As the wound-roll radius grows, shaft speed must fall to hold web speed because v = ωr. Control line speed with web feedback or a suitable drive function, and regulate winding tension separately with a dancer or tension feedback; an encoder on the web can provide the speed measurement. A constant-torque command alone does not maintain constant web tension across changing roll diameters.
Web speed falls at the shaft as roll diameter grows
For a target linear web speed, the winder’s required angular speed is inversely proportional to roll radius: ω = v/r. The shaft turns faster with a small core and progressively slower as material builds up. A fixed motor-speed command therefore cannot keep web speed steady through the roll.
The controller needs a usable signal for actual line speed, roll diameter, or both. A web-mounted encoder measures material motion directly; a drive’s web-control function may instead estimate or manage the changing roll condition using internal feedback. The drive’s capability and feedback method determine whether external sensing is required.
Separate speed variation from tension variation
Web speed and web tension are related but are not the same control objective. Speed control keeps material moving at the commanded linear rate. Tension control manages the force pulling on the web as it enters the roll. A dancer provides a mechanical position signal and a buffer of web; its position can trim the drive speed command to correct tension-related slack or pull.
Without effective tension control, winding can produce sections that are tighter or looser than others. The described consequences include out-of-round rolls and web narrowing where tension is high. Adjusting speed from a dancer can help, but it should not be confused with direct measurement of web speed.
| Quantity | What it decides | Where to read or establish it |
|---|---|---|
Linear web speed, v
|
Whether the material moves at its setpoint | Web encoder feedback or a verified drive speed estimate |
Roll radius, r
|
Required shaft speed for a given web speed | Diameter sensor, calculated estimate, or drive web-control function |
Shaft speed, ω
|
Winder rotation rate | Motor/drive feedback and shaft measurement |
| Dancer position | Whether the dancer is correcting web accumulation or tension | Dancer sensor and its controller/PLC input |
| Web tension | Whether winding force is within the process requirement | Tension transducer or validated process measurement |
Torque must follow the tension target and roll radius
At the roll, web tension produces shaft torque through the roll radius: τ = F × r, where F is web tension and τ is torque at the roll. For a fixed tension target, required torque rises as radius increases. Conversely, holding torque constant while radius grows reduces the web force attributable to that torque. This is why a constant-torque description should not be taken as proof of constant tension; check the actual control mode and torque reference behavior.
Acceleration adds another load: the drive must accelerate the motor and the growing roll inertia in addition to producing process torque. A setup tuned only at steady speed may behave differently during starts, stops, or speed changes. Confirm the actual motor, drive, roll inertia, and required acceleration before selecting drive capacity or tuning the loop.
Choose feedback and drive architecture for the accuracy required
A web encoder feeding a PLC and VFD is a viable architecture when the PLC can execute the required speed and tension coordination and the drive accepts a stable command with suitable response. Closed-loop web feedback can improve accuracy and repeatability compared with relying only on an internal estimate. A vector drive with a dedicated web-control function may provide diameter-related speed adjustment and internal feedback, but capabilities vary by drive; verify the selected model’s control functions and feedback requirements.
Servo systems and DC motors with dedicated controllers are also used for rewind applications. The choice depends on the required speed accuracy, tension quality, dynamics, motor/drive matching, and integration resources. The application is more specialized than basic pump or conveyor speed control, so use a drive supplier or integrator with demonstrated winding experience when the process window is narrow or development time is limited.
Configure the speed and tension loops in sequence
- Define the required web-speed range, tension target and acceptable variation, roll/core dimensions, material properties, and acceleration/deceleration demands. These values determine whether the drive, motor, sensing, and mechanical arrangement can meet the process.
- Select the control arrangement: web encoder feedback for measured linear speed, a drive web-control function with confirmed feedback behavior, or a combination. Add a dancer or tension measurement where tension regulation is required.
- Wire and scale feedback signals, then verify direction and engineering units at low speed. Confirm that increasing physical web speed produces an increasing measured-speed value and that dancer position changes in the expected direction.
- Set the base speed command and confirm the winder tracks web speed with a small roll. Configure dancer-based speed trim or the drive’s documented tension/diameter functions according to the device manual; avoid treating a generic constant-torque setting as a tension loop.
- Test controlled speed changes and roll growth. Tune speed and tension responses separately, correcting sensor polarity, scaling, or loop behavior before increasing operating rate.
Verify the full roll range with measured data
Record commanded and measured web speed, shaft speed, roll diameter or estimated radius, dancer position, and tension if measured. At a fixed web-speed setpoint, shaft speed should decrease as the roll grows while linear speed remains within the process tolerance. Confirm that the dancer stays within its operating travel and does not steadily drift toward an end stop.
Compare winding quality near the core, at intermediate diameter, and near the maximum roll. Look for tight/loose bands, out-of-round geometry, and web narrowing associated with excessive tension. Also test start, stop, and commanded speed changes; a steady-state pass does not prove that transient tension remains acceptable. Set pass/fail limits from the material and process requirements rather than assuming a universal tolerance.
Correct recurring causes before increasing gain
| Observed condition | Likely control issue | Next check |
|---|---|---|
| Web speed drifts as roll diameter increases | Fixed RPM command, incorrect diameter compensation, or bad speed feedback scaling | Compare web encoder speed, shaft speed, and radius across roll growth |
| Web speed is steady but winding is uneven | Tension control is absent, poorly adjusted, or not acting on the drive command | Check dancer position/trim or tension feedback and inspect roll profile |
| Dancer runs toward a travel limit | Speed trim has wrong polarity, insufficient authority, or a persistent speed mismatch | Verify signal direction and compare commanded versus measured line speed |
| Performance changes during acceleration | Transient torque and roll-inertia demand differ from steady-state demand | Review acceleration demand, drive response, and tension trend during ramps |
Increasing loop gain before checking encoder scaling, sensor direction, and the control objective can amplify oscillation without correcting the underlying mismatch. Avoid raising tension to hide a speed-control problem; high tension can narrow or damage the web and degrade roll geometry.
Frequently asked questions
How do I keep web speed constant as the roll diameter changes?
Use ω = v/r: for a fixed linear web speed, reduce shaft speed as roll radius increases. Measure web speed with an encoder or use a drive web-control function whose feedback and diameter handling are verified.
How do I add a web encoder to a PLC and VFD?
Feed the encoder to a PLC or suitable drive input, scale its pulses to linear web speed, and use that measurement in the speed-control strategy. Verify signal direction and units at low speed before enabling production control.
How do I control winding tension with a dancer?
Use dancer position as feedback to trim the winder speed command, with correction polarity verified so the dancer returns toward its operating position. Tune the tension-related trim separately from line-speed tracking and keep the dancer within its travel range.
When should I stop tuning and call an integrator?
Stop if the dancer reaches a travel limit, tension or web speed becomes unstable, or the roll shows narrowing or distortion while control changes worsen the condition. Escalate to a drive supplier or integrator with demonstrated winding experience; provide measured web speed, shaft speed, roll diameter, dancer position, and tension data.