Note: Descriptions are shown in the official language in which they were submitted.
CA 02236742 2006-06-05
-1-
Apparatus and Method for Controlling Web Speed
Background of the Invention
1. Field of the Invention
This invention relates to plastic bag fabricating machines and more
particularly to an apparatus and method for controlling the flow of film.
2. Description of Related Art
An intermittently working bag machine stops production after a
predetermined number of bags has been produced to allow a stacking
conveyor to be indexed. After the indexing has occurred, production can be
resumed. This process interruption is generally known in the industry as cycle
interrupt. As a result, the next predetermined number of bags can be
produced and stacked on an adjacent stacking device after the cycle interrupt
has been completed.
A bag machine may employ a driven nip roll system to purl film from an
unwind stand. The speed of these nip rolls is controlled by a dancer. The
dancer is capable of not only storing a certain amount of film, it may also
provide tension for the film.
Referring to Figure 1, there is shown a prior art system for controlling
the infeed of the film or web 10 to a bag making machine 12. The web 10
may be fed through a plurality of rolls 14 and through a pair of nip rolls 16.
The web 10 may then be fed through rolls 17 on a dancer 18 having a dancer
arm 19 and then through a plurality of rolls 20 prior to entering the
remainder
of the bag making machine 12. An eccentric cam 22 may be disposed by the
pivot point 21 of the dancer arm 19 and an analog proximity switch 24 may be
disposed near the cam 22. Instead of using proximity switches and cams or
eccenters, potentiometers, either rotary or linear, may also be used to
measure the position
CA 02236742 2006-06-05
-2-
of the dancer 18. The cam 22 may provide speed command voltage to a
drive for the nip rools 16. When, for example, the dancer 18 goes upward, the
cam 22 may provide speed command voltage such that the drive for nip rolls
16 increases the speed of the nip rolls. When, for example, the dancer 18
goes downward, the cam 22 may provide speed command voltage such that
the drive for the nip rolls 16 decreases the speed of the nip rolls. That is,
the
cam 22 may provide an appropriate signal for providing a variable speed for
the nip rolls.
As the bag machine 12 stops production for a short time during the
cycle interrupt, the dancer 18 will start to move downward and the nip rolls
16
will gradually slow down as the speed of these nip rolls is controlled by the
position of the dancer 18 via the cam 22 and the analog proximity switch 24.
The dancer 18 further stores a certain amount of film 10 due to the slowdown
in the nip rolls 16. At the end of the cycle interrupt, the dancer 18 will
start to
rise because of the demand from the draw rolls 23 disposed within the bag
machine 12. Therefore, the speed of the nip rolls 16 will gradually increase
to
the nominal operating speed.
However, typical prior art systems result in a large fluctuation of the
speed of the nip rolls 16 between the nominal operating speed and the cycle
interrupt speed. Such a large fluctuation in speed may adversely influence
upstream processes. For example, hot air longitudinal sealing or ball and die
punching may be adversely affected where a larger fluctuation speed occurs
in the nip rolls 16. As a result, the bags being produced may be of an
inconsistent quality. Therefore, it is desired to have a gag machine having an
infeed system where the fluctuation in the speed of the nip rolls 16 is
reduced.
Summary of the Invention
There is provided an apparatus for controlling the speed of a web
comprising nip rolls for feeding the web, a dancer having a dancer arm for
receiving the web, the dancer arm having upward and downward position, a
sensor for sensing a position of the dancer arm, draw rolls disposed
downstream from the nip rolls, means for providing the nip rolls with a fixed
speed once a cycle interrupt occurs, and means for providing the nip rolls
with
CA 02236742 2006-06-05
-3-
a variable speed once the sensor senses that the dancer arm reaches the
upward position.
There is also provided an apparatus for controlling the speed of a web
comprising nip rolls for feeding the web, a dancer having a dancer arm for
receiving the web, the dancer arm having an upward and downward position,
a sensor for sensing a position of the dancer arm, draw rolls for drawing said
film; a controller for comparing an actual position of the dancer arm for a
given
point in time in a production run with a preprogrammed position corresponding
to the given point in time, and means for correcting a speed in the nip rolls
such that the dancer arm approximately equals a subsequent preprogrammed
position for a subsequent point in time.
There is further provided a method of controlling the speed of a web
comprising providing nip rolls with a fixed speed once a cycle interrupt
occurs,
sensing a position of a dancer arm, the dancer arm having an upward and
downward position, and providing the nip rolls with' a variable speed once the
dancer arm reaches the upward position.
There is also provided a method of controlling the speed of a web
comprising comparing an actual position of a dancer arm for a given point in
time in a production run with a preprogrammed position corresponding to the
given point in time and correcting a speed of nip rolls such that the dancer
arm approximately equals a subsequent preprogrammed position for a
subsequent point in time.
According to one aspect of the present invention, there is provided an
apparatus for controlling the speed of a web of film for producing bags
comprising:
nip rolls for feeding said web;
a dancer having a dancer arm for receiving said web, said dancer arm
operating between an upward and downward position, said nip rolls feeding
said web to said dancer; a sensor for sensing a position of said dancer arm;
draw rolls for drawing said web, said web being provided from said
dancer to said draw rolls;
means for temporarily stopping the flow of said web at said draw rolls,
said means for temporarily stopping defining a cycle interrupt for a duration
CA 02236742 2006-06-05
-3a-
when said web is temporarily stopped;
means for providing said nip rolls with a fixed speed once said cycle
interrupt occurs; and
means for providing said nip rolls with a variable speed once said
sensor senses that said dancer arm reaches said upward position.
According to another aspect of the present invention, there is provided
an apparatus for controlling the speed of a web of film for producing bags
comprising:
nip rolls for feeding said web;
a dancer having a dancer arm for receiving said web, said dancer arm
operating between an upward and downward position, said nip rolls feeding
said web to said dancer;
a sensor for sensing a position of said dancer arm;
draw rolls for drawing said web, said web being provided from said
dancer to said draw rolls;
means for temporarily stopping the flow of said web at said draw rolls,
said means for temporarily stopping defining a cycle interrupt for a duration
when said web is temporarily stopped;
a controller for comparing an actual position of said dancer arm for a
given point in time in a production run with a preprogrammed position
corresponding to said given point in time, during said cycle interrupt; and
means for setting a fixed speed in said nip rolls such that a position of
said dancer arm approximately equals a subsequent preprogrammed position
of said dancer arm for a subsequent point in time.
According to yet another aspect of the present invention, there is
provided a method of controlling the speed of a web for producing bags
comprising:
in sequence providing said web to nip rolls, to a dancer having a
dancer arm, and to draw rolls, said dancer being disposed between said nip
rolls and said draw rolls;
temporarily stopping the flow of said web at said draw rolls, said step
of temporarily stopping the flow of said web defining a cycle interrupt;
CA 02236742 2006-06-05
-3b-
providing said nip rolls with a fixed speed during said cycle interrupt;
sensing a position of said dancer arm during said cycle interrupt, said
dancer arm having an upward and downward position; and
providing said nip rolls with a variable speed once said dancer arm
reaches said upward position.
According to yet another aspect of the present invention, there is
provided a method of controlling the speed of a web for producing bags
comprising:
in sequence providing said web to nip rolls, to a dancer having a
dancer arm with an upward and downward position and to draw rolls, said
dancer being disposed between said nip rolls and said draw rolls;
temporarily stopping the flow of said web at said draw rolls, said step of
temporarily stopping the flow of said web defining a cycle interrupt;
comparing during said cycle interrupt an actual position of said dancer
arm for a given point in time in a production run with a preprogrammed
position corresponding to said given point in time; and
correcting a speed of said nip rolls such that a position of said dancer
arm approximately equals a subsequent preprogrammed position of said
dancer arm for a subsequent point in time.
Brief Description of the Drawing
Figure 1 is a schematic view of a prior art infeed system to a bag
making machine.
Figure 2 is a schematic view of an infeed system to a bag making
machine in accordance with a first preferred embodiment of the present
invention.
CA 02236742 2006-06-05
-4-
Figure 3 is a schematic view of the variation in the position of the
dancerarm.
Figure 4A is a plot of the actual average speed as pulled by draw rolls
during two production runs and during two interrupt cycles. Figure 4B shows
the infeed speed of the nip rolls of a typical prior art system. Figure 4C
shows
the infeed speed of the nip rolls of the present invention. Figure 4D shows a
comparison between the infeed speed of the nip rolls of the present invention
and the draw rolls.
Figure 5 is a schematic view of an infeed system to a bag making
machine in accordance with a second preferred embodiment of the present
invention.
Figure 6 is a schematic view of an infeed system to a bag making
machine in accordance with a third preferred embodiment of the present
invention.
Description of the Preferred Embodiments
Referring to Figure 2, there is provided a schematic view of an infeed
system to a bag making machine 12 in accordance with a first preferred
embodiment of the present invention. Similar components have been labeled
similarly for purposes of clarity.
A motor 30 is operatively connected to nip rolls 16 as is standardly
used in the bag making industry. Further, a flag 32, such as a sheet of metal
or a magnet as is standardly used in the industry, is disposed at one end of
the dancer arm 19. The flag 32 may be used to activate sensors or proximity
switches 34, 36, and 38. Proximity switches 34 and 36 may be connected to
a controller 40 and are preferably on/off proximity switches. The controller
40
is preferably a programmable logic controller. Alternatively, the controller
40
may be a servo controller, a personal computer or an industrial computer. As
will be described, proximity switch 34 informs the controller 40 to switch
back
from a fixed speed to a normal operating speed and proximity switch 36 is
used as a safety control.
CA 02236742 2005-06-29
-4a-
invention.
Figure 3 is a schematic view of the variation in the position of the
dancerarm.
Figure 4A is a plot of the actual average speed as pulled by draw rolls
during two production runs and during two interrupt cycles. Figure 4B shows
the infeed speed of the nip rolls of a typical prior art system. Figure 4C
shows
the infeed speed of the nip rolls of the present invention. Figure 4D shows a
comparison between the infeed speed of the nip rolls of the present invention
and the draw rolls.
Figure 5 is a schematic view of an infeed system to a bag making
machine in accordance with a second preferred embodiment of the present
invention.
Figure 6 is a schematic view of an infeed system to a bag making
machine in accordance with a third preferred embodiment of the present
invention.
Description of the Preferred Embodiments
Referring to Figure 2, there is provided a schematic view of an infeed
system to a bag making machine 12 in accordance with a first preferred
embodiment of the present invention. Similar components have been labeled
similarly for purposes of clarity.
A motor 30 is operatively connected to nip rolls 16 as is standardly
used in the bag making industry. Further, a flag 32, such as a sheet of metal
or a magnet as is standardly used in the industry, is disposed at one end of
the dancer arm 19. The flag 32 may be used to activate sensors or proximity
switches 34, 36, and 38. Proximity switches 34 and 36 may be connected to
a controller 40 and are preferably on/off proximity switches. The controller
40
is preferably a programmable logic controller. Alternatively, the controller
40
may be a servo controller, a personal computer or an industrial computer. As
will be described, proximity switch 34 informs the controller 40 to switch
back
from a fixed speed to a normal operating speed and proximity switch 36 is
used as a safety control.
CA 02236742 1998-05-04
WO. 97/19012 PCTIUS96/18621
-5-
Proximity switch 38 which is preferably an analog proximity switch is
similar in function to proximity switch 24 shown in Figure 1 and may be
connected to a first potentiometer 42. Potentiometer 42 may be, for example, a
ten turn potentiometer. The output of potentiometer 42 is connected to node 44
of a relay 46. The second node 48 of relay 46 is connected to a second
potentiometer 50. The relay 46 is further connected to controller 40. A
frequency controller 52, such as a frequency controller manufactured by Allen-
Bradley Company, Inc. of Milwaukee, Wisconsin, connects either nodes 44 or 48
of the relay 46 to the motor 30. The setting of potentiometer 42 provides a
variable speed or normal production speed for the nip rolls 16 during normal
operation (i.e., when the bag machine 12 is not in the cycle interrupt phase)
via
relay 46, frequency controller 52 and motor 30. Similarly, the setting of
potentiometer 50 provides a fixed speed or cycle interrupt speed for the nip
rolls
16 when the cycle interrupt operation of the bag machine 12 occurs.
Referring also to Figure 3, there is shown a schematic view of the
variation of the position of the dancer arm 19. Position 54 is the maximum
downward position of the dancer arm 19 and occurs when the nip rolls 16 are at
zero speed or when the bag machine 12 has stopped. Position 60 is the
maximum upward position of the dancer arm. This position occurs when the
speed of this nip rolls 16 are so high that the bag machine 12 may turn off to
prevent damage to the equipment. Arrow 64 indicates the maximum mechanical
displacement of the dancer arm 19 and arrow 62 indicates the normal dancer
displacement during interrupt.
The dancer arm 19 is at a downward position 56 at the end of the cycle
interrupt. Position 56 may be greater than the maximum downward position 54
and less than fifty percent of the maximum mechanical displacement 64 as
measured from the maximum downward position 54. Preferably, position 56 is
between ten to twenty percent of the maximum mechanical displacement 64 as
measured from the maximum downward position 54. However, as will be
discussed, a safety device may be installed to prevent the dancer arm 19 from
bottoming out. As a result, position 56 is preferably at such a level as to
allow
CA 02236742 1998-05-04
WO. 97/19012 PCT/US96/18621
-6-
proximity switch 36 to sense the failing of the dancer arm 19 and stop the nip
rolls 16 prior to the dancer arm 19 reaching position 54.
The dancer arm 19 is at an upward position 58 when the bag machine 12
is operating at normal production speed prior to the end of a given production
run. Position 58 may be greater than fifty percent of the maximum mechanical
displacement 64 as measured from the maximum downward position 54 and less
than the maximum upward position 60. Preferably, position 56 is between eighty
to ninety percent of the maximum mechanical displacement as measured from
the maximum downward position 54.
Referring also to Figure 4, there is shown a plot of film speed versus time
from start through two production runs. Point 66 on the time line indicates
the
start of the bag machine 12. Point 68 on the time line indicates that the nip
rolls
16 reach the maximum speed and that the dancer 18 is at its maximum working
position. Points 70 and 72 on the time line indicate the start and end of a
first
interrupt cycle, respectively. Point 74 on the time line indicates where the
nip
rolls reach the maximum speed and the dancer is working at its maximum
working position for a prior art system. Point 76 on the time line indicates
where
the dancer 18 reaches its maximum position in the present invention. Point 78
on the time line indicates the start of the second interrupt cycle.
As shown in Figure 4A, the actual average speed of the draw rolls 23 is
approximately a constant from the start of the bag machine 12 until the cycle
interrupt. During the cycle interrupt, the speed of the draw rolls 23 may be
zero.
As shown in Figure 4B, the speed of typical prior art nip rolls 16 is shown.
The
speed of the prior art nip rolls gradually increases from zero to the normal
production speed, as shown by points 66 and 68 on the time line. Once the
cycle
interrupt occurs, the speed of the nip rolls 16 greatly decreases, as shown by
points 70 and 72 on the time line. Further, the speed of the nip rolls 16 may
greatly have to increase to the normal production speed as shown by point 74
on
the time line.
This contrasts to the current invention where the speed of the nip rolls
during the cycle interrupt may not vary greatly from the speed of the nip
rolls
CA 02236742 1998-05-04
WO -97/19012 PCT/US96/18621
-7-
during normal production. As shown in Figures 4C and 4D, during the cycle
interrupt (i.e., between points 70 and 72) the speed of the fluctuation of the
speed of the nip rolls is reduced in comparison with the typical prior art
device.
Figure 4D provides a comparison the speed of the nip rolls 16 and the draw
rolls
23, the draw roll speed being shown in dotted lines in that figure.
Specifically, by the use of the flag 32, proximity switch 34, controller 40,
and relay 46, the control of the speed of the nip rolls 16 will switch from a
variable or normal production speed to the fixed speed at the moment the bag
machine 12 begins the cycle interrupt and returns to the normal production
speed
once the dancer arm 19 returns to position 58. The speed of the nip rolls 16
at
the cycle interrupt may, for example, be between fifty to one hundred percent
the
speed of the nip rolls 16 during normal production. Preferably, the speed of
the
nip rolls 16 is at or as close to the average line speed as possible during
both
normal production as well as during the cycle interrupt, as will be discussed.
Where the speed of the nip rolls 16 is at the average line speed, the plot may
essentially be flat in Figures 4C and 4D after point 68 in the time line is
reached.
As the bag machine 12 is started, the draw rolls 23 begin pulling the film
10 at a speed equal to the cycle speed multiplied by the draw length, as is
standardly done in the industry. This is illustrated at point 66 in the time
line of
Figure 4A. Further, the infeed speed of the nip rolls 16 increases from zero
to
the desired production speed, indicated by points 66 and 68 of Figures 4C. The
operator sets potentiometer 42 such to bring the dancer to position 58 as
shown
in Figure 3 or higher which occurs at point 68 on the time line of Figure 4C.
At point 70, the controller 40 begins the cycle interrupt, sending a signal to
relay 46 and thus energizing relay 46. As a result, the relay 46 changes the
speed of the nip rolls 16 to a fixed speed, set by potentiometer 50 via the
frequency controller 52. That is, the input of the frequency controller 52 is
connected to node 48 of relay 46 rather than to node 44 of relay 46,
connecting
potentiometer 50 rather than potentiometer 42 to the frequency controller 52
during the cycle interrupt. At point 74, the interrupt ends but as the nip
rolls 16
are at a slightly lower speed in comparison to the draw rolls 23 at that point
in
CA 02236742 1998-05-04
WO 97/19012 PCT/US96/18621
-8-
time, the dancer arm 19 will start to rise until it reaches position 58 as
shown in
Figure 3. Preferably, the dancer arm 19 will start to rise and reach position
58 at
any time before the next cycle interrupt is reached.
When the dancer arm 19 reaches position 58, the sensor or proximity
switch 34 informs the controller 40 to de-energize relay 46, thus connecting
potentiometer 42 to the frequency controller 52 rather than potentiometer 50.
Specifically, proximity switch 34 informs the controller 40 to switch back,
for
example, to a conventional cam controlled or eccentric controlled speed. This
occurs at point 76 on the time line of Figure 4. As a result, the motor 30
will drive
the nip rolls 16 at the normal operating speed.
Correct setting of the potentiometer 50 can be visually controlled by
observing the dancer 18 displacement. At the beginning of a cycle interrupt,
the
dancer arm 19 should be in position 58. At the end of the cycle interrupt, the
dancer arm 19 should be in position 56. After the cycle interrupt is
completed,
the dancer arm 19 should begin to rise to position 58. If the dancer arm 19
does
not lift, then the fixed speed provided by the potentiometer 50 may be set too
high. As a result, the fixed speed should be reduced in order to lower the
speed
of the nip rolls 16 during the cycle interrupt. The speed of the nip rolls 16
should
also increase to the production speed prior to the next cycle interrupt as
shown
by points 76 and 78.
Although the speed of the nip rolls 16 in Figure 4A is shown to slightly vary
after reaching point 68 on the time line, the speed of the nip rolls 16 may be
a
constant value. Specifically, the speed of the nip rolls 16 may be set, for
example, to the average line speed. The average line speed may be calculated
by the following equation:
Average line speed = (S x L x K)/(K + I),
where S is the cycle speed, L is the bag length, K is the number of bags for a
given stack, and I is the interrupt count. For example, where the cycle speed
is
three hundred cycles, the bag length is one foot, there are one hundred bags
per
stack, and the interrupt count is five bags, the average line speed is 285.7
feet
per minute.
CA 02236742 1998-05-04
WO 97/19012 PCT/iJS96/18621
-9-
The nip rolls 16 may be set to the average line speed via potentiometer 50
where the production run is long enough in relation to the interrupt cycle
time to
allow the dancer arm 19 to return from position 56 to position 58 prior to the
next
interrupt cycle. If the production run is not long enough in relation to the
interrupt
cycle time to allow the dancer arm 19 to return from position 56 to position
58
prior to the next interrupt cycle, then the speed of the nip rolls 16 should
be
lowered slightly below the average line speed until the dancer arm 19 returns
from position 56 to position 58 prior to the next interrupt cycle. That is,
the lower
the speed the nip rolls 16 are set in relation to the average line speed, the
earlier
the dancer arm 19 returns from position 56 to position 58 prior to the next
interrupt cycle, as shown in Figure 4D. Preferably, the range designated by
arrow 62 in Figure 3 is as large as possible to allow the dancer 18 to gather
the
excess web 10 during the cycle interrupt when the nip rolls are operating at a
speed higher than the draw rolls 23. This may allow for operating the nip
rolls 16
at or near the average line speed.
Where the differences between the fixed speed and the cam controlled
speed are smaller, the bag machine 12 may be better tuned. As a result, if the
dancer allows enough take up of the web 10 during the cycle interrupt, the
speed
of the nip rolls may be maintained at a constant speed during normal operation
as well as during the cycle interrupt.
Auxiliary equipment, such as longitudinal sealers or ball and die punches
can be tuned to the lower constant speed of the nip rolls which occurs after
the
cycle interrupt takes place. This auxiliary equipment should be set to the
lower
fixed speed and back to the normal cam controlled speed at the same time relay
46 is energized and de-energized for altering the speed of the nip rolls 16.
Proximity switch 36 may also be provided as a safety device. If during
cycle interrupt the dancer arm drops below position 56, the speed control is
switched from the preset or fixed value provided by potentiometer 50 to the
cam
or eccentric controlled value provided by potentiometer 42, eventually
bringing
the motor 30 to a normal stop. This safety device may be activated, where, for
CA 02236742 1998-05-04
WO 97/19012 PCT/US96/18621
-10-
example, the fixed speed set by potentiometer 50 is initially set too high.
The
safety circuitry may help prevent the dancer arm 19 from bottoming out.
Referring now to Figure 5, there is shown an alternate embodiment for the
present invention. Instead of using mechanical devices such as potentiometers
42 and 50 as well as relay 46, the circuitry for controlling the speed of the
nip
rolls is encompassed by a controller 40'. The controller 40' may be, for
example,
a programmable logic controller manufactured by Allen-Bradley Company. As
with the first preferred embodiment, the outputs of proximity switch 34 and 36
are
provided to the controller 40'. On the contrary, the output of proximity
switch is
provided to the controller 40' as well. In addition, the controller 40'
directly
provides the activating signal for the frequency controller 52.
The setting during normal operation which would otherwise be set by
potentiometer 42 may, for example, be directly inputted into the controller
via a
control panel, not shown. Further, the setting for the nip rolls 16 during the
cycle
interrupt (as well as until the dancer arm 19 returns to position 58 as shown
in
Figure 3) is also directly inputted into the controller 40'. This is similar
in function
to the setting provided by potentiometer 50 in the first preferred embodiment
(i.e.,
the potentiometer 50 provides the fixed speed to the nip rolls 16 at the start
of the
interrupt cycle).
The user of the bag machine 12 may input the bag length, cycle speed,
the bags for a given stack, and the number of cycle interrupts for a given
operation, as well as the time for a given interrupt into the controller 40'.
As a
result, the controller 40' may calculate the actual speed as well as the
average
line speed. The actual speed is the average actual speed of the draw rolls 23
without reference to the interrupt cycle and the average line speed is the
average
speed of the draw rolls 23 which includes the interrupt cycle. The average
line
speed may be calculated by the above-noted equation:
Average line speed = (S x L x K)/(K + I),
where S is the cycle speed, L is the bag length, K is the number of bags for a
given stack, and I is the interrupt count.
CA 02236742 1998-05-04
WO 97/19012 PCTIUS96/18621
-11-
The controller 40' may directly input a proper signal to the frequency
controller 52 for activating the nip rolls 16 at the average line speed. As
with the
first preferred embodiment, the nip rolls 16 may be activated at the average
line
speed where the dancer arm 19 is capable of returning from position 56 to
position 58 prior to the next interrupt cycle. Otherwise, the controller 40'
may be
inputted with a setting such that the nip rolls 16 are provided with a fixed
speed
lower than the average line speed such that the dancer arm 19 retums from
position 56 to position 58 prior to the next interrupt cycle. Once the
proximity
switch 34 senses the return of the dancer arm to position 58, the cam 22 and
the
proximity switch 38 may be used to provide speed command voltage to drive the
nip rolls 16 via the controller 40, frequency controller 52, and motor 30.
Referring now to Figure 6, there is shown a third preferred embodiment for
controlling the speed of the nip rolls 16. In this embodiment, the average
line
speed can be calculated by the controller 40' and the dancer 18 may be used to
trim or correct the speed of the nip rolls 16 to obtain the desired speed
during
both the production run and cycle interrupt. This embodiment is similar in
construction to what is illustrated in Figure 5 except the proximity switches
34
and 36 and flag 32 are not employed and the position of the dancer arm 19 is
used as a small trim. Further, this embodiment utilizes the average line speed
as
a general guideline for operating the nip rolls 16 and utilizes the controller
40' to
alter the speed of the nip rolls 16 should the position of the dancer arm 19
not be
at a desired location at a given point on the time line.
Specifically, in this embodiment, at the beginning of a production run (i.e.,
at or slightly after point 72 of Figure 4), the dancer arm 19 should be at or
near its
lower position 56. Further, the dancer arm 19 should be at or near its upper
position 58 at the end of a production run (i.e., just prior to the cycle
interrupt or
point 78 of Figure 4). Because the nip rolls 16 would be turning at the
average
line speed, the dancer arm 19 would rise from lower position 56 at the
beginning
of the production run to position 58 at the end of the production run. At the
cycle
interrupt, the dancer arm 19 would then fall to lower position 56 and the
process
would then repeat itself.
CA 02236742 1998-05-04
WO 97/19012 PCT/US96/18621
-12-
The controller 40', via the analog proximity switch 38, is informed of the
current position of the dancer arm 19 at a given point on the time line in
relation
to a given production run. Further, the controller 40' may be programmed with
data reflecting where the position of the dancer arm 19 should be at a given
points in time in relation to a given production run. For example, the
controller
40' may be programmed to know that at two seconds before point 76 on the time
line of Figure 4A, the dancer arm should be at ninety-nine percent of the
value of
position 58. Should, the dancer arm 19 not be in this position at this point
in
time, the controller 40' could lower the speed of the nip rolls 16 such that
the
dancer arm 19 is in position 58 prior to the beginning of the interrupt cycle.
Similarly, if at a given point in time, the dancer arm 19 is too high in
comparison
to the position it should be as inputted into the controller 40', then the
speed of
the nip rolls may be temporarily increased to lower the dancer arm 19 until
the
dancer arm 19 is disposed at a location corresponding to the point inputted
into
the controller 40' for a specified point in time for a given production run.
That is
the speed of the nip rolls 16 will be corrected such that the dancer arm 19
approximately equals a subsequent preprogrammed position for a subsequent
point in time.
As stated earlier, just after the cycle interrupt occurs, the dancer arm 19
should fall to position 56. From this time until the end of the production
run, the
dancer arm should be in position 58. The controller 40' could, for example, be
inputted with points for every bag, cycle, or at regular intervals for a given
production run where the dancer arm 19 should be positioned. Further, the
position of the dancer arm 19 may, for example, increase linearly until
position 58
is reached at the end of the production run. Therefore, each data point for a
given bag or cycle of the production run could be compared to the position
provided to the controller 40' via the analog proximity sensor 38
corresponding to
that given point in time. The speed of the nip rolls 16 may be slightly
decreased
if the dancer arm 19 is too low or increased if the dancer arm 19 is too high.
This
comparison continues each bag or cycle during both the production run as well
as during the cycle interrupt. If after a given bag or cycle, the dancer arm
19 is
CA 02236742 1998-05-04
WO 97/19012 PCT/US96/18621
-13-
still not in the proper position, the process will repeat once again (i.e.,
the nip roll
19 speed will be increased or decreased) until it coincides with the desired
position for that given point in time of the production cycle.
The dancer 18 should be capable of not only storing the film 10 during the
cycle interrupt, it should also be capable of having a range in motion of the
dancer arm 19 such that the dancer arm can begin at position 56 at the
beginning of the production run and end at position 58 at the end of the
production run. As a result, additional rolls 17 disposed on the dancer 18 may
be
added for storing additional film 10 and to help provide the desired range in
motion of the dancer arm 19.
It should be recognized that, while the present invention has been
described in relation to the preferred embodiments thereof, those skilled in
the art
may develop a wide variation of structural details without departing from the
principles of the invention. Therefore, the appended claims are to be
construed
to cover all equivalents falling within the true scope and spirit of the
invention.
