U.S. patent number 3,912,145 [Application Number 05/447,695] was granted by the patent office on 1975-10-14 for web tension control system.
This patent grant is currently assigned to Butler Automatic, Inc.. Invention is credited to Edward F. Meihofer.
United States Patent |
3,912,145 |
Meihofer |
October 14, 1975 |
Web tension control system
Abstract
The present system maintains and controls the tension or speed
of a moving web over a preselected range of web speeds in the
vicinity of the prime mover speed by means of a web draw roll
driven by controlled overspeed and underspeed clutches operating
off the prime mover. A balanced, undamped, low inertia, preloaded
dancer assembly contacts the web and web tension upsets are
reflected in movements of the dancer assembly. Such movements are
detected by a transducer and applied to an electronic controller
which provides a continuous, proportional output signal to either
the overspeed or underspeed clutch. The clutch applies a drive
torque or holdback torque component to the draw roll which changes
the web tension as needed to reposition the dancer assembly to its
reference position.
Inventors: |
Meihofer; Edward F. (Norfolk,
MA) |
Assignee: |
Butler Automatic, Inc. (Canton,
MA)
|
Family
ID: |
23777362 |
Appl.
No.: |
05/447,695 |
Filed: |
March 4, 1974 |
Current U.S.
Class: |
226/44;
101/226 |
Current CPC
Class: |
B65H
23/1888 (20130101) |
Current International
Class: |
B65H
23/188 (20060101); B65H 025/04 () |
Field of
Search: |
;226/44,25,11,37,42 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Schacher; Richard A.
Attorney, Agent or Firm: Cesari and McKenna
Claims
I claim:
1. A system for controlling tension in a moving web comprising
A. a draw roll for engaging said web,
B. a guide roll spaced from and parallel to the draw roll for
engaging said web,
C. a balanced, undamped, low inertia dancer assembly positioned
between the draw roll and the guide roll, said dancer assembly
including
1. a frame, and
2. a pair of spaced-apart dancer rolls rotatively supported by the
frame so that their axes are parallel to the draw and guide roll
axes and for engaging opposite sides of said web,
D. means for pivotally mounting the frame so that it can pivot
about an axis midway between and parallel to the dancer rolls in
response to changes in the tension of the web,
E. means for applying a torque to the dancer assembly so as to
offset the torque applied thereto by a selected tension in the web
so the assembly tends to assume a reference position, said
torque-applying means applying torque by exerting a force on said
assembly which is directed along a line which remains parallel to
said entering and leaving web paths as the dancer assembly deviates
from its reference position,
F. means for detecting movement of the dancer assembly away from
its reference position in response to a change in web tension and
producing an output signal in response thereto, and
G. means responsive to said output signal for applying the proper
torque correction to the draw roll to change the tension in the web
such that the dancer assembly tends to return to its reference
position.
2. The system defined in claim 1 wherein the mounting means include
bearing units which allow the assembly to pivot with minimum
frictional losses.
3. The system defined in claim 1 wherein the draw roll, guide roll
and dancer rolls are arranged so that the web paths entering and
leaving the dancer assembly are parallel.
4. The system defined in claim 3 wherein said entering and leaving
web paths are perpendicular to the plane defined by the dancer
rolls when the dancer assembly is in its reference position.
5. The system defined in claim 1 and further including
A. a nip roll, and
B. means for mounting the nip roll so as to form a nip with the
draw roll to minimize slippage of web on the draw roll.
6. The system defined in claim 1 wherein the torque applying means
comprises
A. a pressurizable loading cylinder having a movable shaft,
B. a stationary support,
C. means for pivotally connecting the loading cylinder and its
shaft between the frame and the support so that when the cylinder
is pressurized, the frame is pivoted about its axis so as to
lengthen the web path between the draw roll and the guide roll,
and
D. means for pressurizing the cylinder so as to maintain the
assembly in its reference position at said selected web
tension.
7. The system defined in claim 6 wherein the cylinder and its
pivotal connections to the assembly are arranged and adapted so
that the axis of the cylinder always remains parallel to the
entering and leaving web paths as the assembly deviates from its
reference position in response to web tension changes.
8. The system defined in claim 1 wherein the detecting means
include a potentiometer whose resistance changes as the assembly
deviates from its reference position.
9. The system defined in claim 1 wherein the torque correction
applying means comprise
A. rotary drive means,
B. one of a controllable overspeed and underspeed clutch coupled
between the drive means and the draw roll, and
C. an electronic controller responsive to the output of the
detecting means to provide control signals for the clutches so that
said proper torque correction is applied to the draw roll.
10. The system defined in claim 9 wherein
A. the other of the overspeed and underspeed clutch is also coupled
between the drive means and the draw roll,
B. the clutches are current controlled, and
C. the controller comprises
1. a first current regulator responsive to the output of the
detecting means for controlling the current in the overspeed
clutch,
2. a second current regulator responsive to the output of the
detecting means for controlling the current in the underspeed
clutch, and
3. means for coupling the output of the detecting means
alternatively to the overspeed and underspeed current regulators so
that said proper torque correction is applied to the draw roll.
11. The system defined in claim 10 wherein the coupling means
include
A. means for amplifying the output of the detecting means,
B. means for differentiating the output of the detecting means,
and
C. means for summing the outputs of the amplifying and
differentiating means so that the signal applied to the current
regulators reflects the instantaneous position and velocity of the
dancer assembly.
12. The system defined in claim 11 wherein the coupling means also
include a lag circuit connected between the summing means and the
current regulators.
13. The system defined in claim 10 and further including
A. means for sensing the current in the overspeed clutch and
developing a first signal in response thereto,
B. means for sensing the current in the underspeed clutch and
developing a second signal in response thereto,
C. means for summing the first signal at the input of the first
current regulator, and
D. means for summing the second signal at the input of the second
current regulator, so as to provide negative feedback to more
closely regulate the currents in the overspeed and underspeed
clutches.
14. A system for maintaining uniform tension in a moving web
comprising
A. a draw roll for engaging the web,
B. a dancer assembly positioned adjacent the draw roll, said dancer
assembly including
1. at least one dancer roll rotatively supported parallel to the
axis of the draw roll and for engaging the moving web,
2. means for mounting the dancer roll so that it can pivot about a
pivotal axis which is parallel to the axis of the draw roll in
response to changes in the tension of the web engaged by the dancer
roll, and
3. means for applying a torque to the dancer assembly at said
pivotal axis so as to offset the torque applied thereto by a
selected tension in the web engaged by the dancer roll so that the
assembly tends to assume a reference position about said pivotal
axis,
C. means for detecting movement of the dancer assembly from its
reference position in response to a change in web tension and
producing an output signal in response thereto,
D. drive means connected to the draw roll for applying tension to
the web,
E. a controllable, variable slip overspeed clutch connected to the
drive means to provide a drive torque to the drive means,
F. a controllable, variable slip underspeed clutch connected to the
drive means to provide a holdback torque to the drive means,
and
G. an electronic controller connected between the detecting means
and the clutches for controlling the clutches in accordance with
the output signal from the detecting means so that the draw roll
controls the tension in the web as needed to maintain the dancer
assembly in its reference position.
15. The system defined in claim 14 wherein the controller is
comprised of
A. a position amplifier connected to amplify the output signal from
the detecting means,
B. a differentiator connected to differentiate the output signal
from the detecting means,
C. means for summing the outputs of the amplifier and
differentiator so as to produce a signal representing the
instantaneous position and velocity of the dancer assembly,
D. means for controlling the slippage in said clutches in
accordance with the output of the summing means so that the torque
applied by the clutches to the drive means is continuously
trimmed.
16. The system defined in claim 15 and further including an
adjustable lag circuit connected between the summing means and the
controlling means for imparting a negative phase shift to the
signal from the summing means.
17. The system defined in claim 15 wherein the controlling means
include means for applying the output signal from the summing means
alternatively to said clutches.
18. A system for controlling tension in a moving web comprising
A. a driven roll for engaging the web,
B. a prime mover,
C. a continuously controllable clutch whose torque coupling varies
in response to a control signal connected between the prime mover
and the driven roll, said clutch being arranged to operate at
maximum torque coupling at a speed slightly different from the
speed of the prime mover,
D. means for sensing a tension change in the web and developing a
control signal corresponding thereto, and
E. means for coupling the control signal to the clutch to vary the
torque coupling from the prime mover to the driven roll to produce
a compensating tension change in the web.
19. The system defined in claim 18 wherein
A. the clutch is current-controlled, and
B. the coupling means include a current regulator responsive to
said control signal for controlling the current in the clutch.
20. The system defined in claim 19 and further including
A. a second current controlled clutch,
B. a second current regulator responsive to the output of the
sensing means for controlling the current in the second clutch,
C. means for applying said control signal alternatively to the
first and second current regulators so that the proper torque is
coupled to the driven roll.
21. The system defined in claim 18 wherein said slightly different
speed is within the range of .+-. 2% to 10% of the speed of said
prime mover.
Description
BACKGROUND OF THE INVENTION
This invention relates to a system for controlling the tension or
speed in a continuous web moving to or from web processing
apparatus.
Web tension may vary because of variations in the modulus of
elasticity of the web along its length, changes in web thickness,
speed of the web-consuming machine and for other reasons.
In many instances, then, it is necessary to control the tension in
a moving web as it is drawn into or out of a printing press,
embosser, coater or other such machine through which the web must
move under controlled tension to be processed properly.
A number of systems have been devised to provide this tension
control and they often include a floating dancer assembly in
contact with the moving web which changes position in response to
web tension changes. Dancer position is then used to control the
speed of a driven roll in contact with the web. The dancer is
normally preloaded so that, in operation, it tends to assume a
selected reference position which is indicative of the desired web
tension. In the case of the infeed device, any increase in tension
causes the dancer to move in one direction from its reference
position. This movement initiates an increase in the speed of the
driven roll which tends to decrease the tension in the web so that
the dancer returns to its reference position. On the other hand, a
decrease in web tension causes the dancer to move in the opposite
direction from its reference position, producing a decrease in the
speed of the driven roll so that web tension increases sufficiently
to return the dancer to its reference position. Thus, any web
tension upsets are reflected in movements of the dancer which, in
turn, control the speed of the driven roll to compensate for the
tension upsets.
The outfeed device operates in a comparable way to maintain uniform
tension in web exiting such processing apparatus.
There are several known types of floating dancer tension control
devices. One type, shown in U.S. Pat. No. 3,087,663, requires a
separate regulated d.c. motor driving a differential drive system
to control the speed of the driven roll to maintain proper web
tension. The added motor and differential increase the initial cost
of the system and its space requirements.
In another type of system we are aware of, the speed of the draw
roll is changed by over and underspeed clutches operating off the
main line shaft. The clutches are controlled by limit switches
which sense extreme positions of the dancer. In response to a web
tension increase, the dancer moves up until it trips one switch
which then actuates the overspeed clutch, resulting in an increase
in the speed of the draw roll which lessens web tension so that the
dancer moves down until it trips the other limit switch. This
actuates the underspeed clutch, resulting in a decrease in speed of
the draw roll and an increase in web tension, causing the dancer to
travel upwards again, and so on.
While such systems work fairly well at slow web speeds, their
performance at high speeds, i.e. on the order of 2,000 fpm and
more, is not satisfactory. When the dancer roll reaches the ends of
its path of travel, which may be as long as 10 inches, it must
immediately reverse direction. This is reflected in a change in the
pressure in the dancer loading cylinder. At fast web speeds, these
pressure pulses upset the stability of the system unless a large
and expensive surge tank or accumulator is incorporated into the
system.
Additional instabilities are introduced into that type of prior
infeed because it cannot compensate for changes in web tension on
the downstream side of the infeed. Even worse, it will translate
any web speed change on the downstream side into an unwanted
tension change. Also, if the web slips on the draw roll as little
as 0.1%, control problems arise.
A variation of that type of infeed arrangement is shown in U.S.
Pat. No. 3,659,767. There, dancer position is used to control a
variable ratio transmission or variator coupled to the draw roll.
While its performance at high speeds is better than the arrangement
just described, it still is not entirely satisfactory. Stability
problems still arise because of load cylinder pulsing and the
infeed is difficult to maintain in fine tune. Further, the
transmission is quite large and expensive. Also, the transmission,
being composed of many mechanical parts which must be moved to
change drive speed, responds too slowly. Further, the system tends
to control web speed over a particular narrow range of, say,
one-half percent. This causes various parts in the transmission to
become worn or develop flats so that the system can no longer
maintain continuous control over speed. Rather, it must, in effect,
over-compensate in order to make the transmission respond beyond
that very narrow speed range.
Thus, for these reasons and others, prior tension control apparatus
which sense web tension and respond by changing the speed of the
draw roll to restore the proper tension value do not operate
entirely satisfactorily in many applications.
SUMMARY OF THE INVENTION
Accordingly, the present invention aims to provide a web tension
control system which maintains accurate control over the tension in
a moving web over a wide range of web speeds, e.g. from 50 fpm to
2,500 fpm and higher.
A further object of the invention is to provide a web tension
control system which is relatively inexpensive to make, maintain
and operate.
Another object is to provide a web tension control system which
corrects for even small tension upsets.
A further object of the invention is to provide a system of this
general type which maintains uniform web tension over a wide range
of tensions.
A further object is to provide a web tension control system which
can compensate for downstream changes in web tension and speed.
Yet another object of the invention is to provide a system of this
type which is easily constructed from standard electromechanical
components.
A further object of the invention is to provide a web tension
control system which is quite stable in operation even at high web
speeds.
Yet another object is to provide a system of this type which is
relatively efficient as compared with those requiring separate
control motors because energy is coupled back into the mechanical
drive system rather than being dissipated.
Other objects will in part be obvious and will in part appear
hereinafter.
The invention accordingly comprises the features of construction,
combination of elements and arrangement of parts which will be
exemplified in the construction hereinafter set forth and the scope
of the invention will be indicated in the claims.
For purposes of discussion, we will describe the present system in
terms of an infeed controlling the tension of web entering a press,
for example. It should be understood, however, that it is equally
applicable as an outfeed device and, indeed, in any application
where it is desirable to maintain uniform tension and/or speed in a
moving web.
Briefly, web from a roll or other source passes over a driven roll
and thence under a guide roll to the press which normally runs at a
uniform speed. A balanced, low inertia, undamped, floating roll
dancer assembly is positioned in the web path between the driven
roll and guide roll. The dancer assembly is comprised of a pair of
parallel rolls which engage the web on opposite sides thereof and
which are spaced from each other in the direction of web travel.
The rolls are rotatively mounted in a frame which, in turn, pivots
on low friction bearings about an axis which is parallel to the
axes of rotation of the rolls.
The dancer assembly is suitably preloaded so that it assumes a
position of maximum web storage. During operation of the system,
when the web is under tension, the dancer assembly tends to assume
a reference position midway between its maximum and minimum storage
positions.
Any tension change in the web downstream from the driven roll
causes the dancer assembly to move from its reference position to
accommodate the tension change. These dancer movements give rise to
corresponding electrical signals which cause a controller to vary
the torque coupling in a pair of variable slip over and underspeed
clutches connected between the machine line shaft and the driven
roll.
Since the dancer assembly is balanced, no preloading is required to
compensate for the weight of the dancer assembly. This is important
when it is necessary to process web under low tension because ten
pounds of loading pressure just to raise the dancer may represent
as much as fifty pounds of web tension. Also, the assembly's small
mass and low friction mounting means that the dancer has relatively
low mechanical inertia. Thus, it can change position very quickly
in response to a web tension change to develop the proper
corrective signal for the controller.
Also, as will be described later, the dancer assembly is specially
designed so that its movements do not affect the relationship
between the torques applied to the assembly by the tensioned web
and the loading device.
The controller develops electrical signals corresponding to both
dancer position and dancer velocity. These signals are summed and
the resultant signal is applied to a lag circuit which compensates
for the small amount of mechanical inertia in the system. The
output of the lag circuit is applied to continuously control the
overspeed and underspeed clutches modulating between them,
depending upon whether drive torque or holdback torque must be
applied to the draw roll in order to restore the proper web tension
that will cause the assembly to return to, and remain in, its
reference position.
In other words, if there is a momentary tension increase in the
web, the dancer assembly pivots so as to shorten the web path
between the driven roll and the guide roll. This dancer movement
produces a corresponding electrical signal which causes the
controller to increase the torque coupling in the overspeed clutch,
thereby increasing the driving torque on the driven roll. The
tension in the web beyond the driven roll is thus decreased, as is
the torque component on the dancer assembly due to web tension,
sufficiently to return the dancer assembly to its reference
position.
Conversely, when the web suffers a tension decrease, the dancer
assembly pivots so as to lengthen the path of the web between the
driven roll and the guide roll. This dancer movement, in turn,
causes the controller to increase the torque coupling in the
underspeed clutch so that the necessary holdback torque is applied
to the driven roll to increase web tension just enough to return
the dancer assembly to its reference position. In other words, the
present system senses the tension in the web and responds to a
change therein by continuously modulating the torque on the driven
roll, rather than its speed, to effect tension correction. This
torque control is maintained over a predetermined range of web
speeds in the vicinity of the speed of the line shaft, e.g. .+-.
4%. The system can tolerate web slippage, web elongation, and even
downstream web speed changes over this preselected speed range. Any
energy savings during torque modulation is coupled back into the
machine line shaft so that the rating of the drive motor can be
kept to a minimum.
The controller responds to both dancer position and rate so that it
exercises very fast and continuous control over the torque applied
to the driven roll. Any change in that torque is reflected almost
immediately (i.e. five Hz or less) in a corrective change in web
tension that will tend to return the dancer to its reference
position.
Moreover, the subject dancer assembly in conjunction with the
electronic controller and over and underspeed drives constitutes a
true integrating type of system. As a consequence, even a small
change in dancer position due to a small tension upset results in a
corrective torque component being applied to the driven roll.
Finally, since the system responds quickly and is quite stable and
exercises control only over the relatively short preselected web
speed range, the excursions of the dancer assembly about its
reference position are very short (e.g. 1/16 inch) and web storage
provided by only the two offset dancer rolls is quite adequate even
at high web speeds.
Thus, the present arrangement provides continuously variable torque
control over the draw roll up to the full rating of the clutches to
maintain a constant trim on web tension from a web-up speed as low
as 50 fpm to normal operating speeds as high as 2500 fpm. Actually,
the tension can be maintained to an accuracy of as high as 1% and
even lower over the entire web speed range.
BRIEF DESCRIPTION OF THE DRAWINGS
For a fuller understanding of the nature and objects of the
invention, reference should be had to the following detailed
description taken in connection with the accompanying drawings, in
which:
FIG. 1 is a diagrammatic view of the web tension control system
embodying the principles lf this invention; and
FIG. 2 is a schematic diagram showing in greater detail certain
elements of the FIG. 1 system.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1 of the drawings, since this is a balanced
system, the web can travel in either direction through the system.
However, in the infeed application specifically shown, the web W is
traveling from left to right. Web is trained around a driven draw
roll 10 and thence passed through a dancer assembly indicated
generally at 12. From there, it is trained under a guide roll 14
and drawn into the web-consuming machine (not shown). In response
to tension changes in the web W, the dancer assembly 12 moves from
its reference position to lengthen or shorten the web path as
necessary to maintain uniform tension in the web.
The dancer motion is detected by a control section shown generally
at 16 which responds by applying a drive torque or a holdback
torque to the draw roll 10. That, in turn, decreases or increases
the tension in the web W entering the dancer assembly as needed to
return the dancer assembly to its reference position.
The draw roll 10 has a shaft 10a which is coupled to the control
section 16. Web slippage relative to roll 10 is minimized by means
of a nip roll 18 which is supported via its shaft 18a at the ends
of a pair of elongated end plates 22. The opposite ends of plates
22 are connected by pivots 24 to the machine frame (not shown). Nip
roll 18 is pressure loaded against roll 10 by suitable means
illustrated herein as pressure cylinders 26 acting between end
plates 26 and the machine frame.
The dancer assembly 12 is comprised of a pair of parallel rolls 32
and 34 which are spaced apart in the direction of web travel and
engage web W on opposite sides thereof. The opposite ends of rolls
32 and 34 are journaled in a pair of end plates 36a and 36b. A pair
of stub shafts 38a and 38b project laterally from the centers of
plates 36a and 36b, respectively, and these shafts are journaled in
a pair of bearing assemblies 42a a 42b, respectively.
The components of the dancer assembly 12 are arranged with respect
to the draw roll 10 and guide roll 14 so that it is a completely
balanced system. In other words, when the assembly is in its
neutral position, the stretch of web between draw roll 10 and roll
32 is parallel to the stretch of web between roll 34 and guide roll
14. Both of these are perpendicular to the plane defined by the
dancer rolls when the assembly is in its reference (e.g.
horizontal) position. When the dancer assembly pivots on its
bearing assemblies 42a, 42b in one direction or the other from its
reference position, it either lengthens or shortens the path of the
web W between rolls 10 and 14. Since the dancer is balanced, no
loading is required to compensate for the weight of the dancer and,
therefore, the system can operate at very low web tensions.
The dancer assembly is biased toward its web storage position by
suitable means illustrated herein as a loading cylinder 44. The end
of cylinder 44 is provided with an ear 46 which is connected by a
pivot 48 to an ear 52 attached to the machine frame. The loading
cyclinder shaft 44a is pivotally connected at 49 to one of the
dancer assembly end plates, i.e. plate 36a. The pivotal connection
49 between shaft 44a and the end frame is situated outboard of
shaft 38a so that the cylinder can impart an appreciable torque to
the dancer assembly.
Fluid under pressure is fed to cylinder 44 by way of a pressure
regulator 50 and the cylinder pressure is adjusted so that the
torque exerted by the cylinder on the dancer assembly just offsets
the opposing torque due to the desired tension in web W so that the
dancer assembly tends to assume a reference position as seen in
FIG. 1. Of course, if need be, a duplicate loading cylinder can be
stalled at the opposite side of the assembly, i.e. at plate 36b, to
minimize racking of the assembly. Also, the loading cylinder 44
could just as well be arranged to push down on plate 36a (and/or
36b) on the other side of bearing assembly 42a, (42b).
The present dancer assembly is specially designed so that no
changes in web tension arise because of changes in the relationship
between the torques on the assembly due to web tension and cylinder
44 at different angular positions of the assembly. More
particularly, one of the shortcomings of many prior dancer roll
systems stems from the fact that as the dancer assembly moves from
its reference position, the forces on the dancer due to web tension
and dancer loading vary as a function of the angle to which the
dancer assembly pivots due to a tension upset. Consequently, in
those prior systems, the dancer assembly itself introduces unwanted
tension changes in the web which adversely affect the stability of
the overall system. The dancer assembly shown in FIG. 1 maintains
constant web tension with a constant loading force from cylinder 44
through all positions of the dancer assembly and there is no load
cylinder pulsing when the dancer changes direction. This is
accomplished by arranging the loading cylinder 44 and its shaft 44g
or other loading devices so that the force is always applied
parallel to the stretches of web W entering and leaving assembly
12. This condition prevails provided the triangle defined by points
A, B and C in FIG. 1 is geometrically similar to the triangle
defined by points A, D and E in that figure. It can easily be shown
mathematically that as long as the above similarity persists, the
force on the dancer assembly from cylinder 44 will equal the
product of a constant and the force on the dancer due to web
tension for all deviations of the dancer assembly from its neutral
position.
Still referring to FIG. 1, any rotation of the dancer assembly on
shafts 38a and 38b is sensed by a suitable detector illustrated
herein as a potentiometer 56 which is connected electrically to the
control section 16. More specifically, the potentiometer 56 applies
a signal to an electronic controller 58 in section 16 which
corresponds to the deviation of the dancer assembly from its
illustrated reference position. The controller 58, in turn,
provides an output to a variable slip overspeed clutch 62 or a
variable slip underspeed clutch 64 coupled to draw roll 10,
depending upon whether drive torque or holdback torque should be
applied to the roll to correct web tension.
The clutches 62 and 64 are driven by the machine line shaft 66. For
this, a toothed pulley 68 rotating with the line shaft 66 is
connected by a timing chain 72 to a toothed pulley 74 affixed to
the input shaft 62a of clutch 62 and also to a toothed pulley 76
connected to the input shaft 64a of clutch 64. The pulley 74 has
fewer teeth than pulley 68 and pulley 76 has correspondingly more
teeth than pulley 68 to obtain the desired speed differential
between the two clutches over the desired speed range of, say
.+-.4% of the line shaft speed. The two clutches also have output
shafts 62b and 64b which carry toothed pulleys 78 and 82,
respectively, which are connected by a timing chain 84 to a toothed
pulley 88 on the end of the draw roll shaft 10a. The clutches are
set to magnetically couple a selected torque to the draw roll 10 as
needed to maintain the dancer assembly in its reference position.
The torque coupling changes required to compensate for web tension
variations are made by controlling the current in the clutch coils.
A variable slip magnetic particle clutch suited for this purpose is
manufactured by W. J. Industries Inc. and the Vickers Division of
Sperry Rand Corp. although other comparable electric, pneumatic or
friction clutches can also be used, as long as they permit
continuously varying control over the torque applied to the draw
roll.
The dancer assembly 12 and control section 16 together function as
an error position integrator so that the infeed responds to even
very small tension changes. Also, the control section 16 includes
feedback between the clutches and the controller and within the
controller itself to keep instabilities within the system to a
minimum. As a consequence, the infeed maintains very close control
over web tension i.e. within 1% of the desired tension. Also, the
excursions of the dancer assembly 12 from its reference position
are very small, i.e. on the order of 1/16 inch during normal
operation and only 1 to 2 inches during an emergency stop
situation. Since the dancer and overall system have very fast
response and only controls web speed within .+-.4% of line speed,
the dancer assembly does not require much storage capacity even at
web speeds in excess of 2500 fpm.
Turning now to FIG. 2, the controller 58 contributes substantially
to the basic stability of the overall system. It includes a
regulated, dual polarity power supply 82 connected to the
potentiometer 56. The output of the potentiometer, taken from the
wiper 56a thereof, is applied to a d.c. amplifier 84 and to a
differentiator 86.
When the torque applied to the dancer assembly 12 (FIG. 1) by the
loading cylinder 44 exactly offsets the torque due to the desired
web tension and the dancer assembly assumes its horizontal
reference position,, the voltage at the potentiometer 56 is 0
volts. However, when the dancer assembly moves in response to a
tension change in the web, a signal is applied to amplifier 84 and
differentiator 86.
The output of the amplifier is a signal corresponding to the
position of the dancer, while the output of the differentiator is
proportional to the dancer velocity. These two signals are summed
and applied to a lag circuit 88 illustratively consisting of a high
gain amplifier 89 and an RC circuit 92 connected between the
amplifier 89 input and output. The lag circuit is basically an
integrator and provides a negative phase shift to compensate for
the mechanical inertia that remains in the infeed system.
The circuit 88 output is applied directly to a diode 94 and by way
of an inverter 96 to a diode 98, both diodes being arranged to pass
only positive going signals. When the dancer assembly moves from
its reference position, a positive going signal is applied either
to diode 94 or diode 98, depending upon the direction of dancer
movement. For purposes of discussion, we will assume that an
increase in web tension which causes the dancer assembly 12 to
rotate clockwise in FIG. 1 produces a positive going signal at
diode 94.
The output of diode 94 is applied by way of a voltage regulator 102
to an overspeed firing circuit 104. The output of circuit 104 is
used to trigger a pair of SCR's 106 and 108 connected to pass
current during alternate half cycles from a center tapped
transformer 110 to a series circuit consisting of the coil 62c of
overspeed clutch 62 and a resistor 112. The output of the regulator
102 determines the firing angle of the SCR's and therefore controls
the current in the clutch coil. A diode 111 is connected across
coil 62c to accomodate back EMF due to a collapsing field in coil
62c. The voltage at the junction of coil 62c and resistor 112 which
is proportional to the current through the coil 62c is fed back and
summed with the signal from diode 94. Thus, the signal applied by
voltage regulator 102 to firing circuit 104 makes the current
through the clutch coil 62c at all times proportional to the output
of the lag circuit 88. Thus, very fast control is exercised over
the overspeed clutch 62 in response to dancer movements due to a
web tension increase.
Still referring to FIG. 2, similar circuitry follows the diode 98
to control the underspeed clutch 64. Thus, the diode 98 output is
applied via a voltage regulator 114 to an underspeed firing circuit
116 whose output is used to trigger a pair of SCR's 118 and 122.
These SCR's are arranged to conduct current from a transformer 124
during alternate half cycles to a series circuit consisting of the
underspeed clutch coil 64c and a resistor 126, with a diode 127 in
parallel with the coil. The output of the lag circuit also controls
the firing angle of these SCR's and thus the current through coil
64c. A voltage proportional to the current through the coil 64c
appears at the junction of that coil and the resistor 126 and is
fed back to the input of regulator 114 where it is summed with the
signal from diode 98. Thus, the signal applied to the firing
circuit 116 controls the current through the underspeed clutch coil
64c so that it is always proportional to the output of the lag
circuit 88. Resultantly, very close control is maintained over the
torque coupling between the underspeed clutch 64 and the draw roll
10 in response to counterclockwise dancer movement indicative of a
web tension decrease.
In summary, then, the present system controls web tension by
sensing web tension changes using a balanced, specially loaded,
undamped dancer having low mechanical inertia and thus fast
response to tension upsets. The dancer motion is sensed and a
signal is applied to a controller which continuously develops
proportional control signals for the continuously controllable
overspeed and underspeed clutches. Thus, these clutches are
modulated so as to apply just the proper torque component to the
draw roll to maintain the dancer in its reference position.
Since the clutches are set to always drive the draw roll at speeds
slightly less than or slightly more than line shaft speed, the
excursions of the dancer are quite small, e.g. 1/16 inch, and
system operation is quite stable. Also, there is no dancer loading
cylinder pulsing problem. Thus, web tension can be controlled with
great accuracy over the full range of web speeds of from 50 fpm to
2500 fpm. As noted above, in a typical case, the clutches are set
to run 4% underspeed and 4% overspeed and this amount may vary in
different applications. As a general rule, however, it has been
found that if the speed difference is much less than .+-.2% of line
shaft speed, tension upsets take a fairly long time to correct.
Also, if the difference is much more than 10% of shaft speed, there
is relatively large energy dissipation incident to tension
control.
The present system can accommodate downstream web speed changes
because the clutches operate off the main line shaft, as does the
downstream web-consuming machine so they compensate for any such
change. Further, any speed upsets are not translated into web
tension upsets as is the case with some prior infeeds.
The present infeed is much less expensive than prior systems,
costing only about one-half as much as the type shown in U.S. Pat.
No. 3,087,663 and on the order of one-third less than the type
depicted in said U.S. Pat. No. 3,659,767 and its mechanical
clutch-type drive is only on the order of one-third the size of the
differential drives and motors in those prior infeeds, yet it is
easier to operate and control.
It will thus be seen that the objects set forth above are
efficiently attained. It should also be understood that certain
changes may be made in the above description without departing from
the scope of the invention. For example, using this equipment, one
can control draw roll speed to impart a precise amount of
elongation to a web by maintaining a precise underspeed torque.
This is useful, for example, to orient polyethylene. As another
example, in those applications such as commercial printing where
the tension in the web downstream from the infeed is always greater
than the tension in the web upstream therefrom, the overspeed
clutch 62 and related control elements may be omitted. Conversely,
in outfeed applications where the upstream web is under greater
tension than the downstream web, the underspeed components of the
system may be dispensed with.
Also, the same principle can be applied to accurately control web
speed. This involves sensing web speed changes instead of tension
using a roll tachometer as the input to the controller.
It will also be understood that the following claims are intended
to cover all of the generic and specific features herein
described.
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