Separate Prestretch Control from Dancer Control
The two roller speeds set film prestretch; the dancer absorbs transient film demand and indicates web tension. Do not treat dancer position as the prestretch setting. Assuming the downstream roller pulls film faster than the upstream roller, calculate prestretch as Prestretch (%) = ((v2 - v1) / v1) × 100, where v1 and v2 are roller surface speeds. For rollers with different diameters, use v = pi × D × n; matching motor frequency alone will not establish the required surface-speed ratio.
The slower roller must produce restraining torque while the faster roller pulls the film. Before selecting the control method, verify whether motor, gearbox, friction, and drive arrangement can sustain that condition without requiring unsupported regenerative or bidirectional power operation.
Compare the PLC-Free Architectures
| Architecture | Prestretch control | Dancer function | Engineering decision |
|---|---|---|---|
| Mechanical gearing and one inverter | Fixed by gear and roller ratios | Buffers film demand | Provides deterministic synchronization but requires a mechanical change. |
| Two inverters with analog cascading | First drive reference is scaled from the second drive signal | Can command the master roller speed | Works only if the available analog output represents the required speed reference and both analog mappings can be calibrated. |
| Dancer connected directly to speed reference | Maintained by the roller-speed ratio | Directly trims speed | Simple, but dancer gain and travel limits must prevent excessive speed changes and oscillation. |
| Dancer feedback through internal PID | Maintained by the roller-speed ratio | Regulates dancer position around a setpoint | Use only if the selected drive documents the required PID, reference, feedback, scaling, and output-limiting functions. |
Configure and Commission the Two-Drive Scheme
- Confirm from the 3G3MV documentation that the installed units provide the required analog input, a suitable analog output, scaling or inversion functions, and any proposed PID function. The evidence does not identify terminals or parameters.
- Measure both roller diameters and record the gearbox ratios. Calculate the commanded speed ratio from roller surface speeds, not motor speeds alone.
- Select the downstream roller as the line-speed reference and scale the upstream reference to produce the required prestretch ratio. Confirm that minimum and maximum commands remain inside the usable speed range of both drives.
- Assign dancer position to line-speed trim or PID feedback. Establish the correct response direction by moving the dancer manually at zero web speed. Use documented drive scaling or inversion; do not reverse a potentiometer supply unless the analog circuit documentation explicitly permits it.
- Begin with restrained speed and film installed. Increase line speed while checking prestretch, dancer travel, oscillation, film breakage, motor current, and drive faults.
- Apply rapid film-demand changes and verify that the dancer returns toward its operating position without reaching either travel limit. Also verify that the slower roller can maintain restraining torque throughout the operating range.
Choose Between Direct Trim and PID
Use direct dancer-to-speed trim when the dancer mechanism and film elasticity provide adequate damping. A potentiometer connected to a speed reference does not inherently create integral control; it creates proportional speed correction unless the drive is configured to perform integration or PID regulation.
Use PID only when direct trim cannot hold the dancer within its usable travel without sustained offset or excessive oscillation. The supplied evidence contains no 3G3MV terminal assignments, PID parameter identifiers, ratings, or tuning values, so exact wiring and parameter settings must come from the manual for the installed drive variant. If repeatable prestretch is more important than electronic adjustability, mechanically coupling the rollers fixes their ratio and reduces the synchronization problem to one inverter.
FAQ
How do I calculate film prestretch from two roller speeds?
Assuming the second roller is faster, use ((v2 - v1) / v1) × 100. Calculate each surface speed from roller diameter and rotational speed before applying the formula.
Can a dancer potentiometer connect directly to an inverter speed input?
Yes, if the drive accepts the potentiometer signal and provides the necessary scaling and response direction. Limit the commanded speed range and verify dancer response during commissioning.
Does direct dancer feedback require PID control?
Not necessarily. Start with direct speed trim when the mechanism is adequately damped; use PID only when the drive supports it and direct trim cannot maintain stable dancer position.