1. Overview
Tension control on a winder or unwinder requires the drive to operate as a torque source rather than a speed source. The motor must deliver just enough torque to keep the strip under a defined line tension while matching the line speed set by the upstream or downstream process section. The Siemens SINAMICS S120 with a CU310DP Control Unit is a common platform for this task because it combines a closed-loop speed controller, a closed-loop torque limiter, and the optional Drive Control Chart (DCC) environment for direct tension control loops.
This reference covers the two most common implementations:
- Speed-setpoint + extra-setpoint (open-loop torque mode with saturated speed controller).
- DCC winder technology block (closed-loop tension, diameter calculation, inertia compensation).
Both methods rely on the same physical torque/tension equations, so the mechanical model is presented first.
2. Tension Control Theory for Winders
A winder must hold a constant web tension F on the material between two process points. The torque the motor has to deliver is:
Tmotor = F × D/2 × (1/GR) × 1/η
Where:
- F = web tension in N
- D = current winding diameter in m
- GR = gearbox ratio (motor speed / winder speed)
- η = gearbox efficiency (typically 0.92 to 0.97 for helical gearboxes)
The corresponding winder speed for a given line speed v:
nwinder = v / (π × D) [rev/s] → nwinder = 60 × v / (π × D) [rpm]
The motor speed is then nmotor = nwinder × GR. As D increases from core to full roll, the winder speed falls (typically in the ratio Dmax/Dmin, e.g. 6:1) while the required torque rises by the same ratio. A torque-controlled drive handles this naturally; a speed-controlled drive cannot.
3. Mechanical Preliminaries
3.1 Gearbox Ratio (GR)
The gearbox ratio is usually stamped on the gearbox nameplate (e.g. "i = 24.5"). If it is not, count the teeth on the input and output pinion: GR = Zoutput / Zinput. You can also lock the output shaft and rotate the input a measured number of turns to count the output turns. The ratio is then GR = Ninput / Noutput.
3.2 Reflected Inertia
The winder inertia reflected to the motor shaft is:
Jtotal = Jmotor + (Jcore + Jroll) / GR²
The variable term Jroll grows with the square of the diameter. This is the dominant disturbance for tension control during acceleration ramps and is normally compensated by the DCC block Inertia_compensation.
3.3 Diameter Estimation
Diameter can be calculated by line speed / winder speed, by an ultrasonic sensor, or by a tensiometer + integrator. The DCC winder block provides all three options; the speed-ratio method is the cheapest but fails during strip slippage.
4. Torque and Tension Calculations
For a winding process the steady-state motor torque can be split into four parts:
| Component | Formula | Notes |
|---|---|---|
| Tension torque | T1 = F × D / (2 × GR × η) | Set value from PLC or DCC |
| Bending / elastic torque | T2 = E × b × t² × k / (2 × D × GR × η) | Significant for thick steel strip, often ignored for foil |
| Friction torque | T3 = μ × m × g × D / (2 × GR × ηtotal) | Bearing + air drag |
| Acceleration torque | T4 = Jtotal × α | Jtotal varies with D |
For a typical film/foil line, T1 dominates and the drive torque limit is set from the web tension target. For wire winding, T2 and T3 may be 30 to 50 percent of the total.
5. SINAMICS S120 Hardware for a Winder Drive
Typical topology for a CU310DP winder:
- CU310DP Control Unit — handles PROFIBUS to the PLC, encoder evaluation, drive I/O.
- Active Line Module (ALM) or Basic Line Module (BLM) — DC bus supply.
- Single Motor Module (Single Motor Module / Booksize) — power stage sized to the motor.
- Servo or induction motor with incremental/absolute encoder (preferred: DRIVE-CLiQ or HTL/TTL).
The CU310DP firmware is commissioned with SINAMICS STARTER (or Startdrive in TIA Portal). The required firmware version is documented in the CU310DP Operating Instructions; current production line is V5.x. Confirm the firmware version of your CU310DP (read out under Commissioning → Device Info in STARTER) before applying parameter sets.
6. Parameter Setup — Speed + Extra-Setpoint Method (Open Torque Mode)
This is the simplest technique. The drive remains in closed-loop speed control, but the speed setpoint is set slightly above what the line can actually demand, so the speed controller saturates and the drive regulates at the active torque limit.
- Enter motor data and run the automatic motor identification (p1910 = 1, p1960 = 1 for rotating measurement).
- Select the speed setpoint source: p1000 = 6 (fieldbus / PROFIBUS from the PLC).
- Set the upper and lower torque limits to the calculated maximum web tension torque. Allow ±10 percent headroom:
| Parameter | Address | Function | Example |
|---|---|---|---|
| p1520 | Torque limit upper, motoring | Upper torque limit (Nm) | Set to Tmax,Fmax + 10% |
| p1521 | Torque limit lower, regen | Lower (regen) torque limit (Nm) | Symmetric for simple winder |
| p1530 | Motor-mode torque limit | Power-stage specific ceiling | Auto from Motor Module rating |
| p1540 | Speed limit for torque control | Speed clamp when in torque mode | Set to 110% of max line speed |
| p0640 | Current limit | Implicit torque ceiling | Set below Motor Module rated I |
- Calculate the extra setpoint. The winder's no-load speed at a given line speed v and current diameter D is nwinder = 60·v/(π·D). The motor speed is nmotor = nwinder·GR. Add 5 to 10 percent to give the extra setpoint:
nset,motor = nline,motor × (1 + Δ), with Δ ≈ 0.05 to 0.10
- Wire the PLC to write the winder speed setpoint and a constant extra-percent into the relevant PZD words. On PROFIBUS the default telegram is Standard Telegram 5 (PZD-10/10), which contains two 16-bit torque setpoints (actuator side) and a speed setpoint. With Telegram 5, the second torque word can carry the lower torque limit override from the PLC if needed.
- On strip break, freeze the extra setpoint at zero. The drive reverts to the true line speed, the speed controller leaves saturation, and the winder holds (does not overspeed).
7. Parameter Setup — Closed-Loop Torque Mode (p1300 = 22)
If the line is delivered with a master speed reference and the winder must track it exactly without saturation drift, switch the control mode to torque control with speed limit:
- p1300 = 22 (torque control with speed limit) or 23 (torque control without speed limit, not recommended for winders).
- p1500 = 6 (torque setpoint via PROFIBUS).
- Torque setpoint is written directly as a 16-bit value in the PZD (with Telegram 5, the second PZD word is the torque setpoint; scaling: ±100% → ±2·r0331, with r0331 = rated motor torque).
- Enable the speed-limit p1540 to protect the drive on strip break.
Convert the desired web tension to motor torque in the PLC:
Tset = F × D / (2 × GR × η)
Then scale to 4000 hex = 100% of the rated motor torque:
PZD = round(Tset / r0331 × 16384)
8. DCC Winder Application
Siemens provides a ready-made DCC library for winders on the SINAMICS Application Examples page. The "DCC Winder" project ships with:
- Winder_diameter — line-speed / winder-speed ratio diameter calculator with start-up convergence.
- Tension_controller — PI controller on web tension (input from tensiometer or dancer).
- Moment_of_inertia_compensation — adds the acceleration torque T = J·α, with J updated from the current diameter.
- Tapered_tension — optional output that reduces the tension reference linearly with diameter to prevent roll deformation.
Install procedure:
- Insert the DCC winder blocks into the drive topology in STARTER (right-click drive → DCC → Insert block).
- Connect the line speed reference (r2050 inputs or r0898 drive state) and the tensiometer / dancer feedback to the inputs of the tension controller.
- Set p5060 = 1 to enable DCC execution.
- Configure p50075 to select the winder macro and p50076 to choose closed-loop / open-loop torque mode.
- Compile and download to the drive.
9. PID Tuning for a Tension Loop
When the tension is measured with a load cell (tensiometer) and the drive runs in torque-control with torque setpoint from the PLC, the tension control loop is closed in the PLC. The plant is approximately an integrator with a small time delay from the speed-loop response: tdelay ≈ 1.5 × Tn,speed.
Recommended starting points on the PLC PID (e.g. S7-300/400 FB41 "CONT_C" or S7-1200/1500 PID_Compact):
| Term | Value | Rationale |
|---|---|---|
| Gain Kp | 0.3 to 1.0 (unitless on normalized setpoint/feedback) | Conservative start |
| Integral time Ti | 200 to 800 ms | Proportional to mechanical delay |
| Derivative time Td | 0 | Web tension is noisy; leave at 0 |
| Output limits | ±Tmax,motor (scaled to PZD range) | Match p1520/p1521 |
Auto-tuning tip: open the loop, give a 5 percent torque step, log the tension response. Set Kp to 0.5 / (steady-state error slope), then add integral until you see a 1.5 to 2 percent overshoot on a tension step. With dancer position control the loop is a pure integrator and a pure I-controller (Kp = 0, Ti = 100 to 300 ms) is often sufficient.
10. Strip Break Protection
A common failure mode of an open-loop winder is the runaway roll when the strip breaks. With the speed+extra-setpoint method, the extra setpoint becomes the actual speed reference; the drive overspeeds and the speed-limit p1540 must catch it. With closed-loop torque, p1540 provides a hard ceiling.
Recommended wiring in the PLC:
- Monitor tensiometer (or dancer position) and line speed.
- If tension falls below Fmin for > 200 ms, latch a "strip break" flag.
- On strip break, ramp the torque setpoint to zero in 50 ms and disable the run enable to the drive.
- Configure drive fault F07903 (speed limit reached) to latch in stop mode so the operator can acknowledge.
Refer to the CU310DP List Manual for the full fault/alarm list (F07900, F07901, F07902, F07903, F07904, F08501).
11. Commissioning Sequence
- Verify wiring: encoder at X23, motor at the power module, PROFIBUS at X126, line speed reference into PZD1 (Standard Telegram 5), torque setpoint into PZD3.
- Configure motor in STARTER, perform motor identification (p1910/p1960).
- Set p0640 (current limit) to the Motor Module's rated current.
- Set p1520/p1521 to ±1.1 × Tmax.
- Set p1540 to 1.10 × maximum motor speed at empty core.
- Jog the winder unloaded, confirm direction with p1820 if needed.
- Thread the strip, run with a low tension setpoint, ramp the line speed slowly.
- Log tension, current, torque, diameter; tune the tension PI.
- Test strip-break protection by stopping the line while the winder still runs.
- Save the project to the CF card of the CU310DP.
12. Verification Checklist
| Check | Expected result |
|---|---|
| Unloaded run, no tension | Torque < friction torque, speed tracks setpoint |
| Constant line speed, constant tension | r0080 ≈ Tset, n-act tracks n-set closely |
| Step change in line speed | Tension transient < 10% of setpoint, settles < 500 ms |
| Strip break simulation | Speed clamps to p1540, torque drops, F07903 latches, drive stops |
| Full roll (Dmax) to core (Dmin) | Diameter calculation converges, no oscillation |
13. Troubleshooting Matrix
| Symptom | Likely cause | Action |
|---|---|---|
| Winder runs at line speed, not tension | Speed controller not saturated, extra setpoint too low | Increase Δ; check p1540 not clipping |
| Tension oscillates heavily | Tension PID gain too high or speed loop too slow | Reduce Kp; tune speed loop Kp/Tn |
| Tension drifts at steady state | Friction term missing or diameter stale | Add friction offset; reset diameter; check encoder |
| F07901 "Motor blocked" on ramp | Torque limit below required acceleration torque | Raise p1520 or p1521; check p0640 |
| F07903 "Speed limit reached" on strip break | Expected; verify p1540 setting and reaction | Lower p1540 to 105-110% of nmax,empty |
| Winder accelerates on strip break | Drive in speed mode, not torque-limited | Verify p1300=22 or torque-limit wiring |
| Tension is correct but current very high at core | Diameter stuck at Dmax | Check diameter estimator; reset to Dmin at start |
| Alarm F08501 | PROFIBUS sign-of-life timeout | Check PLC cycle, telegram configuration |
14. Field-Proven Caveats
- The mechanical ratio GR must be measured or read from the nameplate — errors propagate directly into tension error. Sanity-check: at the empty core, the motor speed should be nline,core · GR.
- For dancer position control, the tension loop is purely integrating; a P-only controller with high gain gives a stiffer dancer than a slow I-controller. Tune by stiffness, not by time-domain step response.
- When the DCC winder block is loaded, the drive uses the same control structure under the hood. Do not duplicate the speed controller by enabling both the DCC tension PI and the standard closed-loop speed control in torque mode — the loops will fight each other.
- Always store the parameter set on the CU310DP CF card (Save to memory card in STARTER). The drive will autoload on power-up even after fieldbus loss.
- For hazardous area or e-stop integration, wire the Safe Torque Off (STO) inputs on the CU310DP (X131). The winder must be brought to a controlled stop on E-stop, not a hard stop — otherwise the web can snap.
15. Quick-Reference Parameter Sheet (CU310DP, S120, FW V5.2)
| Parameter | Description | Typical winder value |
|---|---|---|
| p1300 | Open/closed-loop control mode | 22 (torque control with speed limit) |
| p1500 | Torque setpoint source | 6 (fieldbus) |
| p1520 | Torque limit upper | +1.1 × Tmax,web |
| p1521 | Torque limit lower | -1.1 × Tmax,web |
| p1530 | Motor-mode torque limit | Auto / Motor Module peak |
| p1540 | Speed limit for torque control | 1.10 × nmax,empty core |
| p0640 | Current limit | Motor Module rated I |
| p1910/p1960 | Motor identification | 1 / 1 |
| p5060 | DCC enable | 1 (when DCC chart is in use) |
| p50075/p50076 | Winder macro select | Winder macro and mode bits |
What parameter sets the torque limit on a SINAMICS S120 with CU310DP?
Use p1520 (upper, motoring) and p1521 (lower, regenerative) in Nm. Set both to ±1.1 × the maximum web-tension torque computed from F × D / (2 × GR × η). p1530 is the motor-mode ceiling enforced by the power stage, and p0640 limits current which indirectly caps torque.
How do I make a winder operate in torque mode without runaway on strip break?
Switch p1300 to 22 (torque control with speed limit) or keep speed mode and add an extra setpoint to saturate the speed controller. In both cases set p1540 to 105-110% of the motor speed at empty core / maximum line speed. On strip break, ramp the torque setpoint to zero in the PLC and latch the drive via the run enable.
Do I need a DCC engineering license for the winder application?
No. The SINAMICS DCC Winder macro ships as a free-of-charge technology block on the SINAMICS Application Examples page. You can configure its inputs, parameters, and interconnections without a DCC authoring license. You only need a DCC engineering license if you want to add new DCC blocks or change the chart's internal structure.
How is the winder torque calculated from web tension?
The motor torque required to hold a web tension F at a current winding diameter D is T = F × D / (2 × GR × η). For a tension of 200 N, D = 0.30 m, GR = 12, η = 0.95 the motor torque is 200 × 0.30 / (2 × 12 × 0.95) = 2.63 Nm. Add friction, bending, and acceleration terms for the full torque limit.
Can I close the tension loop in the PLC instead of in DCC?
Yes. Read the tensiometer signal into the PLC, run a PI controller (e.g. FB41 CONT_C on S7-300/400 or PID_Compact on S7-1200/1500), and write the output as a torque setpoint via PROFIBUS Standard Telegram 5. Keep Kp small (0.3-1.0) and Ti in the 200-800 ms range; the speed loop inside the drive provides the fast inner regulation.
What causes F07901 "Motor blocked" on a winder ramp?
It usually means the torque limit is below the torque required to accelerate the roll. Either raise p1520, or check that the inertia compensation (DCC block or PLC feedforward) is enabled and uses the correct current diameter. Also verify p0640 is not clipping the current below what the Motor Module can deliver at low speed.
How is the gearbox ratio (GR) measured if it is not on the nameplate?
Lock the output shaft, rotate the input shaft exactly N input turns, and count the output turns. The ratio is GR = N_input / N_output. Alternatively count teeth on the pinions: GR = Z_output / Z_input. The ratio belongs in the torque formula as 1/GR and in the speed formula as a multiplier (motor speed = winder speed × GR).