JEC COMPOSITES MAGAZINE - Issue #99 - September 2015 - 53

to be scrapped.
With some carbon fibre costing around
$900 a pound, ensuring you don't end
up with scrap is highly important.
Tension control is the process of maintaining a predetermined or set amount
of stress or strain on a given material between two points to maintain its desired
properties (such as form, appearance,
etc.) and quality. This process often
takes place in specific control zones:
unwind/payout, process (nip), and rewind/take-up, with each having unique
tension control requirements.
So tension control is the process of
ensuring the material gets from point A
to point B in a uniform and consistent
manner without any loss to material
quality. Under this basic definition,
most applications can already get their
material processed from point A to
B, but perhaps it is very manual, or is
slower than it could be. In your current
application, could you double your
speed without sacrificing end product
quality?
Automating the process, creating a
continuous process and increasing
productivity are all going to rely heavily
on proper tension control to ensure
consistent and high-quality results.

So how can tension be
controlled?
With a fixed torque in an unwind application, as the roll diameter decreases,
the tension applied to the web will increase, and it will increase more rapidly
as the roll diameter gets closer to core.
In rewind applications, it is just the opposite. As the roll diameter builds, with
a fixed torque, tension will decrease as
the roll diameter increases (see Figure
2)

Fig. 2: Fixed output

Fig. 3: Tension = torque / radius

Mathematically, tension equals torque
divided by radius. This means that as
a roll diameter increases or decreases,
torque must be adjusted to maintain
tension at the optimal value, or setpoint.
So in a basic sense, by controlling the
torque in a uniform manner in relation
to the roll, we control tension.
There are three primary types of tension
control: manual control, closed-loop
control and open-loop control.

Manual control
Just as it implies, manual control is
the process of visually or physically
determining if tension is correct, and
then making manual adjustments to
maintain the desired tension level. With
manual tension control, a web is subject
to constant up and down spikes as an
operator is always playing with correcting the tension.
Maintaining proper tension gets increasingly difficult as roll diameters decrease
and changes happen more frequently.
This method is also exposed to the subjectivity of different operators, typically
resulting in non-uniform results (see
Figure 4).

Open-loop control

Fig. 5: Automated open loop tension control

sensors to measure, or calculate, roll
diameter. Based on inputs into the
controller, the controller uses any
detected changes in roll diameter to
regulate torque and maintain tension, as
opposed to closed-loop systems, which
receive tension feedback directly from
the web of material.
Although not as precise as closed-loop
systems, open-loop systems still provide
an automated system of consistent tension control that outperforms manual
control. Open-loop systems are generally less expensive than closed-loop
systems because you are not utilizing
additional products, like load cells.

Closed-loop control
Closed-loop tension control refers to
a continuous flow of communication
from a web tension measurement device, to a torque controller, to a torque
device.
Web tension measurement devices
include load cells or a dancer position
feedback sensor that are constantly
relaying tension changes directly from
the web of material to the tension

Open-loop systems utilize products
such as ultrasonic sensors or proximity

Fig. 4: Manual control

Fig. 6: Automated closed loop tension control

No99 September 2015 / jec

composites magazine 53
Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #99 - September 2015
