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Abstract
This paper addresses an economical, supple, continuous monitoring system of underground mine
workers’ protection and security. A module of MEMS based sensors are used for monitoring
underground parameters as per the requirement of the user and automating sequence of measuring
data through digital wireless communication system is projected with high precision, soft control and
reliability. A microcontroller based system is used for collecting and storing data and making decision
accordingly, based on which the mine worker is informed through different alarm tone as well as voice
system. The voice system with both microphone and speaker, converted into digital signal and
successfully communicate wirelessly with the ground control centre computer system. The
communication system is reliable based on ZigBee, IEEE 802.15.4 standard. This is used for
transmission between the hardware circuit fitted with the mine workers and the ground control centre
computer system through some routers.
Introduction
Safety is the most vital part of any type of industry. Negligence in the safety part may cause damaging of high
quality equipment hampering of production or may cause loss of human life also in extreme cases. In the mining
industry safety and security is a fundamental aspect of all. To avoid any types of unwanted phenomena all
mining industry follows some basic precaution and phenomena [1]. Communication is the most vital key factor
today, to monitor different parameters continuously and to take necessary actions accordingly to avoid any types
of hazards related to production, security, managing of human resources. To avoid loss of material and
damaging of human health, security and safety system as well as reliable continuous faithful communication
system is essential in the interior of the underground mines. To enhance security, safety and productivity in
underground mines, a reliable communication system must be established between workers, moving in the mine,
and a fixed base station. The communication network must not be interrupted at any moment and at any
condition. Inside underground mines, the wired communication network system is not so effective. The
reliability and long life of conventional communications systems in harsh mining environments has always been
a problem. Inside mines due to uncomfortable situation the installation cost as well as maintenance cost is high
for wired communication networks. It is very difficult to reinstall the wired communication system inside mines
after a landslide or damage due to any reason. If due to some reason any wire of the communication network
damages, it may cause temporary interruption of the continuous process or may cause a long term break down of
the system.
Due to roof slide, if by any means some workers trapped inside mines, it is very
much required to maintain the continuity of the communication system. It is very much important to know the
actual position and condition of the trapped workers. To monitor other parameters during this condition it is
very much necessary to maintain the communication system as usual. Accordingly, development of mine monitoring system to accurately detect temperature, pressure, flammable and poisonous gas and to track
underground miners and vehicles on real-time has significant meaning to safety production and rescue of
underground mine disaster.
Coal mine safety monitoring system based on wireless sensor network can timely and accurately reflect dynamic
situation of staff in the underground regions to ground computer system [2]. A hybrid tunnel radio propagation
model consisting of the free space propagation and the modified waveguide propagation is proposed in [3]. But,
using this popular radio communication inside mines has some disadvantages. When radio signals are
transmitted, diffraction, attenuation, multi-path and scattering are often very serious [4]. So, wireless
communication is the burning need today for the rapid, precise, flexible, safety, uninterrupted process in
underground mines.
There are different other research ideas proposed by different people on wireless communication. In [5], a
network called chain-type wireless underground mine sensor network (CWUMSN) is recently proposed which
consists of three kinds of sensor nodes: sensing nodes, cluster head nodes, and a base station deployed on both
sides of the tunnel at regular intervals to monitor the underground environment and locate the miners. A new
decision-making approach to coal and gas outburst prediction with multi-sensor information fusion is proposed
in [6]. Two of the multi-sensor information fusion methods- neural network and the dempster-shafter evidence
theory, were taken into account, and the improved combination rules in fuzzy sets was given for decision fusion.
But, those communication methods having specific technology lacks in practical application in underground
mines.
For the successfully wireless data transmission, in this work the ZigBee specification is utilised. There has been
increased interest in the ZigBee standard, in particular for building automation and industrial controls since its
release in 2004 [7]. Though the specification was available publicly in 2005, people prefer using this standard
among different wireless protocol for diversified applications. In [8], an agent-based wireless local positioning
system with ZigBee technology is proposed, mainly for factory level applications. A cost effective ZigBeebased
wireless mine supervising system with early-warning intelligence on methane, temperature, humidity in
mining area is proposed in [9]. Again, another article [10] presents the development of a system integrated to a
ZigBee network to measure the whole human body vibration, for the persons exposed to vibratory environment.
ZigBee specification is incorporated by many manufacturers in their devices because of its low power
consumption and decreasing development cost. In the work presented here, Digi make XBEE24 product is used
here for transmitting and receiving data wirelessly.
2 Description of the scheme
The developed system can be divided into two sections. First is a hardware circuit that will be attached with the
body of the mine workers. This may be preferably fitted with the safety helmet of the workers also which should
be mandatory in the premises of any underground mines. An additional stand by system can be fitted with the
wrist of the underground mine workers if required. The circuit has a sensor module consisting of some MEMS
based sensors that measures real-time underground parameters like temperature, humidity concentration of
different gases, vibration inside mines etc. Gas concentration is meant for the harmful gases like methane and
carbon-monoxide etc. Some of the gases are toxic and some are inflammable. A microcontroller is used with the
sensors to receive the sensor outputs and to take the necessary decision. The microcontroller can store data’s as
required by the user for maintaining of records.
Once temperature is more than the safety level preprogrammed at microcontroller, microcontroller decodes
beep alarms through the headset speaker connected with controller as shown in Fig. 1. Again, once the measured
humidity value is more than the safety level preprogrammed at microcontroller, it decodes different type of beep
alarms. Similarly when gas concentration crosses the safety level, microcontroller decodes siren alarms. In all
such cases, this will send an alarm through an urgent message and alarm sound to the ground control terminal
through zigbee. For the voice CODEC the low size, low power,CMX639 is used which is a continuously
variable slope delta modulation (CVSD) digital voice communication systems. With its robust and selectable
coding algorithms, 8kbps to 128kbps data/sampling rates, supported internal clock signals makes it versatile
[11]. It has analog input interface with encoder that connects the microphone and microcontroller and also an
analog output interface with decoder that connects speaker/headset and microcontroller. Communication
through these encoding and decoding of voice and alarm signals is effectively established with the help of
microcontroller. The microcontroller data is transmitted through two separate boards i.e. ZigBee transmission
module to the data collector or receiver module. The microcontroller used here is PIC 16F877A with 20MHz
operating frequency. It has five I/O ports, eight A/D input channels and 368 bytes data memory.
Data Transmission through Zigbee
The main characteristics of ZigBee network are simple implementation, low power consumption, low cost
interface, redundancy of devices, high node density per physical layer (PHY) and medium access control layer
(MAC). Besides, they allow the network to work with a great number of active devices. ZigBee is based on
IEEE 802.15.4 standard in terms of the PHY and MAC layers [12]. IEEE 802.15.4 defines two kinds of devices:
the Full Function Device (FFD) and the Reduced Function Device (RFD). The FFD has the function to
coordinate the network and consequently has access to all other devices. The RFD is limited to a star topology
configuration, not being able to work as a network coordinator, so it does not have all the protocol services. The
FFD and RFD devices can operate in three different ways at the ZigBee standard as the ZigBee coordinator
(ZC), ZigBee Router (ZR), or ZigBee End Device (ZED). The network layer supports three topologies: star,
cluster tree and mesh as shown in Fig. 4. A star topology consists of a coordinating node and of one or more
FFD or RFD which communicates with the ZC. At the cluster tree, the final devices can be associated to the
network by the ZC and the ZR helping the increasing of number of nodes and the network scope. At the mesh
topology, the FFD can distribute messages directly to other FFD. To enter the network, each device receives an
address given by ZC or a ZR.
X-CTU is a windows-based application provided program designed to interact with the firmware files and to
provide a simple-to-use graphical user interface. Each of the four tabs there has a different function. PC Settings
tab allows selecting the desired COM port and configuring that port to fit the zigbee settings. As shown in Fig. 5
baud rate, type of flow control and no of bits are required to set before the operation. The Test / Query button is
used to test the selected COM port and PC settings. A response is received if the communication between them
is correct. The range test tab is used to verify the range of the radio link by sending a user-specified data packet
and verifying the response packet is the same, within the time specified (Fig. 6). Terminal tab accesses to the
computers COM port with a terminal emulation program. This tab also allows to send and receive predefined
assemble packet data or data in Hex and ASCII formats using suitable commands. A complete list of commands
is available in the product manual [13]. Terminal tab of the X-CTU software is also used to change the RF
module's DL (Destination Address Low) parameter and save the new address to non-volatile memory. Modem
Configuration tab is used to program the device firmware settings via a graphical user interface. It is also utilized to restore default parameter values of the RF module. A view of the transmitter module is shown in Fig.
7.
4 Data Management software
A software, developed here is to make an interactive, reliable monitoring and management of sensed data and
alarm. The system software is made using Visual Basic which helps to form graphical user interface. It can
display the parameters in the forms of bar chart, table and graphical display. Also, it generates and prints the
reports of the parameters. The different environmental parameters received by the ground control PC are
displayed in those manners in the LCD screen. The parameters include the temperature, humidity, concentration
of methane and carbon monoxide gases etc. The computer stores the parameters in the hard disk and ground
staff can choose any of the parameters for recording and replaying. When it is found that the parameters
received have exceeded the limit set, the microcontroller will control the alarm buzzer to ring in time, and the
computer at ground control centre also gives the alarm ring and the alarm pictures.
Based on the alarm received ground staff takes decision and establish voice communication with the
underground workers. The safety department people are sent alarm. Respective control and safety measures are
taken accordingly based on the continuous monitoring of situation and voice communication with underground
people.
Conclusion
Traditional mine security system can be effectively replaced by the surveillance and safety system proposed in
the paper. This paper gives a system related to safety and security of under ground mines. The system is reliable,
faithful, uninterrupted, economical and user friendly. A larger area and more depth inside hazardous
underground mines are now can be covered and potential accidents can be controlled effectively. The system
combined the low power, low cost Zigbee based high frequency wireless data transmission technology with
modern age MEMES based small size sensors. The sensor and zigbee module can be preferably installed over
the helmet of mine worker. Proper monitoring and conversation is possible between the workers and the ground
staff which can help to take appropriate actions more rapidly and smartly. The system also can be easily
extended with ZigBee wireless image transmission facility in future; it will improve scalability of underground
environment and extend accurate position of miners.