Winding is the discipline where drives stop being speed knobs and become force instruments. The physics is compact: tension × radius = torque. Everything else is compensation terms.
The control structure
A center winder in open-loop tension mode runs the motor in torque control with a speed limit. The torque setpoint is tension demand × current radius; radius comes not from a sensor but from arithmetic — line speed divided by winder angular speed, filtered sanely. Around that core:
- Inertia compensation: during acceleration, torque must also spin up the growing roll (J grows with r⁴ — the term dominates on heavy coils). Without it, tension sags on every speed change.
- Friction compensation: measured once at commissioning across speed, injected as a feedforward.
- Taper tension: deliberately reducing tension with diameter so outer wraps do not crush inner ones — a recipe parameter, product-dependent.
The speed limiter runs slightly above line speed on a rewinder: if the web breaks, the drive accelerates only to the limit instead of running away. That one detail separates tidy web breaks from wrapped-around-the-roll disasters.
Commissioning order that works
Friction ID first (empty core, sweep speeds), inertia estimate second (known core, controlled accel), then diameter calculation sanity (compare computed vs tape measure through a roll), then closed product runs tuning taper. Document per-product recipes; “the winder is fussy” is usually an untuned recipe, not a drive problem.
FAQ
Dancer or load cell instead? Closed-loop via dancer/load cell adds accuracy and forgiveness at the cost of mechanics — thin or delicate webs usually deserve it; heavy steel strip often runs beautifully open-loop.
Why does tension dip at speed changes only when the roll is full? Inertia compensation missing or under-scaled — J at full roll is orders beyond core.
Zone Otomasyon commissions winders on slitting and processing lines — it is home turf. See the Galva Makina project.