An unwinding application involves the payoff of a web or cable from a roll into the tension process. If the tension is not properly controlled in a web application, issues such as feathering, telescoping, or product breakage could occur. Likewise, in a cable unwinding application, improper control or diameter variations could result in breakage.
There are various methods to control tension in the unwinding application, but the most accurate method is to control the tension through direct load cell measurement. To monitor the tension in the product, load cells are used to measure the actual tension of the product in the unwind zone. The load cells will typically be mounted on one of the idler rolls downstream of the unwinding roll.
To control the tension in the unwind zone, a brake is utilized on the unwinding roll. The two ends of the closed-loop control are the input of the load cell and the output to the braking device. When a roll is first introduced at its maximum diameter, the maximum amount of braking action is required due to the inertia of the roll. Vice versa, as the roll diameter reaches its core size, the output requirement is at its minimum. Over the coarse of the diameter change, the response of the controller will need to dynamically change – allowing more aggressive control response at the beginning roll than at the end. The tension controller accommodates for this type of dynamic change.
If the process is stopped and restarted numerous times, the output at each starting point will also require continuous adjustment to avoid large spikes in tension during startup. This Auto Start functionality must also be built into the tension controller.
The Hardy Tension Controller and LVDT-based and Strain Gage-based load cells provide a complete solution to the above application.
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