Generally speaking, there are two types of tension control systems to choose from (see last week's tech tip). Open loop and closed loop.
Open loop tension controls control torque of a drive, brake or clutch to compensate for roll radius changes at an unwind or rewind. As roll radius decreases at an unwind station, open loop controls sense that change and decrease torque at a linear rate. Conversely they increase torque at a rewind. Open loop controls include follower arm, roll radius calculators (based on RPM) and ultrasound.
- Follower arms use a mechanical arm and wheel loaded against the roll O.D. As the roll changes radius the arm moves and a sensor tells the control to change torque based on sensor and arm position. Follower arms are a good replacement of manual control for unwind and rewind stands. However, their biggest disadvantage is the mechanical arm being in the way of roll changes.
- Roll radius calculators use RPM sensors at the unwind or rewind shaft and at an idler or driven roll in the machine. By comparing the RPM of the unwind or rewind to the idler or driven roll, unwind or rewind radius is calculated. The control adjusts torque based on the calculated roll radius. These controls are good replacements for manual controls. However, their biggest disadvantage is that they cannot calculate roll diameter at machine start, because RPM's are essentially zero at start up.
- Ultrasound controls measure roll radius with a sensor that uses ultrasonic sound waves to measure distance. As the unwind or rewind change radius the sensor sees the change and tells the control to adjust torque. Ultrasound controls are an excellent, cost effective method of replacing manual controls. Their biggest disadvantage is that they may not operate properly with sound absorbing materials.
Closed loop tension controls control torque of a drive, brake or clutch by measuring actual web tension. They include dancer roll assemblies and load cell tension controls.
- Dancer rolls use moving idler roll assemblies. All dancer rolls are loaded in one direction and the web has a tendency to pull the dancer in the opposite direction. A sensor detects the dancer position and always tries to maintain a constant position by adjusting a drive, brake or clutch.
- Load cells use strain gauges to detect the "weight" applied to an idler roll by the web as it is wrapped around the idler roller. The machine operator sets a value for tension, the load cell measures actual process value and controls torque to keep web tension the same at all times.
Generally speaking open loop controls are very stable but not as accurate as closed loop systems. Closed loop tension controls are extremely accurate, but not as stable as closed loop.