DP to Fiber Optic: Tension Control Accuracy from ±5% to ±1.2%
In a steel rolling mill, achieving precise tension control is critical for product quality. This article explores how a DP to fiber optic conversion solved communication issues, improving accuracy from ±5% to ±1.2%.
The Challenge: Unstable Profibus Communication
A metallurgical plant specializing in high-strength structural steel and alloy plates faced significant production challenges. The core rolling workshop, responsible for 80% of the plant’s output, relied on a control architecture consisting of a central Siemens S7-400 PLC, eight four-high reversing rolling mills, and six infrared temperature sensors. The PLC needed to receive real-time data from each mill’s tension controller—including rolling force (1000-3000 kN), roll gap position, and roll surface temperature (800-1000°C)—to dynamically adjust parameters like reduction and rolling speed, maintaining thickness tolerance within ±0.1 mm.
The workshop’s distributed layout, with the control room up to 150 meters from the farthest mill, made traditional copper-based Profibus cables problematic. Two major issues plagued the system:
- Signal Attenuation: Over 150 meters, signal integrity loss exceeded 30%, causing delayed or lost control commands and impairing real-time control.
- Electromagnetic Interference (EMI): High-power mill startups generated transient currents, and VFDs emitted high-frequency radiation (50-200 MHz), disrupting differential signals and causing frequent bus communication dropouts.
These issues led to tension control deviations exceeding ±5%, resulting in localized thickness errors (as much as -0.5 mm or +0.6 mm) that required costly rework. Temperature data acquisition delays over 100 ms prevented timely cooling adjustments, causing roll overheating, cracks, and surface defects, keeping product qualification rates stubbornly low.
Key Problem: Traditional Profibus cables couldn’t handle the distance and EMI, leading to poor tension control and high scrap rates.
The Solution: DP to Fiber Optic Conversion
After evaluating options, the engineering team implemented a DP to fiber optic solution using industrial-grade Profibus-to-fiber converters. Fiber optics offer inherent immunity to EMI and extremely low signal loss—less than 0.1% over 150 meters with single-mode fiber, compared to 30% with copper cables.
The deployment was tailored to the workshop’s layout:
- Topology: A central converter with dual optical ports (supporting cascading and four fiber branches) was installed near the PLC. Remote converters (single optical port) were placed in control cabinets for each group of two mills and one or two temperature sensors, creating a full fiber optic star/tree topology.
- Communication Settings: All converters were set to 12 Mbps (selectable 6/12 Mbps) to handle the high data rates—tension controllers transmit 10 times per second, temperature sensors 5 times per second. The built-in DP bus signal enhancement feature compensated for any residual loss, ensuring zero-latency data exchange.
- Rugged Design: The converters featured corrugated aluminum housings with 3x the impact resistance of plastic, withstanding vibrations from 5-50 Hz. Redundant power inputs (18-36 VDC) with isolation protected against voltage fluctuations (15-30 V) during mill startups. The wide input range allowed direct connection to the existing 24 VDC system, enabling installation in just three days.
| Parameter | Before (Copper) | After (Fiber) |
|---|---|---|
| Signal Attenuation | >30% | <0.5% |
| Daily Communication Interruptions | 3-4 times | 0 |
| Tension Control Accuracy | ±5% | ±1.2% |
| Temperature Data Delay | >100 ms | <30 ms |
| Product Qualification Rate | 92% | 98.5% |
Results: Precision and Reliability Restored
The fiber optic upgrade delivered immediate and dramatic improvements:
- Stable Communication: Signal attenuation dropped below 0.5%, with zero data packet loss. The bus operated interruption-free, synchronizing PLC commands perfectly with field devices.
- Enhanced Control Accuracy: Tension control deviation narrowed to ±1.2%, ensuring consistent plate thickness within ±0.1 mm. Real-time temperature data allowed precise cooling control, eliminating roll overheating and extending roll life by 30%.
- Reduced Maintenance: The robust fiber links eliminated the need for daily cable inspections (previously 2 hours/day), cutting maintenance time by over 60%. Built-in relay alarms for fiber breaks enabled fault location within 15 minutes, down from 2 hours.
- Extreme Environment Resilience: The converters operated reliably in ambient temperatures from -10°C to 45°C and were unaffected by EMI from adjacent motor testing, proving their suitability for harsh industrial settings.
Bottom Line: The DP to fiber optic conversion boosted product qualification from 92% to 98.5%, delivering significant cost savings and paving the way for future digitalization.
Why DP to Fiber Optic is Ideal for Heavy Industry
This case highlights the advantages of fiber optic communication in electrically noisy, long-distance industrial environments. Unlike copper, fiber is immune to EMI, has much lower attenuation, and provides galvanic isolation, protecting equipment from ground loops and surges. Modern Profibus-to-fiber converters are designed for harsh conditions, with wide temperature ranges, vibration resistance, and redundant power inputs, making them a drop-in upgrade for legacy systems.
For plants looking to improve control system performance, migrating critical bus segments to fiber is a proven strategy. It not only solves immediate reliability issues but also lays the foundation for advanced automation and IoT integration, where high-speed, deterministic data transfer is essential.
Frequently Asked Questions
Q: What is a DP to fiber optic converter?
A: It’s a device that converts Profibus-DP electrical signals to optical signals for transmission over fiber optic cables, and vice versa. It extends communication distance and provides EMI immunity.
Q: Can I use multimode fiber instead of single-mode?
A: Yes, but single-mode fiber is recommended for distances over 2 km or in high-EMI environments. For 150 meters, both work, but single-mode offers lower loss and higher bandwidth potential.
Q: How difficult is it to retrofit an existing Profibus network with fiber?
A: It’s relatively straightforward. Converters are typically plug-and-play, requiring no changes to the PLC program. The main task is running the fiber cable and connecting the converters to the existing Profibus connectors.
