Vertical form, fill and seal machine having constant film pull length

A method and apparatus for a vertical form, fill and seal machine to produce a constant pull length of the film and therefore packages of consistent length, whatever the velocity of the machine. When the film is pulled, and the pulling ceases, there is always a finite overrun of pulled film. The method according to the invention determines the velocity for operation of the machine while accounting for the overrun in order to produce packages of constant length. The apparatus includes a motion controller for calculating the velocity and operating the machine at the calculated velocity during the time that the film is being advanced in the vertical form, fill and seal machine.

BACKGROUND OF THE INVENTION 
This invention relates to vertical form, fill and seal machines, and in 
particular to a vertical form, fill and seal machine and method of 
operating the machine in which variations in the velocity of the film fed 
through the machine do not unintentionally vary the lengths of the 
packages being made by the machine. 
For many years, manufacturers of vertical form, fill and seal machines have 
constructed the machinery with a series of clutches and brakes for 
operating the interlinked portions of the machine. One such machine is 
described in U.S. Pat. No. 4,288,965 assigned to Hayssen Manufacturing 
Company, the disclosure of which is incorporated herein by reference. The 
'965 patent describes a vertical form, fill and seal machine which uses a 
measuring roll to meter the delivery of film to the machine to produce 
constant length packages. The particular system described in the patent 
uses a clutch and brake drive system in conjunction with a shaft encoder 
to operate the measuring rolls within a fixed portion of a 360.degree. bag 
making cycle. 
The system of the '965 patent operates well when the machine is operated at 
a single delivery velocity for the film. However, when the speed of the 
film is changed, the number of degrees required to produce a package of a 
desired length must be changed, as well, in order to achieve that length. 
This is necessary because even though the measuring rolls are commanded to 
operate for the duration of a commanded pull signal, once the signal has 
ceased, the inertia of the measuring rolls causes an over-running 
situation. As the velocity of the rolls increases, the over-running 
situation worsens. While a modification of the commanded pull signal 
resolves the variation in package length, it is impossible to vary the 
velocity over a wide range without readjusting the commanded parameters 
for the machine. As a result, the machine, when operating with a clutch 
and brake drive system, normally is not utilized in situations where 
varying velocities occur. 
However, with the advent of motion control systems, it is now possible to 
utilize mathematical relationships within the motion controllers used in 
such systems to automatically compensate for length variations that would 
otherwise occur due to inertia of the measuring rolls. Thus, it is now 
possible to accurately compensate for deceleration of the measuring rolls, 
which occurs outside the commanded pull signal, in order to produce 
packages of consistent length, without utilizing a shaft encoder or the 
like on the measuring rolls in order to determine the position of the 
rolls, and therefore the amount of film pulled. 
In a typical vertical form, fill and seal machine of the nature of the 
invention, the portion of the 360.degree. bag making cycle in which the 
measuring rolls are operated has three phases, an acceleration phase where 
the speed of the film is accelerated from rest to a desired maximum or 
running velocity, a running velocity phase where the rolls are operated at 
a constant velocity, and a deceleration phase, where, after the commanded 
pull signal has ceased, the measuring rolls are decelerated to rest. In a 
typical machine, the example of which is utilized in connection with the 
present invention, the acceleration phase occupies one third of the time 
of the commanded pull signal, and the running velocity phase occupies the 
remaining portion, or two thirds, of the commanded pull signal. It is only 
the deceleration phase that will vary. If there is a constant 
deceleration, then the amount of over-pull will vary depending on the 
velocity from which the measuring rolls are halted. On the other hand, the 
deceleration can be variable, and, in that instance, the deceleration 
distance always remains the same. Therefore, the time of the deceleration 
will vary inversely to the value of the velocity from which the measuring 
rolls are halted. 
SUMMARY OF THE INVENTION 
The invention is directed to a method and apparatus for providing 
consistent package lengths, irrespective of the rate of making the bags 
and therefore irrespective of the velocity of the measuring rolls. In 
accordance with the method of the invention, the method determines a 
running velocity for the film in a packaging machine. The method comprises 
the steps of determining a desired length of packages to be made by the 
packing machine, then determining a film velocity profile which comprises 
an acceleration phase for the film, a running phase for the film, and a 
deceleration phase for the film. Thereafter, based upon the film velocity, 
the method determines a film pull distance for that velocity profile, and 
the film pull distance is then equated to the desired length of the 
packages to be made by the machine, and the running velocity is calculated 
therefrom. Finally, the packaging machine is set with the running velocity 
for the film to be that of the calculated running velocity. 
In accordance with one form of the invention, the acceleration phase has a 
constant rate of acceleration for any given running velocity, the 
deceleration phase has a constant rate of deceleration, and the 
acceleration phase and a the running velocity phase are operated for a 
fixed period of time, which is the duration of the commanded pull signal. 
Therefore, the film pull distance comprises a total of the distance 
traveled during the constant acceleration phase, the distance traveled in 
the running velocity phase, and the distance traveled in the deceleration 
phase. With the time for the acceleration phase being a predetermined 
amount (such as one third of the fixed period of time), the velocity of 
the running velocity phase can be calculated from the quadratic equation. 
In accordance with a second form of the invention, the acceleration phase 
has a constant rate acceleration for any given running velocity, the 
deceleration phase has a fixed distance to be traveled, and the 
acceleration phase and the running velocity phase are operated for the 
fixed period of time comprising the commanded pull signal. In this 
instance, the velocity of the running velocity phase is calculated from a 
simple linear equation. 
The packaging machine according to the invention includes means for 
providing a signal indicative of package making rate, and means for 
providing a signal indicative of the fixed period of time for advancing 
the film, with the fixed period of time encompassing the durations of the 
acceleration phase and the running velocity phase. Means is provided for 
calculating the running velocity, and means is provided for advancing the 
film at the running velocity. 
In accordance with the preferred form of the invention, a machine 
controller is utilized, the machine controller encompassing both the means 
for providing a signal indicative of package making rate and the means for 
providing a signal indicative of a fixed period of time for advancing the 
film. The invention also preferably includes a motion controller, with the 
motion controller comprising the calculating means. The motion controller 
is connected between the machine controller and the means for advancing 
the film at the running velocity. 
In one form, the deceleration phase includes a fixed rate of deceleration 
and a variable distance of film travel during the deceleration phase. In 
another form, the deceleration phase includes a variable rate of 
deceleration and a fixed distance of film travel during the deceleration 
phase. 
In either form of the invention, the running velocity for the film is 
dependent on the distance traveled by the film during the deceleration 
phase, after cessation of the commanded film pull signal. In either form, 
the running velocity for the film is accurately determined so that the 
film pull distance is extremely accurate and the lengths of the packages 
made by the packaging machine are always consistent.

DESCRIPTION OF EXAMPLES EMBODYING THE BEST MODE OF THE INVENTION 
Turning to FIG. 1, illustrated is a typical profile for a vertical form, 
fill and seal machine which operates with intermittent film pull. Before 
an enable signal is generated, the film is at rest. When the enable signal 
is generated to initiate the pull cycle, the pull begins. In the 
illustrated form of the invention, the enable signal goes low to initiate 
the pull cycle and then goes high to stop the pull cycle. Obviously, the 
opposite could also occur, depending on the nature of the motion 
controller, as described in greater detail below. 
When the enable signal goes low, the film is accelerated at a constant rate 
of acceleration until it reaches a running velocity, identified as VMAX in 
FIG. 1. Then, for the duration of the enable signal, the film is pulled at 
VMAX, and when the enable signal is terminated, pulling of the film is 
terminated. However, the measuring rolls have mass and corresponding 
inertia, and therefore cannot be decelerated instantaneously. Instead, 
there is a finite period of deceleration, until the film actually ceases 
movement. The cessation is either due to a constant deceleration, no 
matter what VMAX may have been, therefore producing a variable over-pull 
of film, or by a variable deceleration, which produces a constant length 
of film regardless of VMAX. Variable deceleration, however, produces 
longer pull times at lower values of VMAX, which is generally not 
acceptable, but can be used in machines needing only slow cycle 
operations. 
Turning to FIG. 2, illustrated for the form of the invention having 
constant deceleration is the change of the curves for varying values of 
VMAX. In each, the constant rate of acceleration to reach VMAX increases 
as VMAX increases so that the time of reaching VMAX is always the same. 
However, when the enable signal goes high, the deceleration is constant, 
but the deceleration distances increase as VMAX increases. While a 
constant rate of acceleration is illustrated and preferred for reaching 
VMAX, obviously the rate of acceleration can vary, so long as VMAX is 
reached at the prescribed time. 
The length of the film pulled is equal to the area under the velocity 
curve. This is readily seen from the following relationships: 
EQU Time to Decelerate=VMAX/DECEL 
where VMAX is the constant or running velocity 
DECEL is the deceleration rate. 
Also, 
EQU D.sub.D =1/2(VMAX).cndot. Time to Decelerate 
where D.sub.D is the deceleration distance. 
Thus, 
EQU D.sub.D =(1/2)(VMAX).sup.2 /DECEL. 
Thus, the deceleration distance varies as the square of VMAX, and therefore 
the over pull varies in relation to the square of VMAX, as well. This is 
exactly the problem which occurs in current vertical form, fill and seal 
machinery, and in the past, in order to accommodate for the increasing 
over pull as VMAX increases, the duration of the enable signal must be 
shortened accordingly. This, however, is disadvantageous since the most 
efficient operation of the machine is to have a constant duration enable 
signal, therefore having a fixed duration of the 360.degree. bag making 
cycle. 
Turning to FIG. 3, illustrated is a curve similar to that illustrated in 
FIG. 1, but where the acceleration to VMAX occupies one-third of the total 
commanded pull signal. Therefore, the running velocity or constant 
velocity phase occupies two-thirds of the cycle, followed by the 
deceleration phase. The acceleration phase of the cycle is illustrated as 
a linear acceleration from zero velocity to VMAX in 1/3 of the total 
enable signal. The percentage of cycle time and the acceleration profile 
can be modified as required so long as the velocity transitions from zero 
to VMAX in the allotted time. The calculations must obviously be modified 
to reflect the change in film pull which occurs during the acceleration 
phase in the allotted time with the selected profile. 
The length or distance that the film is pulled is equal to the area under 
the velocity curve. Thus, for each of the three segments: 
EQU D.sub.A =(1/2)(VMAX)T/3=(1/6)(T)(VMAX) 
where D.sub.A is the acceleration distance, 
EQU D.sub.R =VMAX(2T/3)=(2/3)(T)(VMAX) 
where D.sub.R is the distance traveled in the running velocity or constant 
velocity phase, and 
EQU D.sub.D =(1/2)(VMAX).sup.2 /DECEL. 
Therefore, the total film pull distance, D.sub.TP, is 
EQU D.sub.TP =(1/6)(T)(VMAX)+(2/3) (T)(VMAX)+(1/2)(VMAX).sup.2 /DECEL. 
or, 
EQU D.sub.TP =(5/6)(T)(VMAX)+(1/2)(VMAX).sup.2 /DECEL. 
At this point, there are two unknowns, the total distance D.sub.TP and 
VMAX. D.sub.TP can be calculated utilizing parameters from the vertical 
form, fill and seal machine. This is accomplished by first determining the 
pull time. The pull time commences at the initiation of the enable signal 
until the pull is terminated when the enable signal goes high. This time 
represents a portion of the bag making cycle. If the pull time is 120 
milliseconds (0.120 seconds), the pull per degree is 0.1 inch per degree 
and the bag making rate is 80 bags per minute, then the following 
relationships are true when the acceleration time is one-third of the 
enable cycle: 
80 bags/min.cndot.360.degree. /bag=28800.degree. /min=480.degree. /sec 
480.degree. /sec is 0.0020833 sec/degree. 
Thus, for a 0.120 sec. pull time, 
0.120 sec.cndot.480.degree. /sec=58.degree.. 
58.degree. @ 0.1 inch/degree=5.8 inches. 
Therefore, the bag length is 5.8 inches. 
The number of revolutions for the measuring roll, per bag, is bag 
length/roll circumference=5.8/6.25=0.928 revolutions for a measuring roll 
of 6.25 inches in circumference. If the measuring roll is operated with 
25400 counts per revolution, then 0.928 revolution=23572 counts. These 
counts must be delivered in the allotted 0.120 sec. pull time. 
Since the deceleration rate is constant, and taking the deceleration rate 
DECEL to be 6 million counts per second per second, VMAX can be determined 
utilizing the information above and the quadratic formula: 
EQU D.sub.TP =(5/6)(T)(VMAX)+(1/2)(VMAX).sup.2 /DECEL 
or 
EQU (1/2)(VMAX).sup.2 /DECEL+(5T/6)(VMAX)-D.sub.TP =0 
Using the quadratic formula, 
EQU VMAX=(-B+(B.sup.2 -4AC).sup.1/2)/2A 
where A=1/2.cndot. DECEL 
B=5T/6 
C=D.sub.TP 
then, using the values calculated above, 
VMAX=201,787 Counts/sec. 
For 6,000,000 counts/sec/sec deceleration rate, 
Time to Decelerate=201,788 counts/sec.div.6,000,00 counts/sec .sup.2 =0.033 
seconds. 
Thus, VMAX can readily be calculated in the software for the motion 
controller, and the vertical form, fill and seal machine can be controlled 
to always generate packages having a consistent bag length. 
A similar analysis applies to the situation where the deceleration distance 
is fixed, and therefore the rate of deceleration increases or decreases 
depending on whether VMAX increases or decreases. Given the example above, 
if the maximum rate of deceleration is 6,000,000 counts per second with 
the highest VMAX being 300,000 counts per second, the distance traveled 
during deceleration would be 7,500 counts, and would take 0.05 seconds. 
FIG. 4 illustrates this form of the invention where VMAX varies from a 
maximum value to a minimum value, and it can be seen that as VMAX varies, 
the deceleration time becomes excessively slow. To calculate VMAX in this 
form of the invention, and given the values explained above, 
EQU D.sub.TP =D.sub.A +D.sub.R +D.sub.D 
EQU =(5/6)(T)(VMAX)+(2/3)(T)(VMAX)+D.sub.D 
EQU =(5/6)(T)(VMAX)+D.sub.D 
or 
EQU (5/6)(0.120)(VMAX)=23572-7500 
EQU VMAX=160,874 counts/sec 
and 
Time to Decelerate=0.0933 sec. 
Thus, it will be seen that although, in this form of the invention, the 
maximum velocity is slightly slower than in the first form of the 
invention, the deceleration time is three times longer than in the first 
form of the invention. Therefore, in most instances, a constant rate of 
deceleration analysis provides quicker completion of the film pull, 
leaving more time for subsequent action (sealing and severing) for each 
360.degree. bag making cycle. 
FIG. 5 illustrates a typical machine configuration according to the 
invention. In FIG. 5, the vertical form, fill and seal machine is 
designated generally at 10. It includes a forming shoulder 12, a forming 
tube 14, and a pair of pull belts 16 and 18. A pair of measuring rolls 20 
are located upstream of the forming tube 12 for feeding film 22 through 
the machine 10. 
Each of the pull belts 16 and 18 is driven by a respective servo drive 
motor 24 and 26, each motor 24 and 26 having its associated motor drive 28 
and 30. It is important, for proper pulling of the film 22, that the pull 
belts 16 and 18 operate at exactly the same speeds, and therefore the 
drive motors 24 and 26 are typically driven in parallel. 
Similarly, the measuring rolls 20 are driven by a servo drive motor 32 
controlled by a motor drive 34. No shaft encoders or the like are needed 
for either the measuring rolls 20 or the pull belts 16 and 18. 
The motor drives 28, 30 and 34 are connected to a motion controller 36. The 
motion controller can be any conventional motion controller, and for the 
purposes of the examples above, the Galil model DMC-1530 motion controller 
manufactured by Galil Motion Control Incorporated, 203 Ravendale Drive, 
Mountainview, Calif., 94043, was used. The motion controller 36, in turn, 
receives signals from a conventional machine controller 38, such as the 
programmable control described in incorporated U.S. Pat. No. 4,288,965. 
The machine controller 38 generates the enable signal (FIGS. 1 through 4) 
as well as an analog signal which represents the number of packages per 
minute on an analog scale. The third signal, an encoder emulation signal, 
is fed to the machine controller 38 from the motion controller 36. 
During operation of the machine 10, the measuring rolls 20 comprise one 
axis, and the pull belts 16 and 18 comprise a second axis. The measuring 
roll axis is the master, and the pull belt axis is the slave. When the 
film 22 is pulled by the pull belts 16 and 18, it is metered by the 
measuring rolls 20. If the measuring rolls have a diameter of 2.5 inches 
and the pull belts have sheaves of a 3.75 inch diameter, then, if the 
motion of the measuring rolls 20 and the pull belts 16, 18 are exactly 
matched, one revolution of the measuring rolls is 7.854 inches, and one 
revolution of the pull belt sheaves is 11.781 inches. Therefore, the ratio 
of the measuring roll axis to the pull belt axis is 7.854/11.781=0.6667. 
Thus, for exact matching of speed between the master axis and the slave 
axis, the pull belts must be operated at a rate of 0.6667 times the speed 
of the measuring roll axis, assuming that the rolls of the measuring roll 
axis are driven directly from the motor. While this is an ideal situation, 
typically the surfaces of the pull belts 16 and 18 wear, leading to slight 
slip between the pull belts 16, 18 and the film tube over the tube 14. To 
overcome the slippage, the pull belts 16 and 18 are driven an additional 
over-pull percentage, up to 10 percent greater than the direct match. This 
avoids any slacking of the film between the measuring rolls 20 and the 
pull belts 16, 18. Tension is maintained in the film 22, and the pull 
belts 16, 18 slip slightly before the film 22 is torn. 
The invention provides a simple, yet effective, manner to operate a 
vertical form, fill and seal machine with exact bag lengths by precisely 
controlling VMAX. Various changes can be made to the invention without 
departing from the spirit thereof, or scope of the following claims.