Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
2001-05-25
2003-02-25
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
C347S234000, C347S248000
Reexamination Certificate
active
06525751
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a raster output scanning system for producing a high intensity imaging beam which scans across a movable photoconductive member to record electrostatic latent images thereon, and, more particularly, to an apparatus for providing registration of the beam in the process direction of the photoconductive member.
BACKGROUND OF THE INVENTION
In recent years, laser printers have been increasingly utilized to produce output copies from input video data representing original image information. The printer typically uses a Raster Output Scanner (ROS) to expose the charged portions of the photoconductive member to record an electrostatic latent image thereon. Generally, a ROS has a laser for generating a collimated beam of monochromatic radiation. The laser beam is modulated in conformance with the image information. The modulated beam is reflected through a lens onto a scanning element, typically a rotating polygon having mirrored facets.
The light beam is reflected from a facet and thereafter focused to a “spot” on the photosensitive member. The rotation of the polygon causes the spot to scan linearly across the photoconductive member in a fast scan (i.e., line scan) direction. Meanwhile, the photoconductive member is advanced relatively more slowly than the rate of the fast scan in a slow scan (process) direction which is orthogonal to the fast scan direction. In this way, the beam scans the recording medium in a raster scanning pattern. The light beam is intensity-modulated in accordance with art input image serial data stream at a rate such that individual picture elements (“pixels”) of the image represented by the data stream are exposed on the photosensitive medium to form a latent image, which is then developed and transferred to an appropriate image receiving medium such as paper. Color laser printers may operate in either a single pass or multiple pass system.
In a single pass, process color xerographic system, three ROS stations are positioned adjacent to a photoreceptor surface and selectively energized to create successive image exposures, one for each of the three basic colors. A fourth ROS station may be added if black images are to be created as well. In a multiple pass system, each image area on the photoreceptor surface must make at least three revolutions (passes) relative to the transverse scanline formed by the modulated laser beam generated by a ROS system. With either system, each color separation image must be registered to within a 0.1 mm circle or within a tolerance of ±0.05 mm. Each color image must be registered in both the photoreceptor process direction (slow scan registration) and in the direction perpendicular to the process direction (referred to as fast scan or transverse registration). Registration in the transverse direction of a single pass ROS system is known in the prior art and a preferred registration technique is disclosed in U.S. Pat. No. 5,237,521 issued on Aug. 17, 1993, assigned to the same assignee as the present invention and hereby incorporated by reference.
In a color printer, the alignment of the lead edge of the color image is made difficult if a Raster Output Scanner (ROS) is used to expose the photoreceptor (PR). Typically as the PR travels into the position where the first scanline is to be imaged, it is sensed by a hole sensor detecting a hole in the belt. It is desired to image the first scanline immediately when this occurs and to repeat this for all four colors to achieve perfect lead edge color registration. However, because of the scanning nature of the ROS imager, the ROS spot, more than likely will not be at the SOS (start of scan) position as the PR hole arrives at the hole sensor. If this is the case, the system must wait until the next scanline crosses the SOS sensor to begin imaging. During this delay, the PR will have traveled and the first scan will be misregistered by a maximum of one full pixel or for a 600 spi single-beam ROS, over 40 microns. A typical prior art registration technique is disclosed in U.S. Pat. No. 5,381,165 showing registration by a feedback loop in which the phase and frequency of SOS signals and a reference signal are compared to produce an error signal representing frequency differences between rotating polygons associated with each Raster Output Scanner in a system of multiple raster output scanners. U.S. Pat. No. 5,808,658 discloses a technique for adjustment of a ROS imager relative to a hole sensor to adjust for registration of an image in wholepixel increments. This technique involves a simplified mathematical procedure not requiring the use of a divide capability in a micro-controller.
SUMMARY OF THE INVENTION
In one embodiment, the present invention is directed toward a method and apparatus for sub-scan image registration adjustment of a ROS imager relative to a hole sensor. In this embodiment, the invention is particularly well suited toward single pass multi-ROS imaging systems, allowing precise image-on-image registration by compensating for differences in distance between each ROS imager and its corresponding hole sensor. In another embodiment, the present invention is directed toward a method and apparatus for minimizing an appropriate adjustment during positional rephasing in the rotational position of a polygon within a ROS imager.
According to various embodiments of the invention, a method and a controller for performing the method are provided for calculating image registration in a process direction by minimizing a required phase shift of a rotating polygon in a raster output scanner. The method having the step of determining a requested phase shift in clock cycles of said rotating polygon to align a beam reflected from a first facet of said rotating polygon to a location on a photoconductive surface. If the requested phase shift in clock cycles is greater than a number of clock cycles equal to one-half of a period between the first facet and a neighboring, subsequent second facet, subtract a number of clock cycles equal to a period between the first facet and the second facet from the requested phase shift in clock cycles to determine a required phase shift and decrement a line counter by one. However, if the requested phase shift in clock cycles is less than a negative number of clock cycles equal to one-half of a period between the first facet and the second facet, add a number of clock cycles equal to a period between the first facet and the second facet from the requested phase shift in clock cycles to determine the required phase shift and increment the line counter by one. Although, if the requested phase shift in clock cycles is greater than or equal to a negative number of clock cycles equal to one-half of a period between the first facet and the second facet and is less than or equal to a number of clock cycles equal to one-half of a period between the first facet and the second facet the required phase shift is equated to the requested phase shift. In these embodiments of the invention, the line counter represents a number of scans before a start of image registration on the photoconductive surface.
According to another embodiment of the invention, a raster output scanner imaging apparatus is provided, having a photoconductive surface adapted to move in a process direction relative to a frame, a first rotating polygon rotatably mounted to the frame and having a plurality of facets adapted to reflect a beam onto the photoconductive surface in the form of first scanlines oriented transverse to the process direction, a first sensor corresponding to the first rotating polygon and mounted to the frame and near the photoconductive surface to enable positional information of the photoconductive surface to be detected. Also provided are a second rotating polygon rotatably mounted to the frame and having a plurality of facets adapted to reflect a beam onto the photoconductive surface in the form of second scanlines oriented transverse to the process direction, a second sensor corresponding to the second rotating
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