Resources / Open-Loop vs Closed-Loop Tension Control
Open-loop tension control calculates a command without directly measuring the controlled web tension. Closed-loop control uses load-cell or dancer feedback to compare the process with a target and adjust the command. The appropriate method depends on the machine zone, disturbances, drive capability, material and commissioning requirements.
The key distinction is whether the control loop receives a measured process value, and whether that feedback represents web tension directly or dancer position.
The labels describe the feedback architecture, not one universal level of quality. An open-loop or indirect method calculates the required command from known machine variables. A closed-loop method measures a process value and uses the difference from its target to adjust torque or speed.[1]
No direct web-tension measurement is used to close the loop. In a centre-driven winder, the tension setpoint can be converted into a torque value using roll diameter and gear ratio, with friction and acceleration compensation affecting the result.[1]
A load cell directly measures web tension. The controller compares that actual value with the setpoint and can adapt a torque limit or an additional speed command.[1]
A dancer system normally closes the loop on dancer position rather than directly on web tension. Dancer movement provides feedback, while the mechanical assembly can also buffer brief changes in web length.[1]
In an indirect torque-based method, the system calculates a torque command instead of measuring the controlled web tension. Roll diameter and transmission ratio are core inputs because the torque needed for a target tension changes with roll radius. Friction and acceleration torque also need suitable compensation for the real machine.[1]
This architecture can avoid a dedicated tension-measurement system, but it shifts more responsibility to the accuracy of the diameter signal, machine model and compensation values. It cannot directly confirm that the actual web tension equals the target.[1]
With load-cell feedback, the measured web tension becomes the controller's actual value. This allows the controller to respond when the measured value moves away from the target, including when open-loop torque accuracy is insufficient or disturbances enter the controlled zone.[1]
A dancer loop solves a related but different problem. It measures dancer position and adjusts speed or torque to maintain the position target. The dancer force and geometry contribute to the resulting web tension, and its travel can absorb short disturbances in web length.[1]
Rockwell Automation's centre-driven winder example uses torque control with a load cell or dancer as the tension-feedback device. The same example states that the drive must support torque control and that surface-driven applications require adaptation, illustrating why a feedback sensor alone does not define a complete solution.[2]
Published roll-to-roll research models web tension and velocity as a nonlinear dynamic system affected by disturbances. That does not prove that every line needs closed-loop control, but it explains why selection must consider transients and coupling instead of only the steady production speed.[3]
Frequent changes in roll diameter, acceleration, material, friction or downstream load increase the importance of checking how the chosen method detects or compensates for those changes.[1][3]
A centre-driven roll, sectional drive and surface-driven roll do not share the same control assumptions. Tension-zone boundaries, available drive modes and web-path mechanics must be confirmed first.[1][2]
Load cells need suitable roller geometry, mounting and signal conditioning. A dancer adds moving mechanics, position sensing and travel that must fit the web path.[1]
Open-loop performance depends on diameter and compensation settings. Closed-loop performance also depends on feedback quality, control direction, scaling and tuning. Both methods require machine-level verification.[1]
A machine can use different methods in different zones. For example, the unwind, process and rewind sections may have different feedback needs, so the complete web path should be divided into real mechanical control zones before the architecture is chosen.
KRD treats the following items as an engineering scoping checklist. They help define the application but do not replace on-machine testing, safety review or commissioning.
For product selection, share this information with KRD together with a web-path drawing, control-panel details and clear photos of the proposed sensor and actuator locations. KRD can then narrow the controller, load-cell, dancer, brake, clutch or drive-interface options for engineering review.
No. Closed-loop control adds measured feedback, but it also requires suitable sensing, mechanics, signal quality and tuning. A well-commissioned open-loop method may be appropriate when the process is predictable and its verified tension variation is acceptable.
No. A closed loop may use direct load-cell tension feedback or dancer-position feedback. The two methods regulate different measured values and have different mechanical requirements.
Yes, an indirect winder method can adapt its torque command using a calculated or measured diameter. Its result still depends on the accuracy of the diameter value, gearing and compensation for friction and acceleration.
Send the web-path drawing, material and width, target tension, speed range, core and maximum roll diameters, roll mass, existing drive or brake information, available feedback signals, mounting space and a description of when the problem occurs.
Share the machine zone, material, speed, roll dimensions, existing actuator or drive, available feedback and expected acceptance criteria. KRD can help narrow open-loop, load-cell and dancer-control options for further engineering review.
