Wireless sensor networks for mine environment monitoring

Abstract: This paper proposes a method of mine environment detection using wireless sensor networks, and gives the network framework, topology and network protocol of wireless sensor networks suitable for mine environment detection.

In the process of mine environmental monitoring, it is usually necessary to detect parameters such as mine wind speed, mine dust, carbon monoxide, temperature, humidity, oxygen, hydrogen sulfide and carbon dioxide. The existing monitoring and detection system requires the establishment of communication lines in the mine to transmit monitoring information. The structure of the mine is constantly changing during the production process, and some tunnels are small, which puts high demands on the extension and maintenance of communication lines. Once the communication link fails, the entire monitoring system may be paralyzed. To solve the above problems, this paper proposes to use wireless sensor networks to monitor the mine environment. There are three significant advantages of using wireless sensor networks for environmental monitoring: (1) The sensor nodes are small in size and the entire network only needs to be deployed once, so the deployment of sensor networks has little human impact on the monitoring environment; (2) The number of sensor network nodes is large and the distribution density is high. Each node can detect detailed information of the local environment and summarize it to the base station. Therefore, the sensor network has the characteristics of comprehensive data collection and high accuracy; (3) The wireless sensor nodes themselves have certain computing and storage capabilities, and can perform more complex monitoring according to changes in the physical environment. Sensor nodes also have the ability to communicate wirelessly, which can be used for collaborative monitoring between nodes [1]. The computing power and wireless communication capabilities of the nodes enable the sensor network to be reprogrammed and redeployed, and to respond promptly to environmental changes, changes in the sensor network itself, and network control instructions. Even if the mine structure is damaged, it can still automatically restore the network and transmit information, providing important information for mine rescue. These characteristics of wireless sensor networks are particularly suitable for mine environment monitoring.

1 Framework of wireless sensor networks

Sensor network systems usually include sensor nodes, aggregation nodes, and management nodes. A large number of sensor nodes are randomly deployed inside or near the monitoring area and can form a network through self-organization. The data monitored by each sensor node is transmitted hop by hop along other sensor nodes and routed to the aggregation node after multiple hops. Users configure and manage the sensor network through the management node, publish monitoring tasks, and collect monitoring information. Each node collaborates to complete the monitoring task.

The wireless sensor network used for mine environment monitoring must meet the requirements of mine environment monitoring in terms of system structure, topology, node structure, software and hardware working environment, network protocol and positioning mechanism. During the mine environment monitoring process, randomly distributed sensor nodes regularly send the monitored data (such as gas concentration, carbon monoxide concentration, wind speed, temperature and humidity in the well, etc.) to the aggregation node outside the well. The aggregation node transmits the data to the management node, namely the manual control console and the automatic control console, through optical fiber, the Internet or satellite. The manual control console analyzes and processes the data, monitors the underground environment indicators in real time and accurately, and issues early warning messages in a timely manner.

1.1 Network system structure

A sensor network system structure suitable for mine environment monitoring is shown in Figure 1. This is a hierarchical network structure, with the sensor nodes deployed on the mine working face at the bottom layer, followed by the transmission network and base station. Depending on the scale of the mine, the base station information can also be connected to the mine environment monitoring center through the Internet. In order to obtain accurate data, the deployment density of sensor nodes is usually relatively large, and they are deployed in several non-adjacent monitoring areas (such as several mine working faces), thus forming multiple sensor networks. The transmission network is a local network responsible for coordinating the gateway nodes of each sensor network and integrating the information of the gateway nodes. The base station is responsible for collecting all the data sent by the transmission network, sending it to the Internet, and saving the log of the sensor data in the local database. For large-scale centralized mine environment monitoring systems, the data collected by the sensor nodes is transmitted to the central database for storage via the Internet. The central database provides remote data services, and researchers can use remote data services through terminals connected to the Internet to further analyze and process the data.

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1.2 Topology

The most basic requirement for mine environment monitoring is to transmit information in a timely and effective manner, issue early warning messages, and ensure underground safety. To this end, the wireless sensor network adopts a mesh topology. Complete mesh topology control consumes more energy from sensor nodes. In order to save energy as much as possible while meeting network connectivity, a small number of nodes deployed on each working face of the mine are selected as backbone network nodes, and their communication modules are turned on. The communication modules of non-backbone nodes are turned off, and the backbone nodes establish a mesh fully connected network to be responsible for data routing and forwarding. This not only ensures data communication within the original coverage area, but also saves energy to a large extent. The network topology is shown in Figure 2.

The backbone node needs to adjust the work of non-backbone nodes and is responsible for data fusion and forwarding, which consumes relatively large amounts of energy. Usually, the network itself periodically monitors the energy status of each sensor and automatically replaces the backbone node to balance the energy consumption of each node in the network. A few nodes with energy greater than a certain set value are selected as backbone nodes, and the remaining nodes select the backbone node closest to them as their control nodes. If the working faces of the mine are far apart or there are many working faces, a node with strong energy can be deployed on each working face as the backbone node of the backbone node of the working face to complete the information transmission between the working faces.

2. Node hardware and software structure

2.1 Hardware Structure

The node hardware structure is shown in Figure 3 [1]. The sensor node consists of four parts: sensor module, processor module, wireless communication module and energy supply module. The sensor module is responsible for collecting information and converting data within the monitoring area; the processor module is responsible for controlling the entire sensor node, processing the collected data and data sent by other nodes; the wireless communication module is responsible for wireless communication with other sensor nodes, exchanging control information and sending and receiving collected data; the energy supply module provides the energy required for the operation of the sensor node, using micro batteries.

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A dedicated sensor board is loaded through an expansion board. The board is equipped with multiple sensors such as gas concentration, humidity, wind speed, carbon monoxide and carbon dioxide. You can select and switch between multiple sensors to meet different monitoring tasks.

The main controller is an 8-bit low-power microcontroller ATMEGA128L from Atmel. Compared with other general-purpose 8-bit microcontrollers, it has more abundant resources and extremely low energy consumption. It has 128KB of on-chip program memory (Flash), 4KB of data memory (SRAM, which can be expanded to 64KB) and 4KB of E2PROM. In addition, it has 8 10-bit ADC channels, 2 8-bit and 2 16-bit hardware timers/counters, UART, SPI, and I2C bus interfaces. The JTAG port provides a convenient interface for development and debugging. In addition to the normal operating mode, it also has 6 different levels of low-energy operating modes, which are suitable for the energy-saving needs of wireless sensor networks. The wireless transceiver CC1000 is a single-chip UHF (Ultra-High Frequency) transceiver designed for low-voltage wireless communication applications. It is connected through peripheral interface lines to complete the construction and functions of the node hardware part.

2.2 Software Structure

TinyOS is an operating system for sensor networks. It adopts an efficient event-based execution method and uses a component model to achieve efficient modularization and construct component-based application software. The upper-level components send commands to the lower-level components, and the lower-level components send signals to the upper-level components to notify the occurrence of events. The lowest-level components directly interact with the hardware. The component structure of the sensor application that supports multi-hop communication is shown in Figure 4. In response to the hardware circuit and application needs, peripheral hardware drivers are added, mainly for sensor control and data sampling.

3 Network Protocols

3.1 Multipath Routing Mechanism and SPEED Routing Protocol

In mine environment monitoring, detection data needs to be transmitted regularly, in real time and accurately. However, sensor nodes have failure problems due to limited energy and harsh working environment. The routing protocol must ensure that the entire system can work normally even if some nodes fail. Reliable routing protocols mainly consider the following two aspects: (1) using the redundancy of nodes to provide multiple paths to ensure the reliability of communication; (2) establishing a transmission reliability estimation mechanism to ensure the reliability of each hop transmission.

The multi-path routing mechanism is an effective mechanism to ensure communication reliability. Its basic idea is: first establish a main path from the data source node to the aggregation node, and then establish multiple backup paths; data is transmitted through the main path, and the backup path is used to transmit data at a low speed to maintain the effectiveness of the path; when the main path fails, the suboptimal path is selected from the backup path as the new main path.

In order to meet the real-time requirements, the SPEED[3] routing protocol can be used, which can achieve end-to-end transmission rate guarantee, network congestion control and load balancing to a certain extent. The SPEED protocol first exchanges the transmission delay of the nodes to obtain the network load situation; then the nodes use local geographic information and transmission rate information to make routing choices, and at the same time use the neighbor feedback mechanism to ensure that the network transmission rate is above a globally defined transmission rate threshold.

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According to the actual situation, a trade-off is made between the multi-path routing mechanism and the SPEED routing protocol. In the daily regular monitoring data feedback, the accuracy and reliability of the data are emphasized, and the multi-path routing mechanism can meet the requirements. When an emergency occurs and it is necessary to accurately understand the underground situation in real time, the SPEED routing protocol is required. According to the actual situation, the routing protocol autonomous switching module can be used to switch freely between different routing protocols.

3.2 Cluster-based TDMA MAC Protocol

Since the sensor network adopts the topology of backbone nodes and non-backbone nodes, that is, the clustered topology, its underlying MAC layer protocol is also based on this clustered structure design. In the specific environment of the mine, the nodes will not be easily moved, that is, once the topology is stable and the node position is stable, the probability of new nodes joining is very small, so the MAC protocol based on the TDMA mechanism can be used.

In the cluster-based TDMA mechanism MAC protocol, the node status is divided into four states: sensing, forwarding, sensing and forwarding, and inactive. When the node is in the sensing state, it collects data and sends it to its adjacent nodes; in the forwarding state, it receives data sent by other nodes and sends it to the next node; in the sensing and forwarding state, the node needs to complete the above two functions; when the node has no data to receive or send, it automatically enters the inactive state.

Non-backbone nodes send monitored data to backbone nodes in their respective time slots. After a period of data transmission, the backbone node collects the data sent by non-backbone nodes under its jurisdiction, runs a data fusion algorithm to process the data, and sends the results directly to the upper-level backbone node or aggregation node.

In practical applications, the failure of sensor nodes will cause dynamic changes in the topology. In order to make the time slot allocation adapt to such dynamic changes, a time frame is divided into four periodic phases: data transmission phase, refresh phase, refresh-induced reorganization phase, and event-triggered reorganization phase. The MAC protocol reallocates time slots in the refresh and reorganization phases to adapt to changes in the topology of nodes in the cluster and changes in node status.

4 Positioning Mechanism

When a gas leak occurs underground, the gas leak point must be found as soon as possible for repair. At this time, the node with the highest gas concentration must be the node closest to the gas leak point, and the node must send location information to the management node.

In order to obtain detailed location information of the nodes, three or more beacon nodes are installed on each working surface. The beacon nodes periodically transmit radio frequency signals and ultrasonic signals. The radio frequency signal contains the location information of the beacon node, while the ultrasonic signal is just a pure pulse signal. Since the transmission rate of the radio frequency signal is much higher than that of the ultrasonic signal, when the node receives the radio frequency signal, it also turns on the ultrasonic signal receiver and calculates the distance from the node to the beacon node based on the interval between the arrival times of the two signals and their respective propagation speeds. After calculating the distance to three or more beacon nodes, each node uses the triangulation method to calculate the coordinates of the node [1]. Finally, corrections are made to obtain accurate node coordinates.

Wireless sensor networks have low power consumption, can form networks by themselves, and have good reliability and maintainability. Its emergence provides a new means of simple deployment and high reliability for mine environment monitoring.

References

1 Sun Limin. Wireless Sensor Networks. Beijing: Tsinghua University Press, 2005

2 Arisha KA, Youssef MA, Younis M F.Energy-aware TDMA-based MAC for sensor networks.In: Proc IEEE work-shop on integrated management of power aware communications, computing and networking, New York, NY, 2002
3 Kumar R ,Wolenetz M, Agarwalla B et al.Dfuse: A framework for distributed data fusion.In:Proc 1st ACM conf on embedded networked sensor systems, Los Angeles, CA, 2003