Web Lines | How the Open Loop Torque Control Sets Winding Tension

There are many winding machines that do not use closed-loop tension control in the winding process. Most of these machines request input of starting and taper functions of winding tension and calculate the complex function desired tension versus roll diameter or winding speed.

These programs convert the calculated tension into motor torque applied at the torque to create the outer roll tension vector. Hopefully these open-loop torque control winding systems take care to consider torque losses, such as rolling resistance or inertial losses during acceleration and deceleration.

For rolling resistance, some systems include a setup feature to perform individual winding spindle tests to measure and record rolling resistance as a function of speed and control winding torque to a value above the baseline of rolling resistance. This is somewhat common, but rarely do these systems look to measure rolling resistance of full diameter or maximum weight rolls, instead assuming rolling resistance is independent of roll weight or winding nip load.

All this capability is great and quite advanced compared to many slitters where tension is controlled by friction torque systems, but it requires a great deal of trust that the slitter control engineers understand what motor control parameters create a given torque output over a span of speeds and loads. This open loop system has no feedback method to indicate process changes from equipment wear, failure, calculation error, except in the quality of wound rolls.

It would be great to be able to verify the system performance with a tension measuring roller. A feature of many slitter/rewinders, including this one, is a short web path from slitting to winding, ensuring good winding roll sidewall, but leaving no consideration for a winding tension measuring roller.

My preference would be to have at least an optional web path from slitting to winding that would include tension measurement. This option would allow for verification trials confirming the open loop torque control system is creating the desired tension and the combination of miscalculations, bearing failure, roll weight, nip load, and inertial factors do not cause significant errors or variations.

Web handling expert Tim Walker, president of TJWalker+Assoc., has 25 years of experience in web processes, education, development, and production problem solving. Tim writes PFFC’s Web Lines column and web handling blog. Contact him at 651-686-5400; This email address is being protected from spambots. You need JavaScript enabled to view it.; www.webhandling.com.

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