Polygon facet error effects elimination in multi-pass color systems

An imaging system for forming multiple superimposed image exposure frames on a photoconductive member moving in a process direction including a rotating polygon having a plurality of facets. A raster output scanner forms a plurality of scanlines in a transverse direction across the photoconductive member by reflecting modulated beams from the rotating polygon. A control provides a start of scan (SOS) signal for each of the facets of the rotating polygon, determines the facet related to the first scanline of a first image exposure frame on the photoconductive member, and initiates the first scanline of each succeeding superimposed image exposure frame on the photoconductive member in relation to the facet related to the first scanline of the first image exposure frame. Similarly, a time period measurement between a given facet occurrence to the same given facet repeat occurrence, relative to subsequent full revolutions of the polygon, provides an `error free` electronic representation of the speed of the polygon.

BACKGROUND AND MATERIAL DISCLOSURE STATEMENT 
This invention relates generally to a raster output scanning system for 
producing a high intensity imaging beam which scans across a rotating 
polygon to a movable photoconductive member to record electrostatic latent 
images thereon, and, more particularly, to the elimination of the effects 
of rotating polygon facet to facet geometric errors. 
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 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 an 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 transferred to an appropriate 
image receiving medium such as paper. Laser printers may operate in either 
a single pass or multiple pass system. 
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. 
Each 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 (skew registration) and in the direction 
perpendicular to the process direction (referred to as fast scan or 
transverse registration). 
Various techniques for multiple image registration are known in the prior 
art. For example, U.S. Pat. No. 5,208,796 discloses a technique wherein 
targets on a photoconductor belt are used for the detection of lateral 
belt displacement to control the transverse location of exposure scan. In 
addition, U.S. Pat. No. 5,381,165 discloses the registration of color 
images in the process direction in a raster output scanner. 
A difficulty, however, in the prior art is that polygon facets are not 
geometrically perfect. Because polygon facets are not geometrically 
perfect, one of the resulting errors is scan length variation from any 
given facet to any other facet. In image on image systems, particularly 
color, scan length variation between first and subsequent scanlines is 
manifested in color misregistration. 
Thus, it would be desirable to provide a multi-pass color polygon scanning 
system that reduces or eliminates the effects of polygon facet to facet 
geometric errors in the scan direction. It is therefore an object of the 
present invention to eliminate scan length color registration error in 
every color on color line. Another object is to improve overall output 
copy color registration error in systems where polygon speed control is 
derived from a source such as a start of scan (start of facet) sensor 
using polygon facets for feedback information on polygon speed. Other 
advantages of the present invention will become apparent as the following 
description proceeds, and the features characterizing the invention will 
be pointed out with particularity in the claims annexed to and forming a 
part of this specification. 
SUMMARY OF THE INVENTION 
There is disclosed an imaging system for forming multiple superimposed 
image exposure frames on a photoconductive member moving in a process 
direction including a rotating polygon having a plurality of facets, a 
raster output scanner forming a plurality of scanlines in a transverse 
direction across the photoconductive member by reflecting modulated beams 
from the rotating polygon, and a control. The control provides a start of 
scan (SOS) signal for each of the facets of the rotating polygon, 
determines the facet related to the first scanline of a first image 
exposure frame on the photoconductive member, and initiates the first 
scanline of each succeeding superimposed image exposure frame on the 
photoconductive member in relation to the facet related to the first 
scanline of the first image exposure frame. Similarly, a time period 
measurement between a given facet occurrence to the same given facet 
repeat occurrence, relative to subsequent full revolutions of the polygon, 
provides an `error free` electronic representation of the speed of the 
polygon, 
For a better understanding of the present invention, reference may be had 
to the accompanying drawings wherein the same reference numerals have been 
applied to like parts and wherein:

DESCRIPTION OF THE INVENTION 
In FIG. 1 of the drawings, an embodiment of the present invention is 
incorporated in a multi-pass xerographic printing system depicted 
schematically and designated generally by reference numeral 10. The system 
10 includes a photoreceptive belt entrained about guide rollers 14 and 16, 
at least one of which is driven to advance the belt 12 in a longitudinal 
direction of processing travel depicted by the arrow 18. The length of the 
belt 12 is designed to accept an integral number of spaced image areas 
l.sub.1 -l.sub.n represented by dashed line rectangles in FIG. 1. As each 
of the image areas l.sub.1 -l.sub.n reaches a transverse line of scan, 
represented by a dashed arrow 20, it is progressively exposed on closely 
spaced transverse raster lines 22 shown with exaggerated longitudinal 
spacing on the image area l.sub.1 in FIG. 1. 
In the embodiment depicted in FIG. 1, the line 20 is scanned by a raster 
output scanner so that a modulated laser beam 24 is reflected to the line 
20 by successive facets 25 on a rotatable polygon-shaped mirror 26. The 
beam 24 is emitted by a laser device 28 such as a laser diode, operated by 
a laser drive module forming part of a control processor generally 
designated by the reference numeral 30. The processor 30 includes other 
not shown circuit or logic modules such as a scanner drive command 
circuit, by which operation of a motor (not shown) for rotating the 
polygon mirror 26 is controlled. 
In the operation of the system 10, as thus far described, the control 30 
responds to a video signal to expose each raster line 22 to a linear 
segment of the video signal image. In xerographic color systems, each 
image area l.sub.1 -l.sub.n, must be exposed in the same manner to four 
successive exposures, one for each of the three basic colors and black. In 
a multi-pass system such as the system 10, where only one raster output 
scanner or head is used, complete exposure of each image area requires 
four revolutions of the belt 12. 
The image areas l.sub.1 -l.sub.n are successively exposed on successive 
raster lines 22 as each raster line registers with a transverse scanline 
20 as a result of longitudinal movement of the belt 12. 
It is to be noted that the length of the transverse scanline 20 in system 
10 is longer than the transverse dimension of the image areas l. Scanline 
length, in this respect, is determined by the length of each mirror facet 
25 and exceeds the length of the raster lines 22. The length of each 
raster line is determined by the time during which the laser diode is 
active to reflect a modulated beam from each facet 25 on the rotating 
polygon 26 as determined by the laser drive module. Thus, the active 
portion of each transverse scanline may be shifted in a transverse 
direction by control of the laser drive module and the transverse position 
of the exposed raster lines 22, and image areas l.sub.1 -l.sub.n, shifted 
in relation to the belt 12. 
Adjustment of the active portion of the transverse scanline 20 for each 
succeeding image is needed to assure precise longitudinal alignment or 
transverse registration of the succeeding images with the first image 
irrespective of the lateral position of the belt during exposure of the 
images. This operation is achieved in substantial measure by the provision 
of targets aligned in the direction of belt travel and of a design to 
facilitate generation of a signal corresponding to the location of each 
target. In particular and in the multi-pass system of FIG. 1, targets 
T.sub.1 -T.sub.n are located along a marginal edge of the belt 12 to be 
aligned in a longitudinal direction and are spaced to be located slightly 
ahead of each image areas l.sub.1 -l.sub.n or upstream from each such area 
in the context of belt travel. A single sensor 36 is located to be aligned 
with targets T1-Tn for the image area passing the transverse scanline 20 
in FIG. 1. 
Downstream from the exposure station, a development station (not shown) 
develops the latent image formed in the preceding image area. After the 
last color exposure, a fully developed color image is then transferred to 
an output sheet. An electronic Sub System (ESS) 32 contains the circuit 
and logic modules which respond to input video data signals and other 
control and timing signals, to drive the photoreceptor belt 17 
synchronously with the image exposure and to control the rotation of the 
polygon by the motor. For further details, reference is made to U.S. Pat. 
Nos. 5,381,165 and 5,208,796 incorporated herein. 
As illustrated any suitable marker on the photoconductive surface or belt 
or any suitable hole provides a reference for each projected image on the 
belt surface. In other words, the detection by sensor of a mark or hole in 
the photoconductive surface establishes the first scanline of the 
projected image and in a multi pass image on image system, helps to 
establish image on image registration. In addition, the start of scan 
signals indicate the scanning laser beam to be at a start of scan position 
with reference to the photoconductive surface. 
Generally, in the prior art, with a polygon of eight facets, the detected 
start of scan signal for each of eight facets on a polygon are used to 
inject a phase shift into the polygon motor. In particular, a polygon 
controller monitors the SOS signals from each of the facets, as the 
polygon rotates, to either speed up or slow down the rotating polygon to 
maintain uniform rotation. Thus there is a closed loop control from the 
SOS detector to maintain a uniform speed of rotation of the polygon. It is 
well known that the individual facets to a polygon, because of fabrication 
tolerances, are not all similar. The facets are different in degree of 
flatness or off center, and there is a non uniformity of the scanning beam 
reflected on to the photoconductive belt due to this non uniformity. 
In accordance with the present invention, for better accuracy and better 
motion quality and to overcome the inaccuracy of facet errors, the 
rephasing and speed control of the rotating polygon is phased or 
referenced to one selected facet of the rotating polygon. 
With reference to FIG. 2A, there is illustrated the misregistration that 
occurs in the prior art due to the fact that polygon facets are not 
geometrically precise. Thus, there is a resulting error or misregistration 
from scan length variation from any given facet to any other facet. In 
particular, in image on image systems, scan length variation between first 
and subsequent scanlines results in color misregistration. As shown in 
FIG. 2A, the lead edge of a first color separation begins with facet 
number 1 (straight, dashed line) and the lead edge of a second color 
separation begins with facet number 2 (solid, curved line). The second 
scanline of the first color separation is facet number 2, (solid, curved 
line) and the second scanline of the second color separation is facet 
number 3, (dashed, curved line). 
Thus, there is a misregistration error shown by the arrow when using 
different facets to start different color separations. On the other hand, 
FIG. 2B, illustrates facet elimination by using the same facet to start 
each color separation. In other words, with reference to facet number 1, 
the lead edge of each color separation begins with this facet. 
Misregistration errors due to facet tolerances are therefore eliminated. 
It should be understood that in FIG. 2B, the selection of a given facet 
such as either facet number 1 or facet number 2 is arbitrary, but once 
selected, the same facet is used to begin each color separation. For 
example, if facet number 2 is the selected facet, the images are 
superimposed on each other since the same facet is used to start the image 
for each color separation. The same facet is used for the rephasing as 
well as for speed control of the polygon. 
The use of the same facet for each revolution of the polygon is 
particularly applicable to a multi-pass image on image system to both 
reduce or eliminate error in a scanline as well as to eliminate start of 
scan spot signal differences for each facet. This, of course, assumes that 
a start of scan signal for a selected facet is the controlling signal 
throughout the scanning operation. Any arbitrary facet can be selected but 
once selected, the same facet is used for the rephasing and the same facet 
is used for speed control of the polygon. 
It should be noted that there are various methods of identifying and 
tracking a selected facet of the rotating polygon for each rotation of the 
polygon in accordance with the present invention. For example, an index 
timing pulse at the start of scan signal for the first projected image can 
be recorded and a timing period or counting method used to determined the 
passage or reflection of the beam from eight facets for a complete 
revolution and a beginning scan again for the selected facet. In this 
manner, the system is being rephrased to the same facet for each rotation 
of the polygon and for the start of scan or beginning scan cycle of each 
succeeding image in an image on image system. This eliminates the 
tolerance error of the polygon facets. In addition, by using the feedback 
from the SOS signal of the same facet of the polygon, there is more 
consistent feedback speed data accuracy and thus better motion quality. 
This leads to better speed control and a more steady state speed of the 
polygon. 
With reference to FIG. 3, there is an illustration of the elimination of 
facet errors when using a start of scan (SOS) pulse train as a motion 
encoder for speed control. Assuming a polygon is rotating at constant 
speed and the polygon has identical facets, the pulse train will be a 
uniform constant frequency with symmetrical periods between SOS pulses. 
However, if the polygon has facet errors, then the pulse train will have 
slightly different time periods between SOS pulses, although the time 
period will be a symmetrical same period for each particular same facet 
SOS pulse. The period difference for different facets caused inaccuracy 
when used as feedback for a motor polygon assembly speed control. However, 
in accordance with the present invention, the pulse train is divided down 
once per polygon revolution and the period of SOS pulses is measured from 
the same facet. This eliminates inaccuracy from the period measurements 
used as feedback and allows better accuracy feedback measurement for speed 
control when using an SOS pulse train as a motion encoder feedback signal. 
In particular, in accordance with a preferred embodiment at the start of 
scan of the first image to be projected in a multi pass system, a time 
stamp is triggered that is stored in a register. The particular facet of 
the eight facets of the rotating polygon that is being scanned at the 
start of scan location is then the particular facet to be tracked for 
subsequent SOS readings. That time stamp forms the bases of the time 
stamps for subsequent revolutions of the polygon and the difference in 
time stamps provides an error signal used as feedback to control the speed 
of the rotating polygon. 
In particular, a time period measurement between a given facet occurrence 
(SOS n) to the same given facet repeat occurrence (SOS n), relative to 
subsequent full revolutions of the polygon, provides an `error free` 
electronic representation of the speed of the polygon. The accuracy of 
this information allows the speed to be more precisely controlled by 
virtue of having an error free speed representation, thereby not 
erroneously gyrating the polygon motor speed to the smallest of 
magnitudes. The more precisely controlled speed results in line end of 
line position consistency, as shown in FIG. 3. The combined result is 
`color registration improvements`, in two directions, yielding true color 
on color alignment. Speed control is implemented by a period regulator 
method that provides adequate motion quality yet at the same time directly 
extends to the phase shifting function involved with polygon rephasing. 
While the invention has been described with reference to the structure 
disclosed, it will be appreciated that numerous changes and modifications 
are likely to occur to those skilled in the art, and it is intended to 
cover all changes and modifications which fall within the true spirit and 
scope of the invention.