Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
2000-09-14
2001-09-04
Pendegrass, Joan (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
C347S234000
Reexamination Certificate
active
06285383
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of controlling print registration in an electrophotographic machine, and, more particularly, to a method of controlling laser scanner phase in a multicolor electrophotographic machine.
2. Description of the Related Art
In an in-line color laser image printing process, the print medium typically passes through four color developing stations in series, with the colors being black, magenta, cyan and yellow. In order for the multi-color laser printer to print at the same speed as a monochrome laser printer, photoconductive drum exposures must occur for all four colors simultaneously. The color planes must be properly registered with each other for proper rendering of images without undesirable color shifts and artifacts.
Pipeline color printers which image using laser print heads are limited by the granular one picture element separation between successive scan lines. Without a method of controlling the phase relationship between multiple laser scanners, color planes would be randomly shifted relative to each other by a distance between zero and the width of a picture element, thereby creating color shifts. Thus, alignment of the four color developing stations in both the process direction (feed direction of the print medium) and scan direction (across the page) is critical.
The process location of each scanning laser beam must overlap in order to prevent color offset in the process direction. Each color must have an adjustment to correct for process direction misalignment because each color has a scanning laser beam following a separate optical path. Without such adjustments, after a scan line is transferred from a first photoconductive drum to the transfer belt, and that point on the transfer belt is rotated to a second photoconductive drum, the scan line on the transfer belt from the first photoconductive drum will not align in the process direction with a scan line to be transferred from the second photoconductive drum.
What is needed in the art is a method of aligning scan lines of multiple photoconductive drums in a process direction in order to avoid undesirable color shifts and print artifacts.
SUMMARY OF THE INVENTION
The present invention provides a method of precisely controlling the relative phase between multiple laser scanners so that color planes can be registered to within a fraction of the width of a picture element in the process direction.
The invention comprises, in one form thereof, a method of aligning print images of an electrophotographic machine on a print medium. A first laser beam is reflected off of a first rotating reflector to thereby scan the first laser beam across a first photoconductive drum to produce a first scan line in a scan direction. The rotation of the first reflector is cyclically repeated to thereby produce a plurality of substantially parallel first scan lines on the first photoconductive drum. A respective line of first toner is applied to each of the first scan lines. A second laser beam is reflected off of a second rotating reflector to thereby scan the second laser beam across a second photoconductive drum to produce a second scan line in the scan direction. The rotation of the second reflector is cyclically repeated to thereby produce a plurality of substantially parallel second scan lines on the second photoconductive drum. A respective line of second toner is applied to each of the second scan lines. A moving toner transfer medium contacts each of the first photoconductive drum and the second photoconductive drum. Each line of first toner is transferred from the first photoconductive drum to the toner transfer medium. Each line of second toner is transferred from the second photoconductive drum to the toner transfer medium such that the lines of second toner are substantially interspersed with the lines of first toner on the toner transfer medium. The lines of first toner and the lines of second toner are transferred from the toner transfer medium to the print medium. A desired phase difference between the rotation of the first reflector and the rotation of the second reflector is determined. The desired phase difference is such that at least one line of first toner is substantially aligned in a process direction with a corresponding line of second toner on the toner transfer medium. The process direction is substantially perpendicular to the scan direction. A speed of the rotation of the first reflector and/or the second reflector is temporarily changed to thereby achieve the desired phase difference therebetween.
An advantage of the present invention is that the laser beams of a multicolor electrophotograhpic machine can be aligned in the process direction in real time while the machine is operating.
Another advantage is that the relative phase between two rotating polygon mirrors which scan the laser beams can be set and maintained or readjusted in response to a sensed misregistration signal.
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Klement Martin Christopher
Ream Gregory Lawrence
Richey John Parker
Lexmark International Inc.
Pendegrass Joan
Taylor & Aust P.C.
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