Siemens S7-1200 & ET 200SP Wireless Bridge Over 1km Tunnel
In many industrial environments, establishing reliable communication between a central controller and remote I/O stations can be a significant challenge, especially when physical obstacles like mountains, tunnels, or heavy machinery are present. This article explores a real-world case where a mining operation needed to connect a Siemens S7-1200 PLC located in a control room to multiple ET 200SP distributed I/O stations situated 2000 meters away in a crushing workshop. The path was blocked by a mountain and a material transport tunnel, making traditional wired solutions impractical and expensive.
Project Background and Challenges
The mining company’s beneficiation workshop required real-time data exchange between the control room’s S7-1200 PLC (housed in an electrical control panel) and the ET 200SP stations in the crushing area. The two locations were separated by a mountain and a tunnel measuring 10m wide, 15m high, and 15m long. Three major obstacles made the project particularly difficult:
- Physical Barrier: The mountain and tunnel completely blocked line-of-sight, ruling out direct fiber optic cable or standard wireless links.
- Harsh Electromagnetic Environment: The tunnel contained metal supports and high-voltage power cables, generating significant electromagnetic interference (EMI).
- Cost and Time Constraints: Trenching for cables was estimated to exceed $200,000 and require 45 days of downtime, which was unacceptable for production.
The Wireless Bridge Solution
To overcome these challenges, engineers deployed an industrial-grade wireless bridge system designed for non-line-of-sight (NLOS) applications. The key device used was a specialized 5 GHz wireless bridge featuring advanced MIMO-OFDM technology with a diffraction gain of up to 18 dB. This allowed the signal to bend around obstacles and penetrate through the tunnel effectively.
Key Technical Specifications of the Wireless Bridge
- Frequency Band: 5 GHz with DFS (Dynamic Frequency Selection) to avoid interference
- Protocol: 802.11ac, delivering up to 400 Mbps throughput at 2000 meters
- Environmental Rating: IP67, operating temperature -40°C to +75°C
- Error Rate: Less than 10⁻⁹ thanks to intelligent anti-interference algorithms
- Antenna Options: Integrated or external parabolic antennas with 24 dBi gain for focused beams
System Architecture: Three-Hop Relay Network
Given the 2000-meter distance and the tunnel obstruction, a single wireless link was not feasible. The solution involved a three-hop relay configuration:
| Hop | Location | Distance | Signal Strength |
|---|---|---|---|
| 1 | Control Room to Tunnel Entrance | 800 m | -65 dBm |
| 2 | Tunnel Entrance to Tunnel Exit (through tunnel) | 500 m | -72 dBm |
| 3 | Tunnel Exit to Crushing Workshop I/O | 700 m | -70 dBm |
Site surveys were conducted using spectrum analysis software to determine optimal relay positions. The goal was to keep signal attenuation below -75 dBm at each hop, ensuring a stable link budget. At the tunnel entrance and exit, high-gain directional parabolic antennas (24 dBi) were installed to focus the beam and minimize multipath effects caused by the tunnel walls.
Ensuring PROFINET Reliability
The communication between the S7-1200 and ET 200SP relied on PROFINET, which demands deterministic cycle times and low jitter. To guarantee performance, the wireless network was configured with a 100 ms update cycle, leaving a 300% bandwidth margin to absorb bursts. All wireless bridges and antennas were properly grounded with resistance below 4 ohms to protect against lightning and static discharge.
In the tunnel, where a large crusher motor generated transient interference exceeding 4 kV during start/stop, the system maintained a bit error rate of just 0.001%. This was achieved through the bridge’s DFS capability, which automatically switched channels when interference was detected, and the robust OFDM modulation that spreads data across multiple subcarriers.
Results and Benefits
The wireless solution was deployed in just one day, compared to the 45 days required for cable trenching. The total cost was only 13% of the wired alternative, yielding a return on investment in less than two months. Most importantly, the system delivered 99.99% communication availability, with end-to-end latency from ET 200SP input to PLC processing consistently under 35 ms.
| Parameter | Wired Solution (Fiber) | Wireless Bridge Solution |
|---|---|---|
| Deployment Time | 45 days | 1 day |
| Cost | $200,000+ | $26,000 (87% savings) |
| Communication Availability | 99.9% (typical) | 99.99% |
| Latency (IO to PLC) | < 1 ms | < 35 ms |
| Immunity to EMI | Excellent (optical isolation) | Excellent (DFS + OFDM) |
Lessons Learned and Recommendations
This case demonstrates that industrial wireless bridges can reliably replace wired connections even in extreme NLOS conditions. For similar projects in mining, tunnels, or ports, consider the following:
- Always perform a thorough site survey with spectrum analysis to identify interference sources and optimal antenna placement.
- Use high-gain directional antennas in tunnels to create a “RF pipeline” that minimizes multipath.
- Ensure proper grounding and surge protection for all outdoor equipment.
- Configure the wireless network with sufficient bandwidth headroom to handle peak traffic and interference.
- Monitor environmental conditions (temperature, humidity) inside tunnels to protect electronics and predict maintenance needs.
The success of this installation confirms that with the right technology and design, wireless can meet the stringent demands of industrial control systems, offering a flexible and cost-effective alternative to traditional wired infrastructure.
