My safety helmet design can be divided into three categories. The first category is data acquisition from sensors and processing them to predict the potential explosion. The second is tracking the miners and determine their current or immediately pre-accident location. The third one is communication between the workers and ground operations centre(GOC). This week's blog is related to second and third parts because the tracking system is related to the communication system. There was a delay to shipping kits so I haven't started hands on developments. However, I am glad for the delay because the more I do research the more I need time. There are huge details and I may modify my proposal based on my findings.
Underground wireless communication can be divided into two categories, primary and secondary communications. Primary communication is for daily communication between miners and ground workers. Leaky feeder and node-based systems are examples of the primary communication systems (PCS). The operating frequency of the PCS starts from very-high-frequency (30 - 300 MHz) band to super-high-frequency band (3 - 30 GHz). Secondary communication uses low frequencies (sub 1MHz) therefore it doesn't have the capacity for the for general communication and requires bigger and heavier antennas. Secondary communication is used during the emergency hence it can propagate through the earth or coal. I will use the PCS for this project hence it requires the plenty data transmission (sensory data for multiple workers).
Leaky Feeder Systems
Leaky feeder systems are mainly used in mining areas. They provide two-way communication by using a special type of the coaxial cable. The cable has holes where electromagnetic waves penetrate into or leak out of the coaxial cable. The leaky feeder cable is large transceiver antenna which is established through the tunnels. Hence, the cable can't transmit the obtained signal at infinite length, it requires amplifiers to strengthen the RF signal.
A leaky feeder cable.
Three types of the node-based systems are used in undergrounds; wireless local area network (WLAN), wireless fidelity (Wi-Fi) mesh, and ad hoc mesh. All three use the same standard 802.11b/g. It is open standard so manufacturers can add proprietary features to original WLAN protocol in order to provide the robustness. The ad-hoc mesh network is the most reliable one from them. It has additional features like using end device as a mesh node, communicating autonomously with any node inside the RF range, and forming a network without dependency on the central server. ZigBee is on of the example of an ad-hoc network. ZigBee is a good option if the data rate is sufficient. (20 kbit/s (868 MHz band) to 250 kbit/s (2.4 GHz band)).
Emslie et al. proposed that mine entries act as waveguides for frequencies in the UHF range and upper frequencies. According to proposal 800 to 1,000 MHz range has the minimum path loss. (This turns on the light for TI sub - 1 GHz RF communication systems). This indicates that 800 to 1,000 MHz is very effective design for undergrounds.
UHF propagation path loss modeling [adapted from Emslie et al. 1975].
Traditionally, tracking inside the mine are done manually informing the dispatcher. However, this method is not reliable because a miner can forget (or ignore) to inform dispatcher or the area can be huge therefore it may be impossible to point the exact location. Electronic tracking systems help to solve these problems. There are various methods like RFID tags, node-based systems which measure the signal strength, and dead reckoning. Tracking system needs a communication system with the ground operations centre(GOC) so a node-based system has advantages. If other methods are used, they need a separate communication system.
RFID systems are common in shopping malls. The system has a tag (usually passive) and reader. Cheap tags can be embedded to tracked body and when the reader reads the tag, the items or people are located in that area. There is also a reverse-RFID method where the location is tagged and people or items have the readers. Both methods require a communication link either wired or wireless to inform the GOC.
My project requires node-based wireless communication system (I will use wi-fi. Although, Wi-Fi can be used inside the mine areas some of them requires sub 1GHz system). Therefore, implementing RFID tracking system is wasting resources. RFID system is simpler to implement but that time system cost will increase. Wi-Fi can be used to track miners based on the received signal strength indicator (RSSI). I haven't checked the CC3100MODBOOST specifications yet, I hope it allows me to get RSSI value. Unfortunately, there may be many reflections and interference. This method requires complicated software algorithm for tracking. Therefore, I am planning to combine dead reckoning with RSSI. Dead reckoning uses accelerometer and gyroscope sensor to get user movement and process it. Dead reckoning doesn't require RF signal but sensor outputs can be noisy which result in error. I hope I can eliminate the errors by using both of them.
If you have any suggestion, I look forward to hearing from you.
If you want to learn about communication and tracking at underground, you can check the "Advanced Tutorial on Wireless Communication and Electronic Tracking" from The National Institute for Occupational Safety and Health (NIOSH). They have plenty of useful resources.
.You can see all the links related to this project in the first blog: Safe & Sound Wearables - Trackable Safety Helmet for Miners #1: Introduction to Project