Direct digital synthesis pixel clock generator

Details Direct digital synthesis pixel clock generator Direct digital synthesis pixel clock generator

2000-10-06

2002-02-26

Pham, Hai C. (Department: 2861)

Incremental printing of symbolic information

Light or beam marking apparatus or processes

Scan of light

C347S250000

Reexamination Certificate

active

06351277

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to variable frequency oscillators used in imaging assemblies of electrophotographic printing machines.
BACKGROUND OF THE PRESENT INVENTION
Electrophotographic marking is a well-known, commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a charged photoreceptor with a light image representation of a desired document. The photoreceptor is discharged in response to that light image, creating an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image, forming a toner image, which is then transferred onto a substrate, such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure, thereby creating a permanent record of the original representation. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of other images.
The foregoing broadly describes a black and white electrophotographic printing machine. Electrophotographic marking can also produce color images by repeating the above process once for each color of toner that is used to make the composite color image. For example, in one color process, referred to herein as the REaD IOI process (Recharge, Expose, and Develop, Image On Image), a charged photoreceptive surface is exposed to a light image which represents a first color, say black. The resulting electrostatic latent image is then developed with black toner particles to produce a black toner image. The charge, expose, and develop process is repeated for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say cyan. The various color images, and thus the various colors of toner particles, are placed in superimposed registration such that a desired composite color image results. That composite color image is then transferred and fused onto a substrate.
The REaD IOI process can be performed in various ways. For example, in a single pass printer wherein the composite image is produced in a single cycle of the photoreceptor. This requires a charging, an exposing, and a developing station for each color of toner. Single pass printers are advantageous in that they are relatively fast since a composite color image can be produced in one cycle of the photoreceptor. However, they are relatively expensive since multiple charging, exposing, and developing stations are required. An alternative process is to have the photoreceptor make multiple passes through the printer. During a first process the photoreceptor is exposed to produce a latent image for a first color (black) and that latent image is developed for that first color. During a second pass the exposure station exposes the photoreceptor to produce a latent image for a second color (yellow), and then that latent image is developed for second first color. The process continues for the third and fourth colors. In a multiple pass printer only one charging station and only one exposure station is required.
One way of exposing the photoreceptor is to use a Raster Output Scanner (ROS). A ROS is typically comprised of a laser source (or sources), a pre-polygon optical system, a rotating polygon having a plurality of mirrored facets, and a post-polygon optical system. In a simplified description of operation a collimated light beam is reflected from facets of an optical polygon and passes through imaging elements that project it into a finely focused spot of light on the photoreceptor surface. As the polygon rotates, the focused spot traces a path on the photoreceptor surface referred to as a scan line. By moving the photoreceptor as the polygon rotates the laser spot raster scans the surface of the photoreceptor. By modulating the laser beam with image information a predetermined latent image is produced on the photoreceptor.
Exposing the photoreceptor requires elements in addition to the basic raster output scanner described above.
FIG. 1
illustrates a typical prior art imaging assembly
6
for exposing a photoreceptor. That assembly includes a laser diode
8
that emits a laser beam
10
that is modulated in response to drive signals from a controller
12
that are applied to the laser diode via a line
9
. As emitted the laser beam
10
is divergent. A lens
14
collimates that diverging beam and directs the collimated beam through a cylindrical lens
16
that has focusing power only in the sagittal direction. After passing through the cylindrical lens
16
the laser beam is incident on a polygon
20
that includes a plurality of mirrored facets
22
. The polygon is rotated at a constant rotational velocity by a motor (not shown) in a direction
24
. The mirrored facets deflect the laser beam as the polygon rotates, resulting in a sweeping laser beam. A post-scan optical system
26
focuses the laser beam
10
to form a spot of circular or elliptic cross sectional shape on a moving photoreceptor
28
. The post-scan optical system
26
is typically an F-theta lens design intended to correct for scan line nonlinearity (see below). In
FIG. 1
, the direction of photoreceptor motion would be into (or out of) the view plane. By properly modulating the laser beam
10
as the focused spot sweeps across the photoreceptor a desired latent image is produced. That latent image is comprised of multiple scan lines, each of which is comprised of a plurality of image elements referred to as pixels.
The imaging assembly of
FIG. 1
also includes a start of scan detector
36
. The start-of-scan detector
36
incorporates a fiber-optic element
44
that guides light received at its input end
46
, which is in the scanning plane of the raster output scanner, to a photosensitive element (not shown). In response to a received light pulse produced by the sweeping scan line, the start-of-scan detector produces the start of scan signal on a line
38
. That signal causes the raster output scanner to synchronize the laser diode modulation such that each scan line starts at the same distance from the edge of the photoreceptor
28
.
In a single pass color printer there are four imaging assemblies. Ideally the four imaging assemblies produce geometrically straight scan lines having evenly spaced, identically sized pixels that result in scan lines of identical lengths which start at the same relative position on the photoreceptor. However, obtaining such scan lines from multiple imaging assemblies is very difficult. For example, manufacturing tolerances in producing and assembling exposure stations result in scan lines that, unless corrected, are shorter or longer than ideal. Other errors include, but are not limited to the following:
1. Changes in scan velocity during the scan. Given a uniform rotational velocity of the raster polygon, the ends of the scan line are scanned at a faster rate then at the center of the scan line (assuming that the polygon is centered on the photoreceptor). This is known as scan line non-linearity. It has the effect of displacing image information along the scan line from its desired location. While the F-theta post-scan optical system
26
reduces scan line non-linearity, in a typical system non-linearity errors of +2 pixels are common.
2. Facet to facet jitter. Each facet of the polygon may have slight imperfections that cause the laser beam to have shorter or longer flight times compared to the other facets. In a typical system, with the scan lines synchronized by the Start-of-Scan signal, the ends of the scan lines receive the full effect of the facet to facet imperfections. In a typical imaging assembly errors of +½ pixels are common.
3. Misalignment of an imaging assembly with the photoreceptor. This causes the scan line length to vary and also produces scan line nonlinearities. While scan line nonlinearities can be improved by changing the average frequency of the pixel clock, known as Magnification Correction, there remains a residual error k

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