What are the options for creating and controlling tension in a zone?

Zone-by-zone, we must select what to control, speed or torque, and whether to run in open-loop or closed-loop mode. Lastly, if we close the tension control loop, which feedback device will be used, a dancer roller or a tension transducer roller.

For each zone, there are two or three decisions:

  1. How will tension be created/modified?
    Option 1A – By force (either directly or through torque)
    Option 1B – By speed ratio (a.k.a., draw control)
  2. Will the zone have closed-loop control?
    Option 2A – Open-loop control with no tension feedback
    Option 2B – Closed-loop control with tension feedback
  3. If Option 2B (closed-loop control) – Which feedback will be used?
    Option 3A – Load cell roller feedback
    Option 3B – Dancer roller feedback

What is open-loop tension control?

Open-loop control is sometimes called non-feedback control. Open-loop control may be as simple as setting an input variable to a fixed value, such as setting a brake at one condition, or it may be fairly intelligent, such as varying a rewind clutch based on calculated roll diameter and weight. However, in either case, open-loop control avoids the thing that seems most obvious, it never measures the variable you are trying to control, in this case, web tension.

When is open-loop tension control used?

Though in most cases I will advocate closed-loop tension control as your best choice, I can see a few reasons for the use of open-loop tension control.

  1. Open-loop systems are simple and usually less expensive than a closed-loop system.
  2. In some cases, it may be difficult or impossible to measure tension, so without feedback, by default, you must choose open-loop control.
  3. Your process or product is sensitive to or dependent on consistent control of speed, length, or a timed event.
  4. You need to speed match equipment to the web where there is limited or no traction.

When is closed-loop tension control used?

Closed-loop control, most commonly PID control, is the best way to ensure tension is at the value you desire and will react to tension upsets in a expedient manner.  Closed-loop tension control most commonly uses a dancer roller or load cell roller to sense changes in web tension and provide a signal to compare to a set point. If the measured value varies from the desired setpoint, the closed-loop system will adjust the controlled variable, usually speed or torque, to move the tension back towards the desired value.

A closed-loop tension control usually goes beyond simply measuring tension and adjusting the speed or torque with PID values tuned to the web properties and equipment characteristics; closed-loop tension control will usually include feed-forward to proactively adjust torque or speed to follow known speed changes (such as acceleration and deceleration) and inertia changes (such as changes in winding or unwinding roll diameter, roll material density, or roll width).