How to Build a Stable Tension Control System with Two Servo Drives
Designing a tension control system with two servo drives is a common challenge in web handling applications like printing, coating, laminating, and slitting. The goal is to maintain a constant, stable tension on the material as it moves from one roll to another, even when external disturbances—like a nip roller engaging—are introduced. Many engineers start with a simple torque follower or speed synchronization approach, but achieving truly smooth and wrinkle-free operation requires a deeper understanding of closed-loop tension control principles.
A typical setup uses two servo motors: one acting as the unwind or infeed (master), and the other as the rewind or outfeed (slave). The slave motor is often tasked with maintaining tension by adjusting its torque or speed based on feedback from a tension sensor or dancer roll. When a nip roller presses down, the dynamics change—friction, inertia, and material properties shift, often causing oscillations or slack. Let’s break down how to build a robust system.
Understanding Tension Control Fundamentals
Tension is the force applied to a continuous web of material. In a moving web, tension is proportional to the difference in speed between two adjacent rollers and the modulus of elasticity of the material. The basic formula is:
Tension (N) = (E × A × ΔV) / V
Where E is the material’s elastic modulus, A is cross-sectional area, ΔV is the speed difference, and V is the line speed. This shows that even a tiny speed mismatch can create large tension variations, especially with stiff materials.
There are two primary control methods:
- Open-loop torque control: The slave motor applies a constant torque proportional to desired tension. Simple but sensitive to inertia changes and friction.
- Closed-loop tension control: A tension sensor (load cell) or dancer position sensor provides feedback to a PID controller, which adjusts motor torque or speed in real time. This is far more accurate and stable.
Why Your System Fails When the Nip Roller Engages
In your description, the system runs smoothly until a “rubber wheel” (nip roller) presses down. This is a classic disturbance. The nip roller adds friction, changes the web path, and may introduce an additional driven or idle roller that alters the tension zone. If your control strategy is purely speed-based without tension feedback, the slave motor cannot compensate for the sudden load change. The result is either slack or over-tension, leading to wrinkles or breakage.
Common causes include:
- Insufficient PID tuning: The tension loop may be too slow or too aggressive, causing oscillation when the nip engages.
- Lack of inertia compensation: The slave motor must account for the changing inertia of the roll diameters.
- No dancer or accumulator: A mechanical buffer can absorb sudden tension spikes.
- Incorrect torque feed-forward: Without a proper feed-forward signal based on line speed and roll diameter, the PID has to work too hard.
Step-by-Step Design for a Stable Tension Control System
1. Choose the Right Control Architecture
For two servo motors, the most reliable setup is a closed-loop tension controller with a load cell or dancer roll. The load cell measures actual web tension and sends a 0–10V or 4–20mA signal to the drive or PLC. The slave drive then runs in torque mode with a speed limit, or in speed mode with a tension trim.
A typical configuration with Panasonic Minas A6 or similar servos:
- Master drive: speed control, line speed reference.
- Slave drive: torque control with speed limit, tension setpoint from HMI or analog input.
- Tension feedback: load cell amplifier connected to slave drive’s analog input.
2. Implement PID Tension Loop Tuning
Start with a low proportional gain (Kp) and no integral or derivative. Increase Kp until the system responds quickly but without overshoot. Then add integral gain (Ki) to eliminate steady-state error. Derivative (Kd) can help dampen oscillations but is sensitive to noise. A common starting point for web tension is Kp=0.5, Ki=0.1, Kd=0.01, but this varies widely.
Use the drive’s auto-tuning function if available, but always fine-tune manually while observing the tension signal on an oscilloscope or trend chart.
3. Add Dancer Roll or Accumulator
A dancer roll is a spring-loaded or pneumatically loaded idler roll that moves with tension changes. Its position is measured by a potentiometer or linear sensor. The slave drive adjusts speed to keep the dancer at mid-position. This mechanical buffer greatly improves stability, especially during nip engagement. It also allows for slight speed mismatches without breaking the web.
4. Compensate for Roll Diameter Changes
In winding applications, the roll diameter changes continuously. The torque required to maintain constant tension is proportional to diameter. Use a diameter calculator function in the drive or PLC: measure line speed and motor RPM to compute diameter, then adjust torque feed-forward accordingly. Without this, tension will drift as the roll builds up.
5. Handle Nip Roller Engagement with Feed-Forward
When the nip roller presses down, the friction and inertia change instantly. A feed-forward signal can be added to the torque command based on a digital input from the nip mechanism. For example, when the nip closes, add a preset torque offset (e.g., +5% of rated torque) to compensate for the additional drag. This reduces the burden on the PID loop and prevents a tension dip.
Practical Tuning Procedure for Your Panasonic Servos
Assuming you are using Panasonic A6 or similar drives with built-in tension control functions:
- Set the slave drive to torque control mode (Pr0.01 = 2 for torque control).
- Connect the tension sensor signal to an analog input (e.g., AI1) and scale it so that 0–10V corresponds to 0–100% of rated tension.
- Enable the tension PID function in the drive parameters (often Pr6.00 series). Set the tension setpoint source (analog or internal).
- Configure the speed limit: the drive must not exceed a maximum speed even if tension is lost. Set a speed limit value (e.g., 110% of line speed) in Pr6.06.
- Start with low gains and run the line at low speed. Gradually increase Kp until the tension signal becomes stable but responsive.
- Introduce the nip roller disturbance and observe the tension response. Adjust Ki to recover steady state faster, and Kd if oscillation occurs.
- If oscillation persists, check the mechanical setup: misalignment, worn bearings, or an out-of-round nip roller can cause periodic disturbances.
Advanced Techniques for High-Performance Tension Control
For demanding applications like thin films or elastic materials, consider these enhancements:
- Dual-loop control: Inner current/torque loop and outer tension loop, with a speed observer for better disturbance rejection.
- Adaptive gain scheduling: PID gains change based on line speed or roll diameter to maintain consistent performance.
- Load cell signal filtering: Use a low-pass filter to remove noise from the tension signal without introducing too much phase lag.
- Friction compensation: A static friction model can be added to the torque command to overcome stiction, especially at low speeds.
Common Pitfalls and How to Avoid Them
| Problem | Likely Cause | Solution |
|---|---|---|
| Tension oscillates after nip engagement | PID gains too high or mechanical resonance | Reduce Kp, add low-pass filter, check nip roller balance |
| Web goes slack when nip closes | Insufficient torque feed-forward or slow PID response | Add feed-forward offset, increase Ki, use dancer |
| Tension drifts over time | No diameter compensation or load cell drift | Implement diameter calculator, calibrate load cell |
| Motor overheats in torque mode | Continuous high torque at low speed without proper cooling | Check motor sizing, add external fan, reduce tension setpoint if possible |
Real-World Example: Converting Line with Nip Roller
A typical slitting machine uses an unwind servo, a nip roller driven by a separate motor or mechanically linked, and a rewind servo. The tension zone between the nip and rewind is critical. By installing a 50N load cell and a dancer roll with a 100mm stroke, the system can maintain ±2% tension accuracy even when the nip engages at 300 m/min. The slave drive runs in torque mode with a speed limit of 105% of line speed. PID parameters: Kp=0.8, Ki=0.15, Kd=0.02. A feed-forward torque of 0.3 Nm is added when the nip solenoid is activated, eliminating the tension dip.
Conclusion
Building a stable tension control system with two servo drives is achievable by moving from a simple speed-follower approach to a closed-loop tension controller with proper PID tuning, dancer roll integration, and feed-forward compensation. The key is to treat the nip roller engagement as a known disturbance and proactively adjust the torque command. With the right setup, even standard servos like Panasonic can deliver excellent tension regulation in demanding web handling applications.
