Tips

Often times people ask me, "what's the difference between open loop and closed loop tension controls?".

Open loop tension controls include follower arms, ultrasound and radius calculator controls. They control the torque output of a device (brake on an unwind, drive at a rewind) relative to a measurement of roll radius. For example, in order to maintain constant tension, torque must decrease, linearly, relative to roll radius at an unwind station. Conversely, torque must increase at a rewind station. Open loop controls simply measure roll radius and tell a drive mechanism to adjust torque, based solely on the roll radius. Simply put, as an open loop control sees the roll getting smaller at the unwind, it will decrease the torque output of a brake or drive. Open loop controls have no idea what the tension is in the web itself. They have no idea what the tension should be, they simply know roll radius.

Closed loop controls include load-cells and dancer roll assemblies. They control tension by measuring web tension. Load-cells simply compare a set point value (tension level set by the machine operator) to a process value (the actual tension in the web) and tell a brake or drive to make adjustments to maintain as close as possible to the set value. Load cells use strain gauges to measure the "weight" applied to an idler as the web wraps around the idler. Dancer rolls use position sensing to adjust brakes and drives. All dancer rolls are loaded in one direction. The web has a tendency to pull them in the opposite direction. A position sensor adjusts a brake or drive to keep the dancer in a constant position, thus maintaining constant tension on the web.

Which type of tension control is best for your operation? Tune in next week and we will discuss which type of tension control may be best for you. Or you may email me at any time (damour@epix.net) with specific questions and I will be happy to do my best to answer them.