Energy Efficient Clustering Protocol for Early

Warning System for Miner's Safety in Coal Mines

K. A. Unnikrishna Menon, Deepa Maria, Hemalatha Thirugnanam Amrita Centre for Wireless Networks and Application

Amrita Vishwa Vidyapeetham, Amritapuri Email:kaumenon@am.amrita.edu,deepamaria89@gmail.com,hemalatha@am.amrita.edu

Abstract: Early detection of spontaneous explosions in

coal mines can be made possible by monitoring the concentration of toxic gases such as Carbon monoxide (CO), Methane (CH4) and Hydrogen sulfide (H2S). Methane gas detection is very important for outburst prediction. In order to protect people and to avoid destructive sequence, it is necessary to design a system that can predict methane outburst in coal mines. In order to improve the reliability of gas monitoring in coal mine, we design a wireless sensor network (WSN) with gas sensors. The wireless sensor network is suitable for toxic gas monitoring in the severe environment of underground coal mines. Semiconductor type methane sensor having low power consumption and high sensitivity is used in the proposed system. The chain type clustering of low power sensor nodes along with data aggregation is devised to optimize the power consumption by the WSN. Additional feature of minimum data transmission and state transitions between listening, sleeping and transmitting modes in sensor nodes also guarantee power optimization.

I. INTRODUCTION Mining industry is susceptible to various types of

disasters which lead to heavy loss of human lives,

property and infrastructure. [1] The issue of safety in the

mines is still unsolved almost all over the world. There

are a number of risk factors involved in the underground

coal mines like faulting, mine explosions, mine fires,

suffocation by the poisonous gases formed, mine

inundations (flooding from water table), mine cave-ins or

roof falls and seismic changes (due to natural

earthquakes or due to mining operations). As per survey,

it was seen that the mine explosions are the main cause

for majority of the underground mine accidents. The

main cause for mine explosions is the presence of toxic

methane gas in an insufficient oxygen environment. The

proposed system aims at developing an advanced power-

efficient early warning system to safeguard the miners

from high exposure to the toxic methane gas.

An early-warning system must be established so as to

send signals before a disaster happens. Along with the

technology for an early-warning system, the proposed

system helps to decrease the rate of accidents and to

improve the emergency safety measures. Existing safety

systems use a wired infrastructure which could get

destroyed during some disasters, power failure or

propagation hazards which will lead to an inefficient

system, necessitating deployment of a wireless sensor

network. The objective is to develop a power optimized

and efficient wireless sensor network to sense the

presence of toxic methane gas and to warn the

monitoring station about the increase in gas

concentration. Light, compact, low cost and power

efficient wireless sensor nodes substitute the traditional

wired, heavy and fixed monitoring equipment and leads

to faster, easier and large-scale deployments in a harsh

and sophisticated environment. The system can thus

reduce the chances of mine explosions and its higher

impact on the miners by providing a warning before the

hazard happens. Wireless sensor networks have attracted more and more

research interest in coal mine applications for their

advantages of self organization, low cost and high

reliability. Sensor networks are used in various

applications mainly due to the simplicity and efficiency.

WSN is a combination of sensor technology, embedded

systems and wireless communications. It has been used

in monitoring, data collection of environmental

parameters and transmitted to terminals by self-

organizing wireless networks with relay method. WSN

consists of many tiny sensor nodes with wireless

transmission ability and computational ability. The

distance among nodes of WSN is always very short. It

uses multi-hop wireless communication for long distance

applications. WSN nodes have the following

fundamental functions: data collection of parameters,

data processing and data transmission. Combining WSN

with appropriate communication technology is a perfect

scheme for mine emergency communication. The

monitoring system for mine safety was designed to cater

the problems faced by miners and to monitor various

environmental parameters in the coal mines.

II. RELATED WORK

Although there are a number of mine monitoring

systems, only a few of them make use of wireless

communication technology. Monitoring systems mainly

use the conventional wired infrastructure so as to ensure

minimum packet loss. There are many issues like

explosions, disasters like earthquakes and flooding to

hamper the wired infrastructure and which destroy the

communication links underground. Also, when the

mining process extend, the wiring has to be extended

which increases the cost of the system. Thorsten et.al has

described in the paper [2] about a single semiconductor

gas sensor device operated in a T-cycled mode. In this

they developed a hierarchical identification strategy for

the use in underground coal mines early fire detection

systems. Pramit and others had developed a system in [3]

[4] to detect the exact fire location and spreading

direction and also provide the fire prevention system to

stop the spread of fire to save the natural resources and

the mining personnel from fire. Simulation analysis

Conference Proceeding 2012 21st Annual Wireless and Optical Communications Conference(WOCC)-April 19-21, Kaohsiung, Taiwan

978-1-4673-0941-7/12/$31.00 ©2012 IEEE -99-

results showed that the average network delay varies

almost linearly with the increase in the number of hops.

In another paper [1], a wireless solution for the

monitoring and control system was designed and details

of the network establishment and a software system to

view the details was also observed. They compared three

basic star, mesh and mixed network topology structures

and selected the topology structure more appropriate for

large scale sensor networks. The network, according to

the mixed network topology, which intends to combine

the advantages including the simplicity of the star

network, the multi-hop transmission and self-healing

from the mesh network, was found to be appropriate.

Localization of miners using RFID was discussed in [5].

Localization algorithms to find the exact location of the

miners were designed. Paper [6] deals with an energy

efficient method in gas monitoring but the system makes

use of a wired media.

III. PROPOSED SYSTEM A. Overview In the current research work, we considered research

issues like choosing of an efficient wireless technology

in coal mines, method to devise fast transmission of

warning signals to long range distance and development

of energy and cost efficient hardware module. In the

proposed system, [6]ZigBee technology was chosen as

an efficient wireless technology to be used in coal mines

since it is a low cost, less power consuming technology

which has long battery lifetime. Also, a number of nodes

could be connected in a network by using ZigBee

technology and is an efficient technology when

implemented in coal mines. As a part of implementing

this network, a hardware module is developed for a

sensing toxic methane gas and to transmit the excessive

gas concentrations. Hardware module designed for the

sensor node was of very low cost and thus can be

deployed in large numbers in the coal mines. Power

optimization in the system was brought by decreasing the

amount of wasteful energy consumed, the amount of data

transmitted and by devising some energy efficient data

aggregation and routing protocols. Since the application

is to monitor the gas concentrations underground by

considering power usage minimization, chain type

clustering is a good topology. Also, the clustering

protocol provides an effective method for prolonging the

lifetime of a wireless sensor network and for long range

communication.

Protocol will transfer data from the lower level sensor

nodes to neighboring cluster head nodes (CH) and

propagate this till the central monitoring station in chain

type multi-hops. Chain type clustering of the sensor

nodes is shown in figure 1. The protocol consists of two

parts, one is with respect to cluster management in sensor

area, and the other is about the data aggregation and data

transmission between the sensor area and the central

monitoring station. According to the proposed protocol,

performance after multi-hop communication is much

better than single-hop clustering protocols in terms of

network lifetime and energy consumption when the

monitoring station is located far away from the sensing

area.

Fig 1: Chain Type clustering of sensor nodes

Several sensor nodes are deployed in a particular region

to ensure data accuracy. Few sensor nodes combine to

form a cluster and a CH is chosen as the node with

maximum remaining energy. The nodes in a particular

region which sensed values above a predefined threshold

value (lower explosive level), transmit the gas

concentration values to the CH. Explosive limits of

methane gas were found to be 5%-15%. At the CH, the

different values obtained is aggregated using some

function and is forwarded to the neighboring CH's. At

last, after many multi-hop transmissions, central

monitoring station receives the areas where there is

excessive amount of gas concentration and rescue

operators are informed about the regions where there is

chance for disastrous explosions and fast and tremendous

rescue operations are held. Assumptions taken for the

proposed system are: • Sensor nodes are fixed/ static. • Sensor nodes are synchronized in time with all other

nodes. • Remaining energy of each of nodes is known to the

neighboring nodes and thus it is easy to elect the

CH. • Location of the nodes is known previously. • Interference from other devices is assumed to be

negligible. • Nodes are largely deployed in a region for data

accuracy. Accuracy of data can be obtained by

deploying a number of sensors in a particular

region.

B. System Design In the proposed system we make use of several sensor

nodes deployed in the mines. System Architecture for the

proposed system is shown in the figure 3. Mostly these

nodes are placed along the walls or roof of the mines

where the gas concentration can be measured more

accurately. There is a central monitoring station above

the ground which will have a number of officers

monitoring the safety of the miners working

underground. Whenever an alarm or warning of some

explosion is received at the station, they inform the

rescue operators and quick rescue or evacuation

operations are carried out. Architecture of a single node

is as shown in figure 2. At each sensor node there will be

different modules like sensor unit consisting of various

Conference Proceeding 2012 21st Annual Wireless and Optical Communications Conference(WOCC)-April 19-21, Kaohsiung, Taiwan

978-1-4673-0941-7/12/$31.00 ©2012 IEEE -100-

sensors, a processing unit to perform several

computations, memory to store the values temporarily,

power unit to supply external power and a

communication module to transmit the sensed values

wirelessly. There will be some protocols like data

aggregation and clustering protocols in order to reduce

the power or to make the node more energy efficient.

Power efficiency to the whole system can be brought

about by using some clustering, efficient data acquisition

and data aggregation protocols. Clustering routing

protocol provides an effective method for prolonging the

lifetime of a wireless sensor network. Power

Fig 2: Sensor Node Architecture

Fig 3: Proposed System Architecture

consumption of a node is mainly due to the use of high

power consuming components, processing and for data

transmission. Hence, to reduce the power consumption of

a node, it is designed using low power components and

to send minimum amount of data. Processing power

cannot be reduced since this is a must and essential

activity. Multi-hop clustering of nodes is devised have a

long range communication and to obtain energy

efficiency. The proposed system is designed to be a chain

type and clustered hierarchy in which there will be

number of sensor nodes in a particular region and

aggregation function is performed only at the cluster

heads. Sensor nodes will be grouped into clusters and

elects the CH by considering the remaining energy. The

cluster heads are connected to the neighboring cluster

heads in order to transmit to central monitoring station

which is at a long distance. Sensor node at the lower end

of the mine will have a transmitter module and those at

the intermediate levels will have transmitter and

reception module. Transmitter module has a sensing unit,

processing unit, communication unit and uses external

power. Sensing unit consists of a methane sensor and the

processing of the sensed value is done by a PIC

microcontroller. In order to transmit those values

wirelessly, ZigBee technology is used. ZigBee was

selected due to its low cost and reliable short range

transmission.

In the transmitter module, the sensed data obtained from

the sensors will have to undergo an analog to digital

conversion. The digital data obtained will be compared

with a threshold value and only those values above the

threshold are wirelessly transmitted. At the reception

module the signal is received and a warning alarm or

message should be provided to the miners or disaster

rescue operators.

C. Hardware Module

Sensor node hardware module has a sensing unit,

processing unit, communication unit all powered by

external battery. Sensing unit consists of sensors which

will sense the gas concentration. In this system, we make

use of a [7] semiconductor type MQ4 sensor which can

sense the presence of methane, butane and propane

gases. MQ4 methane sensor uses a sensitive material

SnO2 (Tin Oxide) which will increase its conductivity

when the combustible gas exists. Sensor converts the

conductivity to a voltage value corresponding to the gas

concentration. The output voltage can be obtained by

biasing a resistor across the terminal of the sensor. The

sensor can measure the methane gas concentration

ranging from 200 to 10000 ppm. Resistance value for

biasing of MQ4 sensor is different for different gases.

Sensitivity adjustments are very necessary for the sensor

while using them for particular applications. Hence, the

sensor has to be calibrated for measuring the methane

concentration in air and for this a load resistor of about

24KΩ (10KΩ to 47KΩ) has to be used. After the analog

output from the sensor is obtained, it is converted to

digital value. In this system, we make use of the

PIC16f877A microcontroller to convert the analog to

digital values and to perform some other computations.

The digital outputs are compared with a predefined

threshold value and only those values above the

threshold are transmitted to ZigBee module. The

minimization of the data transmitted will reduce the

power consumed to a large extent. Along with the values

above threshold, a particular message will be transmitted

to indicate the level of danger. In order to transmit those

values wirelessly, ZigBee technology is used where it

makes use of XBee Series2 module. In order to transmit

to XBee module we will make use of UART module of

PIC microcontroller. Since, the values are to be

transmitted serially to the XBee module, we make use of

MAX232 IC and a DB9 female pin. Figure 4 shows the

sensor module which was designed on a PCB board

which is connected to a voltage regulator circuit. The

loop circuit diagram for the sensor is as shown in figure5.

The load resistor of 24KΩ was used for biasing across

the VRL and GND terminals. The resistance across the

Conference Proceeding 2012 21st Annual Wireless and Optical Communications Conference(WOCC)-April 19-21, Kaohsiung, Taiwan

978-1-4673-0941-7/12/$31.00 ©2012 IEEE -101-

H-H pins of the sensor was found to be 27KΩ (31KΩ ±

3KΩ in datasheet). Current drawn by the sensor was

found to be 185mA (180mA in datasheet). Power

consumption was found to be 925mW (900mW in

datasheet).The above readings were taken by providing a

heater voltage supply of 5V to the sensor for half an

hour. When the sensor was exposed to gas, the output

voltage which was almost stable rose abruptly to 4.8V.

This voltage obtained is dependent on gas

concentrations.

Fig 4: Methane Sensor module connected to a voltage regulator

Fig 5: Heater circuit to MQ4 sensor

IV. ALGORITHMS USED IN THE SYSTEM A. Efficient Clustering and data aggregation Protocol Power optimization is brought to the system using chain

type clustering protocol which also performs a data

aggregation technique at the CHs. There are mainly 3

phases for the algorithm: 1.Cluster formation,

2.Sychronization phase, 3.Data aggregation and data transmission phase.

1) Cluster formation: The location and remaining energy

of the nodes are known to the neighbors. Sensor nodes

will form clusters and will designate a region-id. In a

particular region, the node with maximum remaining

energy is elected as the CH. After a CH is elected, it

informs the other nodes by broadcasting the CH

Message. CH Message will have the CHs id and will also

have a unique Cluster ID set for a particular cluster. In

HEED (Hybrid, Energy-Efficient, Distributed Clustering

Approach) protocol, CH rotation is driven by time which

means that CH selection is triggered periodically.

However, in this protocol energy-driven method is used

to elect CH which can reduce the extra energy

consumption in time-driven method. Each CH is rotated

according to its remaining energy and thus due to the

rotation, the remaining processes will not affect all

clusters but will be restricted within its own cluster.

Thus, this procedure can avoid disturbing the normal

operation of other clusters with high remaining energy

and can balance the energy consumption in the network. 2) Synchronization phase: The sink node, CH and lower

level nodes are synchronized in time. Initially, all the

nodes send the SynchMessage to all the neighboring

nodes. SynchMessage will contain the timestamp and the

state information (ON, LISTEN, SLEEP, TRANSMIT,

OFF) in order to adjust the remaining receiving nodes to

change their transitions. CH and sink node can also

change its transition according to the lower level nodes

with the reception of SynchMessage. Lower level nodes

will send the SynchMessage frequently. CH and sink

node will also wait for a particular interval more after

receiving the SynchMessage from other nodes before

adjusting its state information.

3) Data aggregation and data transmission phase: At

the CH, we perform data aggregation procedure. Since

the more priority data has to be transmitted to the central

monitoring station, by the aggregation process the data

obtained with the maximum value of gas concentration is

transmitted to the next neighboring CH. Along with the

maximum gas concentration value, region-id designated

for the CH, timestamp value and state information of the

CH is also transmitted. In this stage, the lower level

nodes sense data and the sensed data is transmitted along

with the region-id, timestamp and state information to

their CH. Each CH aggregates the data receiving from all

its lower level nodes and then sends to its neighboring

CH nodes as a chain till sink node. The CH which has the

sink node as its neighbor will surely transmit to sink

node and thus reliability of data reaching the central

monitoring station is solved. Central monitoring station

can then take necessary actions like reporting to the

rescue operators. Each node will check its remaining

energy after accomplishing its transmission assignment.

B. Prioritization Algorithm Power optimization can be achieved by different methods

like introducing state transitions for each sensor nodes,

minimizing data transmissions by only sending optimal

data values, by data aggregation techniques and by

prioritizing the data send. In our proposed system inorder

to manage the power of the system, we take in to

consideration the following states: ON, LISTEN, SLEEP,

TRANSMIT and OFF. In the initial state all the sensor

nodes which are in ON state will achieve LISTEN state

after getting the enough power to sense. In LISTEN state,

it senses the parameters (methane gas concentrations)

and analyzes whether the data within the threshold value

(1.7V). In this state the transmitter module is switched on

since it might need to transmit. If node doesn't sense data

> threshold for a fixed time (30 minutes) it will move to

SLEEP state. Here, it will only perform sensing

operations and no receiving. In this state the transmitter

module is switched off. If the values sensed exceed the

threshold value, the node will go to TRANSMIT state.

The transmitter module will be switched on in this case

and the data is send to the CH node. In addition to the

transmitter module, the sensing module is also working.

From all the states it can enter in to OFF state due to

power failure. State Transition Diagram is shown in

figure 6. In the case of data transmission, at each CH it

will perform a data aggregation procedure where only the

maximum of all the received data values will be

Conference Proceeding 2012 21st Annual Wireless and Optical Communications Conference(WOCC)-April 19-21, Kaohsiung, Taiwan

978-1-4673-0941-7/12/$31.00 ©2012 IEEE -102-

transmitted.

C. Processing algorithm at the node's transmitter module The methane gas concentration sensed by the MQ4

sensor varies from 200-10000 ppm. Analog voltage is

only obtained from the sensor. Hence ADC (Analog to

digital converter) present in the pic16f877A

microcontroller is used to convert the analog voltage to

digital values. The ADC channel has to be selected

which will input the values to the ADC and digital

Fig 6: State transition Diagram

outputs can be obtained. Inorder to reduce the amount of

transmissions the digital value is compared with the

explosive limit of the methane gas (set as threshold).

Only when the output value exceeds the explosive limits,

value is transmitted to the UART module. Along with the

gas concentration values, a warning message is also

transmitted according to the extent of danger.

V. ADVANTAGES OF THE SYSTEM The proposed system to report the abnormal amount of

methane gas concentrations which can lead to mine

explosions is designed to enhance the safety of the

miners working in the harsh environments in

underground coal mines. Deployment of numerous

sensor nodes in the underground mines will be a cost

effective technique as each sensor node is designed in a

low cost manner. Another feature which is dominantly

looked upon is the power optimization of the whole

system. Since each sensor node is build using low power

consuming components, the power consumption by the

whole system is found to be very less. The power

consumption for the wasteful activities like idle listening,

overhearing are reduced by introducing state transitions

to the sensor nodes in the system. Power consumption for

the data transmission is reduced by sending only the gas

concentrations above a threshold. By introducing the

clustering and data aggregation protocols the power

consumption can be reduced to a greater extent. Since

transmission is along the CHs in multi-hop manner, there

is a maximum probability of the monitoring station

receives the early warning messages and can report to the

rescue operators which can reduce the impact of the mine

explosions. Remote and real time monitoring of the

underground mines is guaranteed with the help of

proposed architecture. Hence the proposed system is a

cost effective, fast and power optimized early warning

system designed using WSN to warn the chances of mine

explosions and to improve the safety of the miners.

VI. IMPLEMENTATION AND RESULTS The sensor module with methane sensor was designed on

PCB board. The methane sensor provides varying output

voltages according to the presence of the gas. As per the

datasheet, load resistor is adjustable and it was taken as

24KΩ. This load resistor at the output end could provide

a voltage fluctuating between 1.0V and 4.99V. The

power consumption was obtained as 925mW which was

found to be low. Voltage across 24KΩ was fluctuating

between 1.0V and 2.6 V. The above readings were taken

by providing a heater voltage supply of 5V to the sensor

for half an hour. After the sensor was heated for 24 hours

(preheat time), the output voltage stabilized to 1.2V.

When the sensor was exposed to gas the output voltage

rose abruptly to 4.8V. Thus, the sensor was found to be

sensitive to the gas and the concentrations of methane

was calibrated accordingly. Program in PIC16f877A

microcontroller was coded using MPLAB IDE v8.63 and

HiTechC compiler was used to compile the program. The

sensor module after interfacing with the PIC

microcontroller was connected serially to the computer to

display output after integration. Results obtained on the

hyper-terminal were as shown in figure 7. The reception

module is planned to be implemented in future.

Fig 7: Output voltage and Message as shown on HyperTerminal

VII. CONCLUSION AND FUTURE WORK The proposed system was designed to develop an

efficient and reliable early warning system which could

be working even at the time of disaster for an ample

period of time. Network lifetime can be increased in this

case by optimizing the power consumption. The

proposed system could provide a low cost, reliable and

efficient early warning system with power optimization

protocols which also helped in increasing the network

lifetime. A transmission module for the proposed system

was developed and the gas concentration values were

seen in the hyper-terminal. In the proposed system only

the methane gas was sensed, while a system which can

monitor all various parameters like humidity,

temperature, [3] oxygen and carbon monoxide can be

implemented. These all parameters will decide the

accuracy in the prediction of mine explosions. Development of an early warning system which can

detect the mine explosions and other problems like

Conference Proceeding 2012 21st Annual Wireless and Optical Communications Conference(WOCC)-April 19-21, Kaohsiung, Taiwan

978-1-4673-0941-7/12/$31.00 ©2012 IEEE -103-

underground roof falls will be taken up as future work.

Also warnings can be prioritized with the level of risks

involved. In the proposed system, the interference from

other systems was not considered and hence a new

system which considers the interference issues can be

implemented. Moreover the research will be extended to

find a more efficient wireless technique which can

reduce the multi-hop delays. Thus, in future an early

warning system which could deal with interference

reduction, delay tolerance, and occurrence of more

specific accidents in mines could be implemented.

ACKNOWLEDGEMENT

We would like to express our immense gratitude to our

beloved Chancellor Shri. (Dr.) Mata Amritanandamayi

Devi for providing a very good motivation and

inspiration for doing this research work.

REFERENCES [1] Qiao Ying-xu, Zhang Zhi-bin, Yang Hong-guo. 2009. Application

of wireless sensor network in the monitoring and control system

of coal mine safety. IEEE Tran, vol. 1, pp. 13–17. [2] A. S. Thorsten Conrad, Peter Reimann. 2008. A hierarchical

strategy for underground early fire detection based on a T-cycled semiconductor gas sensor. Proceedings of the IEEE SENSORS.

[3] Pramit Roy, Sudipta Bhattacharjee, S. Ghosh. 2011. Fire monitoring in coal mines using wireless sensor networks. Proceedings of the IEEE Transaction.

[4] Nuria Oliver, Fernando Flores Mangas . 2006. Healthgear: A real-time wearable system for monitoring and analyzing physiological signals. Proceedings of the IEEE Transaction.

[5] M. Y. Wang Yan, Zheng Ya-ru. 2008. Study on the coal mine personn el position system based on wireless body sensor networks. Symposium on Medical Devices and Biosensors.

[6] H. S. Yuan. 2008. The research of energy efficient MAC protocols in gas monitoring system of mine. International Symposium on Information Science and Engineering.

[7] Y. L. Fei Han, Minming Tong, S. Tang, L. Cai, and H. Dong. 2011. Design of coal mine wireless sensor networks based on piezoelectric sensors for gas monitoring. Proceedings of the IEEE

Transaction.

Conference Proceeding 2012 21st Annual Wireless and Optical Communications Conference(WOCC)-April 19-21, Kaohsiung, Taiwan

978-1-4673-0941-7/12/$31.00 ©2012 IEEE -104-