Until recently, the most advanced mining communications solution was a leaky feeder. Leaky feeders are cables that can emit radio frequencies. They're like coaxial cables which carry a signal from one endpoint to another. Normally, coaxial cables have a copper sheath surrounding the cable itself to prevent the signal from leaking out throughout the length of the cable. If the network uses a long cable, you might not be able to detect a signal from the opposite endpoint.
Leaky feeders are different. Instead of covering the cable in a solid copper shield, a leaky feeder has small gaps in the shielding to let the signal through. The gaps create a limited wireless radio network environment. Line amplifiers and repeaters boost the signal at regular intervals along the cable to make up for signal loss.
Wireless devices can interface with leaky feeders. They can receive radio signals from the cable and transmit data back. The data can include voice, video and computer data. With the right receivers, you can even control stationary equipment like water pumps equipped with radio receivers using signals sent via the leaky feeder. But you still need to be relatively close to the physical cable -- 300 feet (91.4 meters) or so -- to receive and transmit signals [source: Mine Safety Technology and Training Commission].
One advantage of a leaky feeder cable is that you can lay it down as you excavate a mine. You can use splitters to send lengths of cable down different pathways. And since cable is flexible, there's no problem moving the network around sharp corners and turns. You can even feed cable straight down a hole if need be.
There are a couple of downsides to leaky feeder systems. If something severs the cable, communications stop beyond the break. Another problem is that multiple leaky feeder cables can sometimes cause interference within the system. And leaky feeder radio frequencies tend to be on the high end of the spectrum -- these high frequencies don't penetrate rock very well.
The Mine Safety and Health Administration (MSHA), the governmental agency in the United States responsible for overseeing miner safety issues, approves of four leaky feeder systems:
- Flexcom from Mine Radio System
- SmartCom IS from Varis Mine Technology
- RFM 2000 from DAC
- Model VHF-1 from El-Equip Inc.
Mesh networks are an alternative to leaky feeders. A mesh network doesn't require a cable like a leaky feeder system. Let's take a closer look at how wireless mesh networks may make mining safer.