numbers of delay scanning lines of respective light beams are not correlative each other), the position of respective images formed by light beams deviates at most ±(scanning line interval/2) along the vertical scanning direction.
The maximum positional deviation magnitude of the image is the same as the above-mentioned result for the case in which rounding by means of rounding up the fraction or rounding down the fraction is employed instead of the above-mentioned rounding by means of counting fractions of 5 and over as a unit and disregarding the rest, and the maximum positional deviation magnitude along the vertical scanning direction is approximately equal to the value of a scanning line interval regardless of any fraction rounding method selected from among known various fraction rounding methods. Therefore, the correction accuracy of the image positional deviation along the vertical scanning direction is insufficient.
It is possible to correct the image position along the vertical scanning direction in a length unit smaller than a scanning interval by use of a structure having plural respective deflection units corres
Fuji 'Xerox Co., Ltd.
Oliff & Berridg,e PLC
Pham Hai
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