In Brief – Open-Loop Tension Control vs Closed Loop Tension Control

Open-Loop Tension Control versus Closed Loop Tension Control for Unwind or Rewind Applications

When is it best to use simple open-loop control versus the more technically involved closed-loop tension control?

The first question I ask is what is your tension tolerance?  Tension tolerance refers to how much tension variability the material or process can withstand before the material is damaged or falls out of specification and is no longer useable for downstream processing or unsaleable to your customer.

Open-loop tension controls tend to be simple, inexpensive and can be very effective for providing constant unwind tension for your process. Open-loop systems utilize indirect methods for measuring or calculating the roll diameter and automatically regulate the output to the torque device to maintain constant tension in the process.

Because there is no direct web feedback in order to actually monitor and regulate control, the accuracy of the control system depends on the accuracy of the diameter calculation and the linearity of the torque device attached to the roll of material.

In a well maintained system, with a very linear torque device, open-loop tension control can provide tension accuracy within about 10-20% of desired setpoint.  If the system degrades mechanically, or the linearity of the torque device is sub-standard or changes over time, the tension accuracy will be compromised.

If your products or process can withstand this level of tension variability, then an open loop tension control system may be a good option for you.  It is certainly a far cry better than utilizing a manual control system, relying on the operators to make regular and consistent changes to the torque level of the brake as the roll diameter changes.  You can probably guess what the tension variability will be under manual control!

Closed-Loop tension control utilizes the web to provide direct feedback to either load cells or a dancer mechanism in order to regulate the torque device to an exact setpoint.  Tension tolerance values for a well-designed and well-maintained closed loop system are on the order of 2-5% tension variability or better.  Closed loop systems also make it easier to change the tension setpoint to repeatable levels for different materials and web widths.  Another advantage of closed-loop systems is their ability to adjust quickly to speed changes.

So when considering a tension control system as either an upgrade from a manual-control system or as a new design, it is very important to answer the question:  What is my tension tolerance, and proceed from there.

For additional assistance with your application Contact a Montalvo Applications Specialist.