Understanding center-driven web winders - Motion System Design

PRECISION MOTION CONTROL
Unwinders and winders are used in almost
all web-handling industries including
paper, plastics, converting, ferrous
and non-ferrous metals, wire, printing,
and textiles. The winder is often the limiting
factor in machine throughput when
continuous winders and unwinders are
not used. Moreover, close winder control
is essential to quality production.
This article deals with the most common
type — dancer-controlled, speedregulated
center winders — and examines
the difficulties of winder control and
the strategies used to wind and unwind
various web materials.
Need for close control
In general, the problems associated
with winder control include obvious
physical deformities such as crushed
cores, air gaps, starring (caused by tension
fluctuations), telescoping, and necking,
Figure 1. There are also less obvious
winder-related problems, such as
scratches produced by relative movement
within the roll (cinching, Figure 1)
and web breaks. These are frequently
caused by compression forces in a wound
roll, which can crush some materials
causing fractures. Web breaks become
apparent on the machine that uses the
roll, not on the machine that produces
the roll.
Mark S. Dudzinski is chief executive officer
of Amicon, Inc., American Industrial Controls,
Charlotte, N. C.
Understanding center-driven
web winders — Part 1
MARK S. DUDZINSKI, Amicon, Inc.
The quality of web products depends on precisely controlling speed
and tension during winding or unwinding. This article explores the
challenges of winder control and discusses the design parameters of
economical, dancer-controlled winders. A following article will cover
load-cell controlled winders and open-loop winder control.
Cinching
Crushed core
Starring
Air gaps
Figure 1— Improper tension and speed
control can produce these typical
physical deformaties.
Telescoping
POWER TRANSMISSION DESIGN ■ MARCH 1995 3

PRECISION MOTION CONTROL
Types of winders
Winders predominately fall into two
broad categories: center winders and surface
winders. In addition, there are
winders that combine the characteristics
of these two types. An example of a combination
winder is a speed-regulated surface
winder with a torque-controlled centerwind
used to wind slippery materials.
Control considerations for a center unwinder
are similar to those for a centerdriven
winder. Therefore, almost all of
the winder information in this article is
also applicable to unwinders.
In a center winder, a motor drives the
core or shaft of the roll being wound. Although
the motor drive can use any technology,
ac drives and dc drives are most
commonly used on large winders. On
smaller winders, servo drives can be
used.
Center winders are usually controlled
either in a speed mode or in a torque
mode. When controlled in a speed mode,
a speed reference is provided to the drive
by the winder control. In such speed-regulated
systems, the speed regulator commands
torque in the motor.
In torque-regulated systems, a speed
regulator is not used. The motor will run
at the highest speed it can reach with the
available torque.
2
Motor
Motor
One motor
revolution
winds 31.4 in.
10 in.
50 in.
POWER TRANSMISSION DESIGN ■ MARCH 1995
Web in
Dancer
position
feedback
Winder control system
challenges
Position
Winders are often the most difficult
part of a machine to control, because
both the diameter and inertia of the
wound roll are continually changing.
Roll diameter. Consider two different
times during the winding of a single roll,
Figure 2. Assume, at both times, the motor
is turning at 100 rpm. While the roll
has a 10-in. diameter, the machine is
winding at 261.8 fpm. However, the 50-in.
roll is winding at 1,309 fpm. If you
increase motor speed by one
rpm, the 10-in. roll winds 2.6 fpm
faster, but material on the 50-in.
roll is wound 13.09 fpm faster.
Thus, the same speed-reference
command change causes different
web-speed changes depending
on roll diameter. This difference—a
system gain
change—can complicate the
control strategy in speed-controlled
winder applications.
One motor revolution
winds 157.1 in. Figure 2 — Changes in roll
diameter, when winding at a
constant motor speed, causes web
speed changes, which can
complicate speed-controlled
winder applications.
PID control
Speed
reference
Motor
Drive
Rewind
Speed
reducer
Figure 3 — A simple, speed-controlled winder with dancer-position feedback is suitable
for build ratios of 4 to 1, or less.
Roll inertia. A second complicating
factor is roll inertia. For example, a 3.5in.
aluminum shaft weighing 60 lb has a
moment of inertia of 2.5 lb-ft 2 . If 50 in. of
paper is wound on this shaft and the roll
weighs 2,500 lb, its inertia is over 21,000
lb-ft 2 . This is an inertia change of over
8,000 to 1. Such inertia changes affect the
tuning of speed-controlled winders, the
inertia compensation of torque-controlled
winders, and the web-tension regulation
capability of brake-controlled unwinds.
The effect of inertia on speed-controlled
winders can be considerable. For
example, an empty spindle on a winder, if
the drive is tuned properly, will closely
follow commanded speed changes. However,
a large-diameter roll on the winder,
tuned at empty core, will be unstable and
will not closely follow commanded speed
changes. In fact, the roll may be wildly
unstable and may rotate in one direction,
and then in the other, even if zero speed
is commanded. This instability results because
the optimal integral gain at the
core is too fast for the high-inertia load to
follow.
Drive tuning. Similarly, if the drive is
tuned with a full roll, the empty spindle
will be unstable, because the full-roll proportional
gain is much higher than an
empty spindle can tolerate. Therefore,

most winder drives are not optimally
tuned for all roll diameters and inertias.
Tuning is then a compromise. On a
dancer-controlled center winder, this
compromise tuning can be seen by observing
the dancer position during two
different situations. First, when the machine
is webbed and the line is enabled
at zero speed, the dancer will usually go
to its center position if the winder is on a
core. However, with a large roll on the
winder, often the dancer will not go to its
center position.
A second illustration of compromised
tuning of the control is when the machine
accelerates. Compromised tuning
is evident if the machine accelerates
without excessive dancer movement with
some roll sizes, but the dancer moves excessively
at other roll sizes. This effect
can also be caused by an incorrectly sized
drive system.
Speed range. A third complicating factor
is winder speed range. This does not
affect the design of the winder control,
but it does alter the selection of the drive
system. A machine designed to operate
from 100 fpm to 2,000 fpm, has a speed
range of 20 to 1. This is a speed range that
almost any drive can attain.
The winder, however, must accommo-
Electric-pneumatic transducer
0 to 10 Vdc
Web in
Pneumatic
cylinder
Dancer
position
feedback
Pneumatic regulator
Position
Line speed
Tachometer
or encoder
Winder
control
date this speed
range from core to
full roll. If the core
is 3 in., and the
maximum roll diameter
is 51 in., the
winder has a build
ratio of 17 to 1. The
winder must meet
the machine speed
range regardless of
roll diameter.
Therefore, the
winder needs a
speed range of 340
to 1. Many off-theshelf
drives have an
effective speed
range of only 20 to 1 or, at best, 100 to 1.
Dancer-controlled winders
An established, cost-effective method
of providing winder control, dancer-controlled
winders are often selected for applications
with a build ratio of over 20 to
1. Because the dancer provides some material
accumulation, slight feed-forward
errors are easily accommodated and corrected.
Thus, this type of winder is appropriate
when very fast accelerations and
Winder
speed
Speed
reference
Motor
Drive
Web
Pneumatic
cylinder
Rewind
Speed
reducer
Figure 4 — A more sophisticated dancer-controlled winder suitable for build ratios
of 10 to 1.
Accumulator
Accumulator
Pneumatic
regulator
Electric-pneumatic
transducer
Pneumatic regulator
(manually adjusted)
Figure 5 — In a pneumatic system, loading on the dancer
controls web tension.
decelerations are required, and for highspeed,
nonstop dual-turret winder applications.
Usually, some small dancer movement
occurs during steady-state, constantspeed
operation. During acceleration and
deceleration, the dancer moves more.
A dancer’s position is determined by
the difference in speed between the
winder and the preceding web section. If
the winder is faster than the previous
section, the dancer rises; if it is slower,
the dancer falls. Thus, speed control is a
good strategy for dancer-controlled
systems.
In a simple dancer-controlled winder,
Figure 3, the dancer position error is
used by a PID (proportional, integral,
derivative) control loop to establish
winder drive speed. A PID loop maintains
a process variable at a given setpoint by
adjusting an output variable to minimize
the error between the process variable
and the setpoint. This type of simple control
can be used on winders with build ratios
of 4 to 1, or less. For example, if the
core diameter is 3 in., then a maximum
roll diameter of 12 in. can be controlled.
The 4-to-1 build ratio for this type of control
is just a guideline. Fast acceleration
rates or large inertia changes may make a
4-to-1 build ratio unobtainable.
Similarly, if the material being wound
has a low density, or if the roll is narrow,
build ratios of 10 to 1 or even 12 to 1 may
be obtained.
POWER TRANSMISSION DESIGN ■ MARCH 1995 3

PRECISION MOTION CONTROL
A more sophisticated dancer-control
system, Figure 4, has three sensors that
are used by the winder control to calculate
the drive-speed reference. The three
sensors determine line speed, winder
speed, and dancer position. Two of the
three sensors are used to calculate a
feed-forward control term that anticipates
the speed that the winder should
be running. The feed-forward term is determined
by dividing the line speed by
the winder speed to calculate the roll diameter.
The line speed is then divided by
the roll diameter to provide the correct
winder speed. If there are no errors in the
feed-forward calculation, and if the drive
follows the speed reference without delay,
the dancer will always stay in its center
position.
But, in the real world, nothing is so exact.
Time delays, electrical drift, mechanical
variances, drive sizing, and out-ofround
rolls contribute to inaccuracies in
the feed-forward calculation. To correct
these inaccuracies, the dancer position
error is fed to a PID loop. The output of
the PID loop modifies the feed-forward
value to provide the drive with the correct
speed reference.
Systems such as those in Figure 4 can
typically be used on winders with build
ratios of up to 10 to 1. For example, with a
2
50%
Tension 100%
0%
Core Tpr1
Diameter
Tpr2 Full roll
Figure 6 — A tension taper control
changes web tension as a function of roll
diameter. Some web materials require
nonlinear tension taper functions.
POWER TRANSMISSION DESIGN ■ MARCH 1995
3-in. core, a maximum
roll diameter of 30 in.
could be obtained. The
10-to-1 ratio is a rule of
thumb and larger or
smaller build ratios can
be obtained depending
on several factors including
material density
and gear ratio.
Dancer-controlled
winders with build ratio’s
larger than 10 to 1
require a more sophisticated
winder control.
The block diagram for
these winders is the
same as shown in Figure
4. However, the winder
control algorithm is
more complicated.
These winder controls
have adaptive gain functions
that compensate
for changing roll diameter.
Such adaptive functions
also
change the
(a)
(b)
90 deg
52 deg
gain as the wound roll inertia
changes.
Maintaining tension. In
a typical pneumatic system
used with a dancer controlled
winder, Figure 5, a
pneumatic cylinder is tied
to the dancer. Both the top
and bottom of this cylinder
can be pneumatically
loaded. A manually adjusted
regulator, which provides
air to the bottom section of
the cylinder, is adjusted to counterbalance
the dancer weight. The pneumatic
loading of the top cylinder section, controlled
by an electric-pneumatic transducer,
provides the required web tension,
which is based on a tension reference.
This is usually provided by the control
system.
Note that accumulators in the air lines
to both sections of the cylinder enable
the piston to move without compressing
air in the system. If dancer movement
should compress the air, thus changing
Figure 7 — Maintaining a constant dancer position can
minimize variations in web tension due to a web path change
caused by dancer movement. As the dancer changes position
from that in (a) to that in (b), with the same cylinder loading,
reduces web-tension by 29%.
the air pressure, web tension or the counterbalance
pressure will be incorrect. A
rule-of thumb is that the accumulators
should provide at least ten times the volume
of the air cylinder. This will normally
keep web-tension variations due to air
compression to less than 5%. If tension
variations of less than 5% are required,
use larger accumulators.
For large rolls, tension is usually linearly
tapered as a function of roll diameter,
Figure 6. It is not uncommon for
winders to have nonlinear tension taper
functions. The exact taper function required
depends on the material. Some
materials do not require winder taper
tension. Unwinders typically do not have
a taper tension function.
Dancer design. Correct dancer design
is not a trivial exercise. Every time the
dancer moves, it causes a small (or possibly
a large) tension variation on the web.
Although it is often not obvious, the
winder control system does not directly
control web tension. Rather, the winder
control provides a tension reference, but

the control itself regulates dancer position.
Thus, tension variations are often
caused by dancer design.
Theoretically (although not possible),
the dancer mechanical and pneumatic
system should be designed so dancer
movement does not cause web-tension
changes. However, steps can be taken to
minimize the effect of the dancer on web
tension.
First, use only low-stiction cylinders.
The force required to start the cylinder
moving shows up as a web-tension
change. Cylinders that have some air
leakage around the piston work best, and
they are often the least expensive.
Second, make the dancer as
lightweight as possible. The dancer has
inertia, even though it may be counterbalanced,
and the force required to overcome
this inertia will cause a web-tension
variation.
Third, use air pressure to counterbalance
the dancer, not a weight. Weight
adds inertia and this is not desirable, as
explained previously.
Fourth, the web path should not
change as the dancer moves. If it does, be
sure the resulting tension variation is
within acceptable limits. For example,
with a pivoted dancer, Figure 7, web tension,
which depends on dancer position,
varies as the cosine of the cylinderdancer-arm
angle. Because of its low
cost, this is a common dancer design.
Dancer disadvantages. There are,
however, disadvantages to dancer-controlled
winders. Dancer design can affect
web tension. In light-tension applications,
dancer stiction and inertia can
cause major problems. Also, good winder
control is necessary to prevent instabilities,
which can be encountered with
large inertia changes and large speed-
Circle #
range requirements.
Finally, since dancer-controlled
winders regulate dancer position and do
not have tension-measuring devices,
web-tension problems often must be
solved mechanically, not electrically.
However, the problems related to dancer
movement can be minimized by good
winder control.
One way to eliminate dancer-related
problems is to use a load-cell to control
the winder. This approach will be discussed
in the final article in this series
appearing in the next issue. ■
To obtain more information on
winder controls by Amicon, Charlotte,
N.C., please circle 421 on the reader service
card.
If this article is helpful, please circle
422 on the reader service card.
POWER TRANSMISSION DESIGN ■ MARCH 1995 3