WIRELESS COMMUNICATION IN U/G

COAL MINES

RAKESH ROUSHAN

INTRODUCTION

• What is wireless communication?

• Why do we need it?

• Why it is better than wired networking already available in working mines?

THE PROBLEM

• Radio wave propagation for wireless communication throughunderground coal strata suffers from dispersion, absorption,scattering and attenuation of waves due to its naturalproperties and space limitations. The heterogeneous andcomplex structure of coal and rock strata further complicatesthe process of radio propagation. The attenuation of signalmainly depends upon the dielectric constant and conductivityof coal strata. The dielectric constant of different types ofcoal available in Indian underground mines is given below.The conductivity of coal varies from 10–8 to 0.02 mho/mdepending upon the physio-chemical properties of the coal.

Type of coalDielectric constant

Anthracite coal3.2

Bituminous coal2.8

Coal dust2.5

Coal with 15 % moisture content4.0

THE NEED

• Accidents due to roof fall and collapse of side gallery are some of the daily occurrences in coal mines.Hence the wireless communication offers an aid to establish communication with the miners, trappedunder coal debris.

• To meet the intrinsic safety criteria for hazardous zone7 (Indian Standard, IS 5780: 2002), the powerrestriction of transceiver to be used in underground mine is 2W, which further limits thecommunication range. Therefore, it is important to find out the suitable frequency, which is attenuatedthe optimum when passing through strata and a suitable model for implementing the same. This willultimately help in designing appropriate trapped miner locator and other wireless communicationdevices for underground mines.

• Wireless communication systems will also open the door for endless possibilities of applications andintelligent solutions that can be used during daily operation like real time tracking of mine machineriesand equipments, mine ventilation control using centralized control interface and much more.

INTRODUCTION TO THE SOLUTION

• The answer is third-generation wireless mesh technologybased on the Wi-Fi 802.11 protocol.

• A series of multi-radio enclosures, called wireless meshnodes, propagate the signal down the length of the mineshaftand wirelessly connect miners working or traveling nearby.The nodes may be placed along entries, travel ways, beltwaysor in airway intakes and returns to wirelessly link miners atthe working face to the rest of the mine, as well as to theoffice and managers outside the mine. To overcome theproblems of congestion and contention one radio is used tocreate a link to its upstream (nearer the wired source or"root") node. Another radio creates a link downstream to thenext neighbour node. Thus, each node may be sending andreceiving simultaneously to its upstream and downstreamneighbours, unlike most competitive solutions, which mustcontinually "turn around" between sending and receivingupstream and downstream. Because each link is managedindependently, the available channels may be re-used acrossthe network.

THE MESH TOPOLOGY

A mesh network is a network topology in which each node (called a mesh node) relays data for the network. All nodes cooperate in the distribution of data in the network.

METHOD FOR MODELING UNDERGROUND WIRELESS MINE COMMUNICATIONS

• The main focus the study presented aims at suggesting a working model and building a framework that ensures transmission of electromagnetic waves used in wireless communication throughout the mine. The location of fixed nodes are important to ensure that mobile nodes like handsets receive the best service throughout the areas miners will likely be in.

• Creating a model for the propagation of wireless signals allows the optimal communications node locations to be calculated. The locations that will provide the communications network with the best service can be calculated by creating a model to simplify the mine and solving the mine’s communication network. Solving a mine’s communication network will provide a pre-installation mine network design map, create coverage maps of the mine, and allow planning for future communication and mining activities.

METHOD FOR MODELING UNDERGROUND WIRELESS MINE COMMUNICATIONS

• According to the different structures of underground mines, two types of channel model are required.

• The tunnel channel model is used to describe the signal propagation in passageways and mining area tunnels.

• In particular, the signal power and the corresponding power distribution among significant modes are function of axial distance at frequencies of 500 MHz and 1.5 GHz, respectively. In the near region, the received power attenuates fast and fluctuates very rapidly. This is attributed to the combined effect of multiple modes. On the other hand, in the far region, the decrease in the received power is gradual. This is due to the fact that the higher order modes attenuate rapidly as the distance increases.

• The room-and-pillar channel model characterizes the wireless channel of the room-and-pillar mining area.

• The operating frequency, room height, antenna position/polarization, and electrical parameters in the room-and-pillar environment affect signal propagation in a similar way as in the tunnel case. However, their influence is much smaller. Compared to the tunnel case, signals in a room-and-pillar mining area experience extra multipath fading caused by the pillars.

CASE STUDY - I

• Comms is a computer modelling method developed at theVirginia Centre for Coal and Energy Research at Virginia Tech(VCCER/VT) that can be used to model communicationsnetworks in mines.

• Comms utilizes IntelliCAD software and programmed routinesto calculate necessary values to both quantitatively andqualitatively solve for and analyse predicted coverage areas.

• Comms solves a mine’s communication network by buildingthe communication, solving the network, predicting idealcoverage, and optimizing the communications network.

CASE STUDY - I

• Comms builds the mine’s communication network using thepillar/perimeter method and/or the centreline method. Thepillar/perimeter method uses existing line work in the miningdesign (pillars, mine perimeters, etc.) to determine whichareas have been mined out and attempts to locate the centreof those mined out areas. The pillar/perimeter method drawsa search line from the centre of the area of interest, such as apillar, and determines where the search line encounters thepillar or perimeter line from the drawing, placing a point halfway between the edges of the area of interest and the nextpillar or perimeter that is encountered. The figure showndepicts a small portion of a mine’s communication networkwhen using the pillar/perimeter method.

PROPOSED NETWORK MODEL

• Solving the network consists of defining every singlepoint as a potential broadcast point and thencalculating the signal strength range at that point. Itcan be done by two methods, shortest distance andpath of least resistance, which takes into account thecumulative resistance as each link or distance iswalked.

PROPOSED NETWORK MODEL

• For every point in the network, the path to every other point in the network can be determined by starting at a point and assessing the links available and whether the point across the link is the end point. Then recursively every link available can be counted until a maximum search of the endpoint is encountered; hence, returning the path of least resistance or the shortest distance. These paths are stored to text files of comma separated values that also register properties of the path such as resistance, obstacles encountered, and angles of turns made. These two methods give predicted coverage for every point in the network.

PROPOSED NETWORK MODEL

• Thus, the expected coverage areas can be drawn,measured signal strengths that do not match predictedvalues can be searched for, and other problematic areaspinpointed. Signal strength values that are calculatedwhen solving the mine’s communication network maythen be used to draw in the coverage area that would beprovided at each individual point, if that point was abroadcast point. The solve routine can be time-consuming depending on the number of points in thenetwork; if the network does not change then thenetwork only needs to be solved once. This is becausethe paths found are saved in comma separated variabletext files that can be loaded into the program foradditional analysis. Output from the solve routine willreveal optimal locations for broadcast nodes.

CASE STUDY - II

ACCOLADE Wireless Mesh Communications System

PROPOSED NETWORK MODEL

The framework comprises of fixed broadcast nodes and handsets.Broadcast nodes are capable of communicating to each other,handsets, and other communication technologies (e.g., leakyfeeder and fibre optic network). Additionally, wireless handsetsare capable of communication directly with each other andthrough fixed broadcast nodes. Using the anticipatedinterferences and design parameters, pre-installation networkplans and coverage maps can be created. Generalized anticipatedsignal loss parameters can be categorized into clear non-obstructed, beltway, stopping, and corner losses. The signal lossdue to a clear non-obstructed opening is the natural signaldegradation.

ANTENNAE POSITION

• The receiver antenna is placed either at the centre or at one-eighth of the tunnel height and one-eighth of the tunnel width.

• If the receiver is also at the centre, both the signal attenuation and the fluctuation are small.

• If the receiver is placed near the tunnel walls, the attenuation and fluctuation are much more significant.

• The receiver antenna is also placed either at the centre or at the border of the tunnel. Near the excitation plane, higher order modes dominate (over 80% of the total power). In this case, the position of the receiver antenna does not affect the received signal. The attenuation and the fluctuation of the received power are significant.

SUITABLE FREQUENCY

Communication purpose Frequency used in India International practiced frequency

Trapped miner communication 457 kHz 457 kHz and 6.0 MHz

Shaft communication 32 kHz 30–60 kHz

Line-of-sight communication 410–500 MHz 400–1000 MHz

Mine-wide communication 146–170 MHz 136–174 MHz

CASE STUDY - III

• Figure shows the communications and tracking map provided by NIOSH in the Communications and Tracking Instruction booklet.

CONCLUSION

• Wireless communication systems can be used not only to track and communicate with miners but alsoto simplify coordination of production and maintenance tasks. Wireless

• communication systems are developing continually, are less restrictive than hardwired systems, providecommunication coverage areas significantly larger than hardwired systems, and provide multipleredundant paths for communication if one path is blocked.

• Every mine’s local geology, mining conditions, and other circumstances yield a unique workingenvironment. Mine equipment and structures are constantly changing as mining progresses, forcingmine communication systems to constantly adapt to the current conditions. For this reason whenmodelling a mine’s communication network, general categories of tunnel profiles are created toestablish a model that will account for major signal losses that occur. Modelling the mine’scommunication network using general signal loss parameters allows the model to be applied to othermines.

Thank You!