What is open-loop torque control?

Closed-loop tension control requires tension feedback by either a transducer or dancer roller. If you control tension without a transducer roller (a.k.a. load cell or tension roller) or a dancer roller, then you are using open-loop tension control. Torque control and draw control, without tension feedback, are both open-loop tension control methods.

If your control brake torque applied to the shaft of an unwinding roll, you can increase or decrease the web tension by adjusting the brake torque. Similarly, using a torque-limiting clutch control on a winding shaft will create tension, but is not closed-loop control without tension feedback.

Both unwinding or winding torque control systems need to change torque proportional to roll diameter to maintain constant tension. An advanced torque control system will include a diameter sensor to automatically adjust the torque proportional to roll diameter, coming close to constant tension control, but without tension feedback, they are still open-loop tension control.

The greatest weakness of an open-loop torque system is inertial torque. Web tension is proportional to the resisting torque divided by the roll or roller radius. For steady speeds, a diameter-based torque compensation system will do a reasonable job in creating uniform tension; however, during acceleration and decelerations, the roll or roller mass will add or subtract from the applied clutch or brake torque, creating a significant uncompensated tension variation.

Why use open-loop torque control?

  1. Unwinding – Many tension insensitive processes use a constant (or manually adjusted) torque brake to control the tension in the unwinding web. Many unwinding processes use a diameter sensor to maintain a constant ratio of torque to roll diameter, creating a consistent brake-induced contribution to web tension. (Note:  This approach often ignores the contribution of friction and inertia to web tension.)
  2. Rewinding – Constant torque center winding creates good wound roll structure in many cases, especially if the roll buildup ratio is less than 3 or 4:1. Constant torque center winding has a natural tapering with the tension decreasing inversely with roll diameter. This natural taper creates the desirable roll structure of firm layers near the core and more loosely wound outer layers. Constant torque center winding has less trouble with roll cinching and cinching-related telescoping since there is not increasing need for torque transmission.
  3. Differential rewinding – Differential winds multiple rolls on a common shaft, but unlike locked-bar winding, allows each roll to turn independently. To maintain the desired tension on each independently rotating roll, differential winding systems are designed to control the torque transmitted to each core.
  4. Tendency driven roller – A tendency driven roller is a hybrid device, partially web-driven and partially motor-driven. Tendency driven roller are most commonly used where the traction created by wrap angle, tension, and web-roller traction coefficient is potentially insufficient to overcome a idler roller’s bearing and inertial drag. In most tendency driven rollers, the bearings between the shaft and roller shell are used as a low torque clutch. The shaft is driven at the desired roller rpms and the bearing drag transmits enough torque to bring the roller shell up to speed. With almost no differential between the tendency roller bearing’s inner and outer races, the torque required by the web to make slight changes in speed from the shaft’s baseline rpm is minimal and accomplished with the low traction available.
  5. Local tension change – If you want to change the tension from low to high or high to low on one roller, simply connect a clutch or brake to the roller and ensure there is sufficient web-roller traction to prevent slippage.
  6. Combination winding – I think of combination winding as surface winding with a center torque assist. Since there is no space to measure tension between the nipping surface drive roller and the winding roll and the roll diameter and circumference are constantly changing, open-loop torque control is the obvious way to assist the surface winder in creating roll tightness.
  7. Nipping rollers – In most nipped roller systems, one roller is driven (often steel roller) and one roller is idling, driven by through the web or through contact between the rollers outside the web’s width. In some cases, a thicker web prevents contact outside the web width and the torque transmission through the web causes damage or defects, the logical solution is to add a clutch and motor to the nipping roller to provide the turning torque to overcome drag and rolling resistance independent of the web.