U.S. patent number RE35,067 [Application Number 08/037,638] was granted by the patent office on 1995-10-17 for bi-directional registration of servo indexed webs.
This patent grant is currently assigned to FMC Corporation. Invention is credited to Donald J. Bauknecht.
United States Patent |
RE35,067 |
Bauknecht |
October 17, 1995 |
Bi-directional registration of servo indexed webs
Abstract
A bag machine fabricates plastic bags by transversely cutting
and sealing, at regularly spaced preselected locations, an elongate
plastic web having graphic material repetitively printed thereon.
In operation, the bag machine advances the web by a calculated draw
length, and an optical sensor, sensitive to the passage of eyemarks
printed in regularly spaced locations on the web, functions to
ensure that the web is cut and sealed only at the desired
locations. To avoid inaccuracies resulting from a gradual variance
between the nominal draw length and the actual distance between
eyemarks, a control system monitors the actual distance between
eyemarks and sets the nominal draw length to the average actual
spacing between eyemarks as measured over a preselectd number of
successive eyemarks.
Inventors: |
Bauknecht; Donald J. (Green
Bay, WI) |
Assignee: |
FMC Corporation (Chicago,
IL)
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Family
ID: |
23020458 |
Appl.
No.: |
08/037,638 |
Filed: |
March 19, 1993 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
267866 |
Nov 7, 1988 |
05000725 |
Mar 19, 1991 |
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Current U.S.
Class: |
493/11; 226/32;
493/22; 493/24; 493/29; 700/122; 700/167 |
Current CPC
Class: |
B26D
5/00 (20130101); B26D 5/34 (20130101); B31B
70/10 (20170801) |
Current International
Class: |
B31B
19/10 (20060101); B31B 19/00 (20060101); B26D
5/00 (20060101); B31B 001/08 () |
Field of
Search: |
;493/2,3,11,22,24,29,194-196 ;226/27,32 ;83/209,72,75,367
;364/469,474.09 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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54-42266 |
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Apr 1979 |
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JP |
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57-51446 |
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Mar 1982 |
|
JP |
|
0248932 |
|
Feb 1990 |
|
JP |
|
0263729 |
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Mar 1990 |
|
JP |
|
Primary Examiner: Lavinder; Jack W.
Attorney, Agent or Firm: Rudy; Douglas W. Low; Warren N.
Claims
I claim:
1. In a bag making machine having intermittently operated draw
rolls coupled to a main shaft for rotation therewith, said draw
rolls for, drawing by a predetermined nominal draw length, a web
having printed thereon a series of regularly spaced eyemarks, the
improvement comprising means, including an optical scanner for
sensing said eyemarks, an encoder for determining motion of said
main shaft and a central processing unit, for determining the
actual spacing between successive ones of the eyemarks; and
additional means for setting the predetermined nominal draw length
for subsequent bags substantially equal to the actual spacing plus
or minus a measured error quantity determined during the previous
draw plus a portion of a scanning zone between subsequent
successive ones of the eyemarks.
2. The improvement as defined in claim 1 wherein said additional
means changes the predetermined nominal draw length only when the
actual measured spacing between eyemarks differs from the
predetermined nominal draw length by a predetermined difference.
.[.3. The improvement as defined in claim 1 further comprising
display meads for indicating to an operator that the predetermined
nominal draw length has been changed..].
The improvement as defined in claim 1 wherein said additional means
calculates the average actual spacing between successive ones of a
plurality of eyemarks and sets the predetermined nominal draw
length to
said average actual spacing. 5. The improvement as defined in claim
4 wherein said additional means changes the predetermined nominal
draw length only when the spacing between each of the successive
ones of the
plurality of eyemarks exceeds said predetermined difference. 6. A
control system for controlling the operation of a bag making
machine of the type wherein a plastic web, having regularly spaced
eyemarks printed thereon, is advanced by a predetermined draw
length and stopped, the plastic web transversely cut and thermally
sealed to form a bag, said control system comprising:
means for sensing the passage of an eyemark by a predetermined
location;
means for determining the distance between the eyemark and the
predetermined location when the web is stopped for cutting and
sealing;
means for determining the .[.actural.]. .Iadd.actual
.Iaddend.spacing between adjacent ones of the eyemarks;
means for calculating the average actual spacing between the
eyemarks over a predetermined number of consecutive eyemarks;
and
means for setting the subsequent predetermined draw length
substantially equal to the average of said actual measured spacing
plus or minus a measured error quantity determined during the
previous draw plus a portion
of a scanning zone between the eyemarks. 7. A control system as
defined in claim 6 wherein said means for determining the actual
spacing between adjacent ones of the eyemarks calculates the
distance between adjacent ones of the eyemarks in accordance with
the formula:
where PR is the actual spacing between successive ones of the
eyemarks, DL is the predetermined draw length, Y is the distance
between an eyemark and the predetermined location when the web has
been stopped for cutting and sealing, and X is the distance between
the next subsequent eyemark and the predetermined location during
the next subsequent stopping, cutting and
sealing of the web. 8. A control system in accordance with claim 7
wherein the bag-making machine includes a motor for advancing the
web, and wherein said means for determining the spacing between the
eyemark and the predetermined location comprises an encoder coupled
to the motor and
operable to generate a signal indicative of motor revolution. 9. A
control system as defined in claim 8 wherein said setting means
changes the draw length only if said actual spacing between
adjacent ones of the eyemarks differs from the predetermined draw
length by a predetermined distance.
A control system in accordance with claim 9 further comprising a
display for indicating to an operator that the predetermined draw
length
has been set substantially equal to said average actual spacing.
11. A plastic bag machine operable to form plastic bags from a
plastic web having a plurality of regularly spaced eyemarks printed
thereon, comprising;
a draw roll assembly operable to advance the web by a predetermined
draw length;
an optical sensor operable to sense the passage of an eyemark by a
predetermined location;
distance sensing means for sensing a deviation distance Y the
sensed eyemark has moved beyond said predetermined location when
the web has been advanced by said predetermined draw length;
a first register operable to store said deviation distance Y;
a second register operable to store the next subsequent deviation
distance X sensed by said distance sensing means;
first calculating means for calculating a print repeat length in
accordance with the formula:
wherein DL is the predetermined draw length;
second calculating means for calculating the average of a
predetermined number of successive ones of said print repeat
lengths calculated by said first calculating means; and
control means for resetting the predetermined draw length to be
substantially equal to said average print repeat length calculated
by said
second calculating means. 12. A plastic bag fabricating machine as
defined in claim 11, wherein said draw roll assembly includes a
motor and wherein said distance sensing means includes an encoder
coupled to said motor and
operable to provide a signal indicative of rotation of said motor.
13. A plastic bag fabricating machine as defined in claim 11
wherein said distance sensing means, said first and second
registers, said fast and second calculating means and said control
means comprise a
microporcessor-based electronic control circuit. 14. A control
system for controlling the operation of a bag making machine of the
type wherein a plastic web, having regularly spaced eyemarks
printed thereon, is advanced by a predetermined draw length and
stopped, the plastic web transversely cut and thermally sealed to
form a bag, said control system comprising:
means for sensing the passage of an eyemark by a predetermined
location;
means for determining the distance between the eyemark and the
predetermined location when the web is stopped for cutting and
sealing;
means for determining the actual spacing between adjacent ones of
the eyemarks;
means for calculating the average actual spacing between the
eyemarks over a predetermined number of consecutive eyemarks;
and
means for setting the subsequent predetermined draw length
substantially equal to the average of said actual measured spacing
between the eyemarks.
5. A control system as defined in claim 14 wherein said means for
determining the actual spacing between adjacent ones of the
eyemarks calculates the distance between adjacent ones of the
eyemarks in accordance with the formula:
where PR is the actual spacing between successive ones of the
eyemarks, DL is the predetermined draw length, Y is the distance
between an eyemark and the predetermined location when the web has
been stopped for cutting and sealing, and X is the distance between
the next subsequent eyemark and the predetermined location during
the next subsequent stopping, cutting and
sealing of the web. 16. A control system in accordance with claim
15 wherein the bag making machine includes a motor for advancing
the web, and wherein said means for determining the spacing between
the eyemark and the predetermined location comprises an encoder
coupled to the motor and
operable to generate a signal indicative of motor revolution. 17. A
control system as defined in claim 16 wherein said setting means
changes the draw length only if said actual spacing between
adjacent ones of the eyemarks differs from the predetermined draw
length by a predetermined
distance. 18. A control system in accordance with claim 17 further
comprising a display for indicating to an operator that the
predetermined draw length has been set substantially equal to said
average actual
spacing. 19. In a bag making machine having intermittently operated
draw rolls couple to a main shaft for rotation therewith, said draw
rolls for drawing, by a predetermined nominal draw length, a web
having printed thereon a series of regularly spaced eyemarks, the
improvement comprising means, including an optical scanner for
sensing said eyemarks, an encoder for determining motion of said
main shaft and a central processing unit for determining the actual
spacing between successive ones of said eyemarks; and additional
means for calculating the average actual spacing between successive
ones of a plurality of eyemarks and sets the predetermined nominal
draw length to equal said average actual spacing.
. The improvement as deemed in claim 19 wherein said additional
means changes the predetermined nominal draw length only when the
spacing between each of the successive ones of the plurality of
eyemarks exceed a
predetermined difference. 21. The improvement in accordance with
claim 19 further comprising display means for indicating to an
operator that the
predetermined nominal draw length has been changed. 22. The
improvement in accordance with claim 19 wherein said additional
means changes the predetermined nominal draw length only when the
spacing between each of the successive ones of the plurality of
eyemarks exceeds said
predetermined difference. .Iadd.23. A machine having intermittently
operated draw rolls coupled to a main shaft rotation therewith,
said draw rolls for drawing by a predetermined nominal draw length,
a web having printed thereon a series of regularly spaced eyemarks,
the improvement comprising means including an optical scanner for
sensing said eyemarks, an encoder for determining motion of said
main shaft and a central processing unit for determining the actual
spacing between successive ones of said eyemarks; and additional
means for setting the predetermined nominal draw length for
substantially equal to the actual spacing plus or minus a measured
error quantity determined during the previous draw plus a portion
of a scanning zone between successive ones of the
eyemarks..Iaddend. .Iadd.24. The invention set forth in claim 23
further comprising display means for indicating to an operator that
the predetermined nominal draw length has been changed..Iaddend.
.Iadd.25. The invention set forth in claim 23 wherein said
additional means calculates the average actual spacing between
successive ones of a plurality of eyemarks and sets the
predetermined nominal draw length to said average actual
spacing..Iaddend. .Iadd.26. The invention set forth in claim 25
wherein said additional means changes the predetermined nominal
draw length only when the spacing between each of the successive
ones of the plurality of eyemarks exceeds said predetermined
difference..Iaddend.
.Iadd.27. A control system for controlling the operation of a
machine of the type wherein a web, having regularly spaced eyemarks
printed thereon, is advanced by a predetermined draw length and
stopped and subsequently started, said control system
comprising:
means for sensing the passage of an eyemark by a predetermined
location;
means for determining the distance between the eyemark and the
predetermined location when the web is stopped for cutting and
sealing;
means for determining the actual spacing between adjacent ones of
said eyemarks calculating the distance between adjacent ones of
said eyemarks in accordance with the formula:
where PR is the actual spacing between successive ones of the
eyemarks, DL is the predetermined draw length, Y is the distance
between an eyemark and the predetermined location when said web has
been stopped and X is the distance between the next subsequent
eyemark and the predetermined location during the next subsequent
stopping of the web;
means for calculating the average actual spacing between the
eyemarks over a predetermined number of consecutive eyemarks;
means for setting the subsequent predetermined draw length
substantially equal to the average of said actual measured spacing
plus or minus a measured error quantity determined during the
previous draw plus a portion of a scanning zone between the
eyemarks..Iaddend. .Iadd.28. Means for advancing a web a desirable
distance, said web having a plurality of regularly spaced eyemarks
thereon, said means comprising:
a draw roll assembly operable to advance the web by a predetermined
draw length;
an optical sensor operable to sense the passage of a printed
eyemark by a predetermined location;
distance sensing means for sensing a deviation distance Y the sense
printed eyemark has moved beyond said predetermined location when
the web has been advanced by said predetermined draw length;
a first register operable to store said deviation distance Y;
a second register operable to store the next subsequent deviation
distance X sensed by said distance sensing means:
first calculating means for calculating a printed eyemark repeat
length in accordance with the formula;
wherein DL is the predetermined draw length;
second calculating means for calculating the average of a
predetermined number of successive ones of said repeat lengths
calculated by said first calculating means; and
control means for resetting the predetermined draw length to be
substantially equal to said average repeat length calculated by
said
second calculating means..Iaddend. .Iadd.29. The improvement as
defined in claim 1 further comprising display means for indicating
to an operator that the predetermined nominal draw length has been
changed..Iaddend.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to plastic bag fabricating
machines and more particularly to systems for controlling operation
of such machines.
Various machines exist for automatically fabricating plastic bags
with economy and speed. Typically, these machines operate by
drawing a predetermined length of plastic web from a supply roll
and thereafter transversely cutting and thermally sealing the web
to form a bag.
Frequently, it is desired to have labels, instructions or similar
graphic material appear on the finished bags. To this end, the
graphic material is printed onto the plastic web in regularly
spaced locations or fields, and it is necessary to ensure that the
web is cut and sealed only at precise locations between the fields
in order to insure proper placement of the printed matter on the
finished bag.
One technique for ensuring that the web is cut and sealed at the
proper locations is to advance the web by a predetermined distance,
or draw length, equal to the spacing of the printed fields.
However, minor errors, resulting from stretching or shrinkage of
the web as it is printed and wound onto and off of the supply roll,
accumulate and gross misalignment can occur over time as the bag
fabricating machine operates. Another technique for ensuring that
the web is cut and sealed at the desired locations between adjacent
printed fields is to print regularly spaced indexing marks or
"eyemarks" on the web before the web is fed into the bag
fabricating machine. An optical sensor detects the passage of each
eyemark by a predetermined location and signals the machine when to
stop, cut and seal the web. However, it is often desired to include
printed matter between. successive eyemarks. In order to avoid
sensing other marks which appear to be eyemarks, it is preferred to
enable the optical sensor only over a small interval or "window" in
which the eyemark is expected to be seen.
This technique is effective in correcting minor systematic errors
in alignment (such as, an intermittent lengthening or shortening of
the actual distance between eyemarks). Progressive errors can arise
as a result of varying tensions as the web is wound and unwound
from the supply roll, and this can result in the eyemarks falling
outside of the sensing window. These types of errors can cause
improper alignment of the printed matter on the finished bags.
In view of the foregoing, it is a general object of the present
invention to provide a new and improved plastic bag fabricating
machine.
It is a more specific object of the present invention to provide a
new and improved system for controlling the operation of a plastic
bag fabricating machine so as to ensure proper alignment of printed
matter on the finished bags.
It is a still more specific object of the present invention to
provide a system for controlling the operation of a plastic bag
fabricating machine so as to compensate for progressive variations
between the nominal spacing of printed matter fields on the web and
the actual spacings of the fields on the webs.
SUMMARY OF THE INVENTION
The invention provides a system for controlling the operation of a
bag-making machine wherein a plastic web, having regularly spaced
eyemarks printed thereon, is advanced by a predetermined draw
length and then transversely cut and thermally sealed to form a
bag. The control system includes structural features for sensing
the passage of an eyemark by a predetermined location. Other
structural features include means for determining the distance
between the eyemark and the predetermined location when the web is
stopped for cutting and sealing. The control system further
includes structural futures for performing the following functions:
determining the actual distance between adjacent eyemarks,
calculating the average actual distance between the eyemarks over a
predetermined number of consecutive eyemarks and setting the
predetermined draw length substantially equal to the actual average
distance between the eyemarks.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevational view of a bag fabricating system
including a bag machine operable to form plastic bags from plastic
web;
FIG. 2 is an enlarged side elevational view of a bag machine.
FIG. 3 is a simplified block diagram of a control system for
controlling operation of the bag machine;
FIG. 4 is a diagramatic view, useful in understanding operation of
the control system, showing a plastic web in relation to an optical
scanner included in the control system;
FIG. 5 is another block diagram showing in greater detail the
control system shown in FIG. 3; and
FIG. 6 is a flow chart diagram useful in understanding the
operation of the control system.
DESCRIPTION OF THE PREFERRED EMBODIMENT
A system 10 for automatically fabricating plastic bags from a
continuous plastic web 12 is illustrated in FIG. 1. As shown, the
system 10 includes a supply roll 14 containing the web 12, and an
optional print mechanism 16 for repetitively printing graphic
material 17 (FIG. 4) at regularly spaced locations on the web 12.
The system 10 further includes a bag machine 18 for transversely
cutting and sealing the web 12 to form individual plastic bags, and
an optional stacker mechanism 20 for stacking the bags formed by
the bag machine 18. A user-operable control panel 22 provides
user-control over the automatic bag fabricating system 10.
Referring to FIG. 2, web 12 is drawn from the supply roll 14 and is
fed to the bag machine 18 where it is drawn forward between a pair
of infeed rolls 24. After passing through the infeed rolls 24, the
web 12 travels around a plurality of idler rollers 26 which
function to maintain a substantially constant supply of the web 12.
After passing through the idler rollers 26, the web 12 passes
between a pair of draw rolls 28 positioned immediately upstream of
a transverse cut and seal bar 30 which cuts and seals the web 12 to
form the individual bags.
To ensure proper registration of the seal relative to the printed
matter 17 on the web 12, a plurality of eyemarks 32 (FIG. 3) are
printed at regular intervals along the edge of the web 12, and an
optical scanner 34 photoelectrically senses the passage of each
eyemark 32. Because other printed matter 17, detectible by the
optical scanner 34, frequently appears between successive eyemarks
32, the optical scanner 34 is not continuously enabled but, rather,
is enabled only for brief periods during which it is expected that
an eyemark 32 should appear. In one embodiment of this invention
the bag machine 18 advances the web 12 by a predetermined or
calculated distance (DL) which, in this embodiment, is
substantially equal to the nominal distance between the eyemarks
32, as set by the operator on the control panel 22 and as sensed by
the optical scanner. Means for determining the actual spacing
between successive one of the eyemarks includes the optical
scanner, the encoder and the central processing unit (as will be
explained further on) is provided. Additional means for setting the
predetermined nominal draw length substantially equal to the actual
measured spacing between sucessive ones of the eyemarks is also
provided. To this end, in another embodiment the bag machine 18
advances the web 12 by a predetermined or calculated distance (DL),
which, in the illustrated embodiment, it substantially equal to the
nominal distance between the eyemarks 32, as set by the operator on
the control panel 22, plus or minus a measured error quantity
determined during the previous draw, plus one-half the width of the
"window." The optical scanner 34 is then enabled only during a
preset portion (e.g., the last one-half inch) of each advancement
of the web 12. In this manner, the optical scanner 34 is only
responsive to eyemarks 32 appearing within a definite zone or
scanning "window".
The actual distance or spacing between eyemarks 32 can, for a
number of reasons, vary from the nominal distance set by the
operator on the control panel 22. For example, dimensional changes
can result from the printing step itself, as well as from varying
tensions as the web 12 is withdrawn from the roll 14 and advanced
through the system 10. As a result, the eyemarks 32 can fall
outside of the scanning "windows" and result misalignment of the
printed matter 17 on the finished bags.
In accordance with one aspect of the invention, the bag machine 18
is provided with a control system 36 which automatically
compensates for any progressive variation of the actual distance
between eyemarks 32 from the nominal draw length set on the control
panel 22.
Referring to FIGS. 3, 4 and 5, the control system 36 includes the
control panel 22 and the optical scanner 34. The bag machine 18
includes an electrical servo motor 38 which is coupled, by means of
a belt 40 or similar arrangement, to the draw rolls 28. The control
system 36 further includes an encoder 42 which is directly coupled
to the motor 38 and which functions to provide electrical pulses
indicative of the rotation of the motor shaft (e.g., 4000 pulses
per motor revolution). The signal generated by the encoder 42, as
well as the signals developed by the control panel 22 and the
optical scanner 34, are fed as inputs to computer control circuits
44. The computer control circuits respond to these inputs by
instructing a motor controller 46 to drive the motor 38 so as to
advance the web 12 a sufficient distance to provide a desired
orientation of the eyemarks 32 relative to the transverse cut and
seal bar 30. The computer control circuits 48 include a resemble
counter 44a which counts the pulses developed by the encoder
42.
In accordance with another aspect of the invention, the control
system 36 functions to sense the actual spacing between a
predetermined number of successive eyemarks 32. In the event the
distances of such actual spacings show a trend to deviate from the
nominal draw length, the control system 36 functions to change the
nominal draw length to be substantially equal to the average of the
the actual spacings thus sensed. In this manner, the control system
36 functions to ensure that the eyemarks 32 continue to appear
within the scanning "windows" even through the actual spacings
between eyemarks tend to deviate from the nominal draw length.
Because additional printed matter 17 may appear between the
successive eyemarks 32, direct sensing of the distance between
eyemarks is impractical and the actual distance between successive
eyemarks is determined in accordance with the scheme illustrated in
FIG. 4. In FIG. 4, the solid lines depict the relative positions of
the optical scanner 34 and an eyemark 32 during a current
bag-forming cycle, while the phantom figure depicts the relative
positions of the optical scanner 34' and an eyemark 32' during the
immediately preceding bag-forming cycle. The distance X is the
distance the leading edge of the eyemark 32 went past the scanner
34 during the current bag-forming cycle, while the distance Y is
the distance the leading edge of the next preceding eyemark 32 went
past the scanner 34 during the immediately preceding bag-forming
cycle. Distances X and Y are each determined by counting the pulses
generated by the encoder 42 between the time the leading edge of
and eyemark 32 is sensed and the time the web 12 is stopped for the
cut and seal operation. The remaining distance, the calculated draw
length DL, is the total distance the web 12 has been advanced for
formation of the current bag. In the illustrated embodiment, DL is
calculated in accordance with actual current operating conditions
and is equal to the nominal draw length, plus or minus the error
quantity measured during formation of the previous bag, plus
one-half the width of the "window." It will be appreciated,
however, that in other embodiments, the calculated draw length DL
might be calculated in a different manner. Once these quantities
are known, the actual distance or print repear length (PR) between
the successive eyemarks 32 is given by the formula:
In this manner, the actual distance or spacing between eyemarks 32
can be determined even through the optical scanner 34 is disabled
over much of the distance traversed by the web 12 between the
bag-forming cycles. The print repeat length PR thus calculated
determines the draw length for the next bag forming cycle.
The control system 36 is illustreated in greater detail in FIG. 5.
As shown, the computer control circuits 44 include a central
processing unit (CPU) 48 which receives data from the control panel
22, the optical scanner 34 and the encoder 42. Also included are a
first register 50 for storing the current distance X, a second
register 52 for storing the previous distance Y, and a draw length
register 54 for storing the nominal draw length initially entered
on the control panel 22. Calculating means 56 (which may comprised
part of a suitably programmed microprocessor-based computer system)
are provided for calculating the print repeat length in accordance
with the formula set forth above, and a memory 58 is provided for
storing a plurality of consecutive print repeat lengths thus
computed. In the illustrated embodiment, the memory 58 is
configured to store ten such print repeat lengths PR.sub.1 through
PR.sub.10, although it will be appreciated that a greater or lesser
number can also be selected. From the memory 58, the print repeat
lengths PR.sub.1 through PR.sub.10 are provided to a second
calculating means 60 which computes a new nominal draw length equal
to the average of the print repeat lengths PR.sub.1 through
PR.sub.10. Once the new draw length has been thus calculated, it is
loaded into the draw length register 54 and from the draw length
register into the CPU 48 and the first calculating means In
addition, the CPU 48 functions to display (e.g., on a cathode ray
tube 62) the new nominal draw length at the control panel 22 to
show the system operator that the nominal draw length has been
automatically changed.
The control system 36 of the present invention is preferably
implemented utilizing microprocessor-based circuitry in conjunction
with suitable programming. One possible program is exemplified by
the flow chart diagram of FIG. 6. Referring to the FIG., the system
36, after receiving an instruction to begin operation, first reads
and stores the nominal draw length entered by the system operator
on the control panel 22. The control system then awaits the
generation of a "go" signal from the CPU 48. When a "go" signal is
received, the servo motor 38 starts and the system monitors the
advancement of the web 12 by counting pulses from the encoder 42
until it is determined that the web 12 has been advanced to within
a specified distance (one-half inch in this example) of the current
calculated draw length. Once it has been determined that the web
has been advanced to within the specified distance of the
calculated draw length, the optical scanner is enabled and both the
optical scanner 34 and the servo motor 38 are monitored until
either the leading edge of the eyemark 32 is detected by the
scanner 34 or the servo motor 38 stops running. When either event
occurs, the encoder counter 44a is set to zero and the status of
the servo motor 38 is ascertained. It will be appreciated that the
specified distance sets the width of the scanning "window."
While the servo motor 38 is running, the system idles and the
encoder count increases as the motor shaft rotates. When the servo
motor stops running, thereby indicating that the web 12 has been
advanced by the current calculated draw length, a constant R1 is
set equal to the final encoder output. Accordingly, R1 represents
the length by which the leading edge of the eyemark 32 is displaced
from the optical scanner 34 when the web 12 has stopped.
In the event the eyemark 32 is not detected by the optical scanner
34 during the scanning "window", R1 will be substantially zero. If
R1 is substantially zero (less than 0.03 inches in the example
illustrated), the system 36 increase the next repeat length by a
predetermined increment (0.15 inches in the illustrated example)
and the system enters an idle to await the generation of a "go"
signal. Thus, it will be appreciated that the length of each
subsequent advancement of the web 12 will be the nominal repeat
length plus 0.15 inches, and an eyemark 32 will eventually appear
within the scanning window.
When an eyemark 32 does appear within the scanning "window," R1
will be non-zero. When the web 12 and the printed matter 17 thereon
is in proper registration relative to the transverse cut and seal
bar 30, the leading edge of the eyemark 32 should appear
substantially midway between the limits or edges of the scanning
"window". In the example illustrated, such proper registration is
indicated by R1 having a value of substantially 0.25 inches or
one-half the scanning window width. Accordingly, if R1 has a value
between 0.2 inches and 0.3 inches, acceptable registration is
indicated. In this event, a pair of additional variable constants
R2 and R3, are set to zero and the next draw length is set equal to
the nominal draw length, minus the current R1 value, and plus
one-half the width of the scanning "window" (one-quarter inch in
the illustrated example). By calculating the next draw length in
this manner, compensation, in either direction, is automatically
made for the small distance by which R1 differs from the distance
representing perfect registration (0.25 inches in the illustrated
example).
In the event R1 does not fail within the desired range (i.e., R1 is
less than 0.2 inches or is greater than 0.3 inches in the
illustrated example), the variable constant R3 is incremented by
the print repeat length PR calculated in the manner described by
reference to FIG. 4, and the variable constant R2 is incremented by
one. R2 serves, therfore, to indicate the number of times that an
eyemark has fallen within the scanning "window" but not within the
desired range of the "window" midpoint.
R2 is next compared against a predetermined constant representing
the number of consecutive times that R1 fails to fall within the
desired range of the midpoint. In the illustrated example, ten such
consecutive failures can occur before the system takes action to
change or update the nominal draw length. As long as R2 remains
less that the predetermined constant, the next or calculated draw
length DL is set equal to the nominal repeat length, minus R1, and
plus 0.25 inches. For this cycle, however, R2 and R3 are not set
equal to zero and these values are retained as the system returns
to await initiation of the next bag forming cycle.
In the event a discrepancy persists between the actual spacing
between eyemarks and the nominal draw length, the variable constant
R2 will eventually equal the predetermined constant (ten in the
illustrated example). At the same time, the variable constant R3
will substantially equal the sum total of the actual distances
between eyemarks over the preceding ten consecutive bag fabricating
cycles when this occurs, the system 36 calculates the average
actual distance between consecutive eyemarks 32 and changes the
nominal draw length to the calculated average. Thereafter, the
variable constants R2 and R3 are set to zero and the system
recycles to await initiation of the next bag fabricating cycle.
It will be appreciated that only those actural distances that are
determined as a result of an eyemark falling within the scanning
"window" will be utilized in calculation of the average actual
print repeat length. An eyemark can, for example, fail to appear
within the scanning "window" as a result of improper
synchronization of the web 12 relative to the bagging machine 18
rather than occuring as a result of an actual change in the eyemark
spacing. As a consequence of the eyemark not appearing within the
"window" due to asynchronization, the system 36 sets R2 and R3 to
zero thus resetting the consecutive count and compelling the system
to start over in its search for ten consecutive bags that are out
of the midpoint range. Accordingly, any errors sufficient to place
the eyemarks totally outside the scanning window will not be
considered in calculating the average actual print repeat length.
It can be seen that any bag that falls within the midpoint range
will also reset R2 and R3 to zero as described above. In this
manner the control system responds to gradual changes in eyemark
spacing rather than to sudden or abrupt changes.
The system as shown and described herein automatically corrects for
variations in the actual eyemark spacing of a plastic web used in
the fabrication of plastic bags. Accordingly, the system reduces
operator supervision and intervention, and ensures the fabrication
of a high quality product by maintaining a desired orientation and
position of printed matter on the finished bags.
While a particular embodiment of the invention has been shown and
described, it will be obvious to those skilled in the art that
changes and modifications may be made without departing from the
invention in its broader aspects, and, therefore, the aim in the
appended claims is to cover all such changes and modifications as
fall within the true spirit and scope of the invention.
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