A new system supports underground safety requirements more cost-efficiently by using a mine’s operational network infrastructure to manage critical safety functions

By Christoph Müller, Dipl. Ing

Underground mining accidents and resulting legislation have shown the importance of communication systems installed in underground mines. Traditionally, underground communication means telephone and analog communication systems like intercom radios or audio frequency-based analog intercom systems. All these systems have strong disadvantages in terms of coverage distance and transmission quality. Often, multiple different systems are installed in parallel.

For quality and cost reasons, the use of Ethernet has become more and more popular in underground infrastructures. Underground communication has mainly involved data networking and voice communication (Voice over Internet Protocol, or VoIP) leading to the use of Ethernet as a universal “all-over-IP”-based communication system. Wireless LAN (WLAN) systems are being used for wireless voice and data communication using Pocket PCs or handheld VoIP phones as hardware devices. The use of personal mobile devices—and thus the people carrying them—offers the means for workers carrying these devices to be tracked automatically throughout the mine via the nearest access point. WLAN tags also may be used to track workers not equipped with these mobile devices.

Raising Network Resilience
Safety regulations demand that mine communications systems and devices be available at all times. However, as shown in Figure 1, the ‘resilience’ of different communication systems is limited, especially in terms of their cabling:

  • A phone system uses star-structured, point-to-point cabling of each individual phone to the PBX. If something cuts a trunk line, all phones in the area of this trunk line are out of service.
  • Intercom loudspeaker systems and many leaky feeder systems use line-based structures in which all communication devices are connected along a single cable. If the cable is cut, all devices from the cut point to the end of the line are out of service.

Thus, traditional communication systems provide only a limited degree of resilience and availability in an emergency, with a high probability that even a single failure will result in an inability to communicate with workers in certain areas.

A network structure using ring topology provides single-failure safety, which means that a single interruption in the network system does not prevent other components from remaining functional. Meshed ring structures are capable of providing multiple redundant transmission paths between two or more participants in the network, providing redundancy in large parts of the communication system.

Since ring and mesh structures can be realized with Ethernet networks, this leads to improved applicability of Ethernet communications for safety-related applications over traditional communication technology, provided that the active components (network nodes as switches and access points) are equipped with battery-backup power supplies.

Because Ethernet is routinely used in underground operations for voice, video and data traffic, it is already present in many mines and consequently, expanding its functionality to carry mine safety-related information saves substantial cost compared with installing and maintaining separate communication systems—which also may lack resilience.

Traditional communication systems either employ central telephone PBX hardware, or with Ethernet, central servers to control network logic; e.g., assigning IP addresses, for example. Telephone PBX hardware and network servers are generally installed in a surface location. One consequence of this is that these systems—and thus probably most modes of communication—may not be available during an underground emergency that results in damage to the cabling to these central systems.

An additional disadvantage of today’s processing of information acquired underground is the fact that information is available only after having been processed in central computer systems located on the surface. Consequently, this information is not available to the people underground when they need it most: during an emergency when the communication to above ground may be cut.

Staying Alive
Due to these limitations, future unified communication systems supporting underground mine safety should offer the following capabilities:

  • The network must be able to “stay alive” underground even when all connections to the surface are interrupted.
  • Safety-related information must be processed underground, without need of interaction with central systems and allowing distribution of vital information from, say, gas and other environmental sensors, over the underground network. In this context, standardization of the information exchange becomes important.

With the support of the European Union’s Research Fund for Coal and Steel, a system has been developed over the past three years by MineTronics that provides this functionality in underground networks. This system is now being introduced in underground mines with the safety support functions following in a step-by-step procedure.

For broadband network communication in underground mines, a wired backbone is essential. This wired backbone is set up using rugged industrial fiber optic cables because maximum effective length for copper Ethernet cable is limited. In most underground infrastructures today, single mode fiber is a de-facto standard.

To achieve redundancy, most underground fiber optic installations today are designed to connect devices in ring structures. A pure ring, however may not be suitable to provide a highly resilient network because a ring cut in two places may leave a large number of network nodes disconnected between the damaged points. One method to overcome this is to use intelligent network nodes that are not only able to provide ring redundancy but also meshed links. These network nodes, called Mining Infrastructure Computers (MIC), also provide location-based operational functions such as tracking of assets or people (See Figure 2).

Just as a mine physically consists of a meshed network of underground tunnels, a modern network can follow these structures, with some limitations. And, just as miners use an alternate route when one tunnel is blocked, a network can employ alternate routing when the original route is unavailable. The MIC, as an active underground component, is not just a network switch or access point. Each MIC is a distributed independent network node that is able to communicate with miners even if all network connections are down; in other words, MICs keep an underground network “alive” in an emergency. This is the precondition for distributed processing of potential safety information, which then can be transmitted to workers in the network areas available after an emergency.

To supply miners with relevant safety information after lines to the surface are cut, the MIC needs certain static information. Traditionally, all information about locations of emergency exits and emergency equipment (fire fighting equipment, rescue chambers or first aid equipment) is provided by signage on tunnel walls. In the new safety support network, however, this information is downloaded to all underground MICs and can be made available to miners. With this information, MICs can function as a navigation system with two electronic maps available for computation of location based safety information. One map is a static version indicating tunnel layout of the mine (similar to street layout in map software) and the location of safety equipment and exits.

The MICs also create a second, dynamic overlay map indicating real time availability of network links. MICs can use this information to provide underground ‘navigation’ information to miners; for example, an active or usable network link can be interpreted to indicate that the tunnel through which the cable is routed is physically passable. This is extremely important information in a mining emergency. It may help organize evacuation plans as well as speed up search and rescue operations.

Guiding Miners to Safety
Many mining accidents are caused by fire, rockfall or gas explosions. With any of these events, the probability is high that a network cable will be damaged and the logic link lost. Conversely, because there also is high probability that active devices in proximity to these events would be damaged, the system’s indication of a suddenly missing link can be interpreted by miners as “this route may be blocked.”

This emergency localization feature of the MICs results from the combination of downloaded static information and real time sensor data available in the network. It is the basis for a number of safety support functions such as guiding trapped miners to emergency exits, shelters or other safe areas as well as providing information for rescue teams to quickly access the site.

In a regular network, communication is not possible when the network’s connection with central systems on the surface is lost; the underground network is “dead.” However, in such cases the active hardware underground, equipped with battery backup power supplies, often is still functional, and MICs can switch to an independent “emergency mode” when connections to surface-located central systems are lost.

In this mode, all MICs in the disconnected island automatically negotiate among themselves as to which unit will “replace” the central services that have been lost and assign IP addresses or coordinate digital voice communications, for example. In emergency mode, all network traffic is limited to services that are essential for mine safety, such as voice communication, environmental sensor information, and tracking. Other services may be blocked by the MICs.

In any emergency it is important to safely evacuate all workers from the mine and to ensure that no one is left behind. The first step in this phase should be a muster at a dynamically assigned assembly point. The entire procedure is illustrated in Figure 3.

For this example it is assumed that:

  1. Prior to an emergency event the cable in position 1 was taken out of service due to maintenance. However the associated tunnel is passable for people and suitable as an emergency escape route.
  2. The network is cut or interrupted by an event at positions 2 and 3.

Because the tunnel at Position 1 is passable for people and suitable as an emergency escape route, it makes the area close to this exit a suitable meeting point. This information is available in the MIC devices and becomes part of the evaluation of the current situation by the miners, based on information available from the network. When the decision about a meeting point is made, either a related button on a network node is pressed or the location is presented on the displays of wireless devices. The network thus knows where to guide the miners, and MICs will begin transmitting messages to miners, indicating where the proposed meeting point is located.

If the miners are equipped with mobile devices such as pagers or phones, the MICs also can actively guide them to this meeting point by showing the instructions on the LCD display of the MICs and distributed on handheld units. Further information displayed on the MICs or on handheld devices also can show whether workers have been left behind.

When everyone is mustered at the meeting place, they can decide whether to attempt an evacuation or retreat to a rescue chamber, for instance. In the example shown in the figure they may decide to evacuate via connection 1. If they then reach an area where communications are intact, their devices will connect to the network and they can report their situation.

The new intelligent underground network infrastructure using MICs has been used commercially for about three years at several mines in countries throughout Europe. New wireless client devices such as messenger (pagers) and smartphones (See Figure 4), that are capable of emergency communication underground are currently being introduced so that the safety support functions described above can be gradually introduced and thoroughly tested in real world applications. The new system is designed to work in both hard rock and coal mining.

Marginal Cost, Improved Functionality
Due to lack of standardization and interoperability in many existing mines, separate systems often are used for different aspects of mine safety such as ventilation, tracking, electricity, geoseismic etc. In an emergency it is crucial to make the right decisions quickly, because the available time frame for successful rescue efforts may only be a matter of minutes or hours. This is a challenge for future integration of underground devices into the networks—a step that is essential to achieve process-optimized mining. Initial steps, such as standardization of tracking formats through IREDES initiative have already been undertaken.

Currently, network-based communication systems are commonly installed in underground mines and are generally used for standard data and voice communication. If these installations are planned using active components capable of providing the safety support functions illustrated, the additional cost for the safety functionality can be marginal compared with investing in additional or separate safety communication infrastructure.

MineTronics’ new safety-related underground network consists of intelligent network nodes combining switches, WLAN access and safety- and infrastructure-related mining functions. In an emergency, this equipment also acts as supporting devices for localization of the emergency, for localizing people and for supporting a coordinated self-escape of the miners.

In general, this new system has the capability to significantly improve and support underground safety in a cost-efficient way by using a mine’s standard operational network infrastructure for safety support functions.

Christoph Müller is CEO of MineTronics GmbH, Goethestraße 52, D-49549 Ladbergen, Germany. Email: chmueller@minetronics.com.