Field of the Invention
The present invention relates generally to the positioning
of printing surfaces or printing devices and specifically to the use of various
calibration devices and methods for positioning of a printing device relative to
print media, such as paper.
Background of the Invention
Multi-scan printing involves the use of a printing device
smaller than the size of a piece of paper. Therefore, to print on the entire piece
of paper, the printing device is moved relative to the piece of paper during the
process of printing. Multi-scan printing provides many benefits, including low cost
from the use of small printing devices. Also, very large pieces of paper can be
imprinted by the use of multi-scan printing.
One difficulty in multi-scan printing involves relocating
the printing device relative to the piece of paper from one printing swath to the
next. The process of juxtaposing two swaths is called "stitching." Stitching accuracy
must be high for the printed image not to contain undesirable visible artifacts.
Similarly, the use of multiple printing devices to obtain a multi-color printed
image also requires the alignment of one printing device to another to avoid visible
One approach to dealing with the difficulties in multi-scan
printing has been the use of printing devices to create narrow swaths and, therefore,
frequent stitching of the swaths. By the use of narrow swaths, it is possible to
move the printing device relative to the piece of paper a known distance by the
rotation of gears, preferably one rotation per swath. Printing swath widths in this
type of multi-scan printing are typically less than one centimeter wide. However,
this approach reduces printing efficiency by requiring many swaths to print an image.
A more efficient approach to multi-scan printing does involve
the use of larger printing devices, such as printing devices capable of printing
a swath of over 1 cm wide. Multi-scan printing involving wider swaths provides substantial
benefit in increasing the speed of printing. However, one difficulty of this type
of multi-scan printing involves the positioning of the printing device relative
to the paper in order to provide high accuracy in stitching. One approach has been
to use high accuracy encoders to establish a location of the printing device relative
to the paper. High costs of such precise encoders have proven to be prohibitive
in some applications. Furthermore, calibration of such encoders can be difficult.
For example, while factory calibration procedures may initially calibrate the encoders,
by the time a printing device is put in service in the field, the encoders may be
out of alignment, resulting in poor stitching. Even if calibration can be maintained
up to the time of initial use of the printing device, a printing device may experience
a change in alignment characteristics during use due to changes of temperatures
of various components involved with positioning the printing device relative to
the piece of paper. Furthermore, a printing device will likely eventually require
replacement. In any event, requiring the return of a printing device to the factory
for calibration or replacement is typically undesirable.
US 6,155,669 describes a pagewidth inkjet printer including
a printbar mounted encoding system. The printer includes a belt for transporting
a recording medium beneath a plurality of print bars. Fiducial marks are located
directly on the belt at a predetermined spacing such that the location of the recording
medium can be accurately determined when passing each of the print bars.
EP 0915050 A1 describes a web having alignment indicia
and an associated web feeding and working apparatus. The apparatus includes a web
and an associated web feeding and handling apparatus for performing a work operation
on the web. The apparatus feeds the web in a feed direction and the operation of
the drive is controlled by lateral and longitudinal alignment indicia located on
the web. The line indicia as arranged along an axis are of uniform size.
EP0584792-A2 describes a sheet feeding apparatus. A sheet
feeding apparatus detects the position of a sheet during feeding thereof and calculates
a deviation of the detected sheet position relative to a reference position to thereby
enable accurate colour printing. A follower roller for detecting a feeding amount
of the sheet is in contact with the sheet which is fed by a sheet feeding roller
1, so that it is rotated in accordance with the movement of the sheet. A sensor
generates an output signal each time the follower roller rotates a predetermined
angle of rotation.
EP 0917961 A2 discloses a system and a method according
to the preambles of claims 1 and 7.
SUMMARY OF INVENTION
It is the object of the present invention to improve an
image forming system with regard to precision and reliability of the paper positioning
system. This object is achieved by providing an image forming system according to
claim 1 and a method of positioning paper according to claim 7. Embodiments of the
invention are set forth in the dependent claims.
Brief Description of the Drawings
The foregoing and other objects, features and advantages
of the invention will be apparent from the following description and apparent from
the accompanying drawings, in which like reference characters refer to the same
parts throughout the different views. The drawings illustrate principles of the
invention and, although not to scale, show relative dimensions.
Detailed Description of the Invention
- Figure 1 provides a top view of a first example;
- Figure 2 provides a top schematic view of the first example;
- Figure 3 provides a side schematic view of the first example;
- Figure 4 provides a view of one configuration of marks according to a variation
of the present invention;
- Figure 5 provides a view of another configuration of marks according to a variation
of the present invention;
- Figure 6 provides a top view of a variation of the first example.
The present invention overcomes the difficulties of the
prior art by the use of an optical sensor capable determining the position of a
printing device relative to a piece of paper or a paper-handling surface of an image
forming system. The term "image forming system" includes a collection of different
printing technologies, such as electrophotographic, electrostatic, electrostatographic,
ionographic, acoustic, piezo, thermal, laser, ink jet, and other types of image
forming or reproducing systems adapted to capture and/or store image data associated
with a particular object, such as a document, and reproduce, form, or produce an
image. An example of an image forming system can be found in U.S. Pat. No. 5,583,629
to Brewington et al. As used herein, the term "paper" is intended to include a wide
variety of imprintable media.
The present invention, in various embodiments, involves
the use of the optical sensor to reading marks to detect movement and/or direction
of movement or spacing of imprints on the paper.
According to an example, an image forming system 100 is
provided as shown in Figure 1. The image forming system includes a paper-handling
surface 110 adapted to receive a piece of paper 120. The paper-handling surface
110 is preferably configured to move the piece of paper 120 relative to a carriage
130. The carriage 130 is preferably provided with at least one printing device 140.
For ease of discussion, Figure 2 illustrates several reference
directions to aid in description of the present invention. A direction of travel
125 is also described as a positive direction along an X axis. An X direction is
parallel to the X axis. A slow scan direction is also parallel to the X axis. The
carriage 130 travels parallel to a Y axis enabling the printing of a swath 131.
The Y axis is within the same plane as the X axis and is perpendicular to the X
axis. A direction of travel in either direction along the Y axis is known as the
fast scan direction or the Y direction. Also for purposes of discussion, a Z axis
is provided, perpendicular to both the X and Y axis.
As shown in Figure 3, the image forming system 100 may
further be provided with a first vacuum plenum 116 and a second vacuum plenum 118.
The first and second vacuum plenums 116, 118 are located under the paper-handling
surface 110 to hold the paper 120 to the paper-handling surface 110. A first roller
112, second roller 114 and third roller 115 may also be provided to define a path
for a belt forming the paper-handling surface 110. A wide variety of alternative
configurations are available for the assembly of the paper-handling surface 110
and associated devices to hold the paper 120 to the paper-handling surface 110.
As shown in Figure 1, the image forming system 100 further
includes an optical sensor 200 and a plurality of marks 250 arranged so that the
marks intersect an axis 255 that is substantially parallel to the direction of travel
125 of the paper 120. The plurality of marks 250 preferably includes small marks
260 interspersed with at least one large mark 270. Alternatively, or in addition,
spacing between marks within the plurality of marks 250 may be varied. The plurality
of marks 250 may be formed by imprinting on the paper-handling surface 110 or by
cutting holes in the paper-handling surface 110 so as to provide a contrasting appearance
to the paper handling surface 110.
In operation, the example involves locating the optical
sensor 200 over the plurality of marks 250 during movement of the paper-handling
surface 110. The optical sensor 200 is then able to monitor the plurality of marks
As shown in Figure 4, the plurality of marks 250 may be
sized approximately 0.51 mm (0.20 inches) along the X axis, parallel to the axis
255. From leading edge to leading edge, the marks may be spaced 1.0 mm (0.040 inches).
This results in approximately 25 marks per 25.4 mm (inch). The size and spacing
of the plurality of marks 250 was selected as a tradeoff between maintaining a sufficient
number of marks for statistical error reduction while maintaining sufficient space
between the marks so that, for typical velocities of the paper 120 and the sampling
rate of the optical sensor 200, the optical sensor 200 is able to retain a unique
identifier for each of the marks during motion of the paper-handling surface 110.
Because, in the configuration shown in Figure 4, each of the marks appears the same,
the optical sensor 200 must be able to track each mark individually in order to
accurately determine the amount of movement of the paper-handling surface 110.
According to the present invention, the plurality of marks
250 is modified to include both small marks 260 and large marks 270, as shown by
way of example in Figure 5. A wide variety of alternatives are within the scope
of the invention. For example, any combination of small or large marks may be used.
Alternatively, the plurality of marks 250 may include marks of sizes other than
those shown by way of example in Figures 4 and 5, or may involve spacing different
than that shown in Figures 4 and 5. One advantage of the configuration of the plurality
of marks 250 shown in Figure 5 is that spacing between the marks can be maintained
so as to, as discussed above, maintain a balance between statistical error reduction
and maintaining unique identification of each of the marks during movement of the
paper-handling surface 110 within velocities contemplated in the design. Furthermore,
the large marks 270 assist in the ability to determine a direction of travel of
the paper-handling surface 110 because they are distinguishable from neighboring
Preferably, the image forming system 100 is provided with
a controller 300 adapted to obtain readings from the optical sensor 200 to determine
movement of the paper-handling surface 110.
According to an embodiment of the invention shown in Figure
1, the carriage 130 may be located, as shown in Figure 6, away from the paper 120
while the optical sensor 200 is located along the axis 255 and over the plurality
of marks 250. This configuration may result in more efficient operation of the image
forming system 100 when the optical sensor 200 is mounted to the carriage 130. Specifically,
the optical sensor 200 is conveniently located for positioning over the plurality
of marks 250 at the end of printing a swath along the Y axis.
According to this embodiment of the invention, the optical
sensor 200 may be mounted to the carriage 130. According to this embodiment, one
or more printing devices 140 may be mounted to the carriage 130. According to a
variation, one or more heaters 150 may be mounted to the carriage 130 to assist
in drawing ink applied to the paper 120 by the printing device 140. Preferably,
multiple printing devices 140 will be provided so as to print multi-color images
on the paper 120.