Note: Descriptions are shown in the official language in which they were submitted.
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REGISTRATION OF PRESS TO EXISTING PATTERN
Summary of the Invention
This invention deals with a process involving
registration and more specifically with the registration
of a pattern press in regard to a preexisting pattern on
an intermittently moving material.
A frequent requirement in the flooring and
other industries is the registration of a second process
upon a visual pattern already existing upon a web or
other piece of material which is moving through the
production process. When the second process involves a
punch or embossing press, an added requirement is that
the second process activate when the web is not moving,
but after it has advanced into position, so that the
first pattern is properly located. A precise visual fit
between the two patterns is required.
Such processes involve the considerable
complication of properly aligning the web, with the
first pattern upon it, with the apparatus for producing
the second pattern. This alignment or registration must
occur in two dimensionsl the direction of web motion,
called the machine direction, and the web width
dimension, the across-machine direction. Of these two,
the machine direction is far the worst since it is the
direction in which the material is constantly moving
with production speeds.
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A very typical adjustment procedure for such a
process is to place a registration mar]c on the web at
the same time the first pattern is located, and to use
this registration mark to register the second pattern.
The registration mark gives the advantage of being a
standard recognizable mark regardless of changes in
pattern, and, if the registration mark is beyond the
useable area of the web, it may even be a hole in the
webO
Such registration marks are usually sensed by
mechanical or passive optical devices, the latter
operating on either light passing through a punched hole
or reflecting from the web and being changed by the
passage of the registration mark.
In theory, an optical sensor in cooperation
with an electronic control system stops the web in
relation to the position of the reyistration mark and
the first pattern is therefore aligned with the device
producing the second pattern. However, in actual
operation a great many factors can change this
registration. I, for instance, the system requires the
web to stop exactly a certain fraction of a second after
the registration mark is recognized, the exact alignment
will be affected by variations in such items as motor
speeds, stretching or shrinking of the web, process
temperature, error in the electronic timing circuit,
electrical noise, and even the sharpness of the edge o
the registration mark.
For many of these items, for instance,
measurement of time and stretching of the web, the error
will also be cumulative. With frequently repeating
operations a small increase in the elongation of the web
would, for example, quickly add up to cause the
registration to be visibly in error on subsequent
pattern repeats and misregistration or phase error
occurs.
Until now, the correction of this type of
error has usuallv been the task of an experienced
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operator who manually adjusted some parameter. In the
above example, the operator would adjust the position of
the sensor with respect to the patterning apparatus to
compensate for the visible error. In reality/ most
existing systems require constant operator adjustment to
insure that phase error is eliminated and registration
is achieved.
The present invention directly attacks the
problem of cumulative error and phase error by locating
two registration marks associated with the existing
pattern and, once the product is in proximity to the
patterning device, adjusting the spatial relationship
between the registration marks and the patterning device
by moving the patterning device or the product, to share
any error between the leading and trailing edges of the
patterns. In essence, centerline registration is
achieved and no substantial accumulation of error can
occur because each repeat of the pattern is individually
adjusted before the second pattern is applied.
To further enhance the accuracy of this
invention, it uses a crosscorrelation technique after
optically sensing the position and timing of the two
registration marks~ Crosscorrelation, in effect,
locates the center of each registration mark and
measures the time relationship between marks rather
than, as previous systems did, attempting to sense the
position of a leading edge of the single mark. The
difficulty with sensing the leading edge of an
electronic signal is that it actually involves sensing
the point at which a particular amplitude or threshold
value is reached, and, therefore, depending upon the
rise of the slope of the signal, it can vary con-
siderably in time. The slope of the leading edge of the
signal in such situations as reflection from a printed
mark will also vary with the sharpness of the edge of
the printed mark. The center of the mark, that is, the
location of the peak of the electronic signal is a more
precise registration reference, and it is that point
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which, in effect, is found when croscorrelation is used.
Actually, in this system the crosscorrelation technique
mathematically matches one signal to another signal in a
"statistical best~fit" type of comparison and then
measures the timing error between them. Since the
present invention actually uses two optical signals it
ultimately compares the locations of two pattern
reference points to precisely determine the pattern
location, as opposed to evaluating the less reliable
leading edge of a single reference point.
~ The present invention uses two digital line
scan cameras as active scanners. The scanner beams move
in a synchronized pattern, but are 180 degrees out of
phase with each other, The synchronous scans move
toward and then away from one another in an exact time
and position relationship, so that when the two
registration marks are located, one by each scanner, the
timing between the marks can be determined.
To do this, the signals from the scanners are
continuously crosscorrelated as described above to
precisely establish their relative location or phase
relationship. The error is then determined, and a press
control system adjusts the relationship of the press to
the material with the preexisting pattern to exactly
center the press between the two registration marks, and
the second pattern is then applied.
It can easily be appreciated that if, for
instance, the web has stretched one-tenth of an inch
between the registration marks, this system will, by
centering the pattern, spread that error over the whole
pattern, whereas a single registration mark system will
cause a maximum error or misregistration o one-tenth
inch at the edge of the pattern repeat. Such a visibly
obvious error can make an otherwise sa~isfactory product
into a reject.
The present invention, by using two
registration marks, statistical crosscorrelation of
signals, and adjustment of the press iteslf, can
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significantly increase product yield even under the same
error produciny conditions which presently exist. It
does so essentially by assuring that the errors will not
accumulate and are minimized both within a single
pattern repeat and from one repeat to another.
Brief Description of the Drawings
The figure is a simplified schematic diagram
of the preferred embodiment of the invention.
Detailed Description oE the Invention
The figure is a simplified schematic represen-
tation o the preferred embodiment of the invention in
which embossing system 10 operates upon product 12 with
preexisting pattern 1~ upon it. Product 12 is moved
intermittently in direction A by conventional methods
well known in the art, and is stopped so that press 16,
moved vertically by power unit 18, can put a second
pattern upon and in register with pattern 14.
Press 16 can also be used in other instances
to punch holes in or die-cut product 16, also in
register with a preexisting pattern.
To accomplish the desired registration,
product 12 is initially supplied with registration marks
20. These may be printed marks or holes punched through
product 12, but in either case they are produced simul-
taneously with pattern 14 and are therefore already in
register with pattern 14. Scanners 22 and 24 are
digital line-scan cameras which are oriented such that
their beams scan the region of the location of registry
marks 20 and react to them.
Scanners 22 and 24 are attached to press 16 by
holders 23 and 25. Their optical scans oscillate in
synchronism, but exactly out of phase with each other
along a line parallel to the direction of travel of
product 12. Scanning speed is independent of the speed
of the production process. The fixed position of the
scanners is determined by the spacing of registry marks
20 and the pattern repeat length.
Scanners 22 and 24 are spaced apart an even
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integral number of repeats of the pattern and they are
substantially equally spaced on either side of press 16.
Their scanning action is parallel to the movement of
registry marks 20, and the scan range must be sufficient
S to include all possible errors of web positioning that
may occur during normal operation of a production line.
Scanners 22 and 24 are spaced a known distance
from the position of press 16 by being physically
attached to press 16. Therefore, as they each sense
registration marks 20 which have stopped beneath them
when pro~uct 12 stops, the timing of the pulse each
sensor develops indicates the relative position of the
registration marks with respect to the centerline of
press 16. If, for instance, the scanning beams are
separating and scanner 22 develops an electrical signal
from the reflection of light from its registration mark,
before scanner 24 does, it is then apparent that the
registration mark under scanner 22 is closer to the
midpoint of the distance between the scanners than the
registration mark under scanner 24. Crosscorrelation of
the two pulses provides a measure of the timing
difference, or phase error, between pattern 14 of web 12
and press 16 upon which scanners 22 and 24 are mounted.
Press controller 28 then produces a control signal which
2; is sent to press ad~uster 30 which moves press 16
slightly one way or the other, parallel to the direction
of product motion, to center the new pattern between the
existing registration marks and divide the existing
registration error. Once this adjustment is made, press
16 is activated by power unit 18 and the second pattern
is placed on product 12.
The present invention uses crosscorrelation to
compare the signals from scanners 22 and 24 in order to
accurately measure the difference in time between ~he
signals.
Crosscorrelator 26, which is available as
standard instrumentation, mathematically compares the
signals from scanners 22 and 24, and measures the time
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difference between them. As described previously, such
a system furnishes a more accurate measurement of the
time difference between the two signals because it does
not compare the times at which the signals reach a
certain amplitude level. The present system, instead,
compares the whole of each signal to evaluate
statistically the time difference between them. A
visual "best-fit" results, which is, of course, the
ultimate test of pattern registration of printed,
embossed and die-cut media.
The measurement of the time difference between
signals from scanners 22 and 24 which results from the
operation of crosscorrelator 26 given information on the
phase relationship between the "whole" signals. It is
then sent to press controller 28 which converts it to a
distance for which press 16 must be adjusted. Press
controller 28 then activates press adjuster 30 to
actually displace press 16 and thus change the spatial
relationship between press 16 and product 12 to correct
for measured registration error.
The present invention can also be beneficial
in some processes although it is impractical to adjust
press orientation. If, for instance, web rollers 32 can
be reversed, web 12 rather than press 16 can be moved to
correct for registration error. However, even in
applications in which web 12 cannot be reversed to
correct registration, additional forward motion can
correct some errors. Moreover, press controller 28 can
be replaced with web controller 34 which controls web
drive 36, and web controller 34 can adjust web 12 so
that the next pattern repeat will be corrected to at
least prevent the registration error from repeating and
to prevent accumulation o~ error.
The combination of the use of two registration
marks to cont~-ol the adjustment of the press or the
product permits the present invention to minimize
registration error due to variable pattern length.
Additionally, use of crosscorrelation to process the
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signals from optical scanners eliminates errors that
otherwise occur because of variations in signal shapes
and the presence of signal noise.
It is to be understood that the form of this
invention as shown is merely a preferred embodiment.
Various changes may be made in the function and
arrangement of parts; equivalent means may be
substituted for those illustrated and described; and
certain features may be used independently from others
without departing from the spirit and scope of the
invention as defined in the following claims.
For instance, the invention could also be used
on individual tiles on a production line, in a fashion
similar to the web described, and in such a case the
sensors could be operated by the edges of the tiles
rather than registration marks.
Moreover, the invention could also be used to
adjust registration in the direction transverse to the
movement of product.