Research on coal mine safety monitoring system based on wireless sensor network

introduction

    At present, the level of informatization in my country's coal mining industry is low, and the application of information technology is unbalanced. Due to the large number of equipment in the underground continuous mining face, each device operates randomly and independently, resulting in a lack of information communication between each other, and it is even more difficult to achieve comprehensive data network management, thus limiting the radiation of mine automation network technology to the region, which to a certain extent restricts coal mine safety production work. Based on the above reasons, the author of this article designed a coal mine safety monitoring system based on wireless sensor networks, which can collect underground environmental parameters and working condition parameters, and send the collected parameters to the computer above the mine in real time for display, control and over-limit alarm processing.

1 Overall system design

    The coal mine safety monitoring system is mainly responsible for the safety monitoring of the mine production system, including the monitoring of harmful or dangerous components in the mine air (such as methane, carbon monoxide, etc.), the monitoring of the physical state of the mine air (such as wind speed, negative pressure, temperature, etc.), and the monitoring of the operating status of the ventilation equipment (such as equipment start and stop, etc.). In an emergency, it can cut off the power supply and stop the operation of some electrical equipment in the coal mine.

    The system is divided into two parts: aboveground and underground. The underground part mainly uses sensors to collect information on temperature, humidity and gas volume fraction in the mine, and then transmits it to the base station through various nodes through routing; the aboveground part is mainly responsible for receiving data transmitted from the base station, processing and analyzing it through various monitoring subsystems, displaying data in the form of curves, graphics and reports, and supporting various query and statistical functions. The overall structure of the system is shown in Figure 1.

Figure 1 System structure diagram

2 Wireless Sensor Network Design

    This system uses ZigBee technology based on IEEE802.15.4 standard to build a wireless network. ZigBee is a short-distance, low-power, low-cost, high-capacity, and high-reliability wireless network technology . Its operating frequency band is 2.405-2.480 GHZ. It uses direct sequence spread spectrum communication technology, and the data transmission rate is 250 KB/S. The node capacity of the wireless network data transmission module can reach up to 65,000. Each data transmission module can communicate with each other. The distance between each network node can be infinitely extended from the standard 75m, which is very suitable for the collection of underground coal mine information. The network nodes form a mesh topology in a self-organizing form. Each node can collect data autonomously, and the data is sent to the aggregation node through a single hop or multi-hop relay method.

    2.1 Architecture of wireless sensor network nodes

    The wireless sensor network node is the basic unit of the network, responsible for data collection and transmission. Its architecture consists of four parts: data acquisition module (sensor, A/D converter), data processing module (microprocessor, memory ), wireless communication module (wireless transceiver) and power module (battery, DC/DC energy converter), as shown in Figure 2.

Figure 2 Wireless sensor network node architecture

    2.2 Design of sensor nodes

    In the data processing module, the microprocessor is the core component of the sensor node, responsible for processing data and coordinating the entire system. In order to make the system have the characteristics of high performance, low power consumption, and multi-model interface, this design finally uses the 8-bit single-chip microcomputer ATmegal28L produced by ATME Company B as the microprocessor unit. The advanced RISC structure of ATmegal28L gives it high computing performance HJ. It not only has universal interfaces such as UART, SPI, HC, but also has a multi-channel analog-to-digital converter. At the same time, its available open source development software tools are mature and the sensor node operating system TinyOS supports it well.

    Although the ATmegal 28L microprocessor comes with a 4KB EEPROM data storage area, it is very necessary for sensor nodes to have a relatively larger and permanent data storage area to meet the needs of more storage space for automatic update of remote node code, storage of node configuration information, etc. For this reason, an additional 512K AT45DB041 non-volatile FLASH is used as an external data storage in the design of the sensor node.

    In the wireless communication module, considering the data transmission rate, receiving and transmitting power, dormant energy consumption, startup stabilization time and signal modulation of the wireless transceiver, the CC2420 RF chip of Chipcon was finally adopted. The CC2420 RF chip adopts SmartRF technology and is made in 0.18um CMOS process, requiring very few external components. It has stable performance and extremely low power consumption. Since TinyOS already includes CC2420 driver support, it can be connected to the microprocessor more conveniently.

    In the data acquisition module, the temperature and humidity sensor uses SHT11 chip, and the combustible gas sensor uses KGS-20. SHT11 is a highly integrated, low-power, high-precision, and anti-interference digital temperature and humidity sensor chip launched by Sensirion. The chip converts humidity and temperature into electrical signals through two sensitive elements. The electrical signal first enters the weak signal amplifier for amplification, then enters a 14-bit A/D converter, and finally outputs the digital signal through a two-wire serial digital interface. The KGS-20 combustible gas sensor is a semiconductor gas sensor that uses tin dioxide as the basic sensitive material and is specially used for combustible gas concentration measurement. Its basic characteristics are extremely high sensitivity, extremely fast response speed, and low power consumption, which is very suitable for the detection of combustible gas secretion fraction underground.

    In this system, a serial communication module is designed to realize the communication between the base station and the host. The base station collects the data of each node and transmits it to the host through an RS232 serial interface, and the host processes the data.

3 Coal Mine Monitoring and Management Software Design

    The well monitoring management software includes eight subsystems, namely: system management, safety monitoring system, bundle pipe monitoring system, ventilation monitoring system, power supply monitoring system, coal flow metering system, main coal flow monitoring system and GIS system. The software is written in Borland's Delphi7.0, and Microsoft Access 2003 is used as the background database. It can display various parameters in tables, curves, report postures and alarm statistics, as well as generate and print reports of various parameters.

    3.1 System Function Design

    As the core part of the entire coal mine safety remote monitoring system, the safety monitoring subsystem is mainly used to monitor various environmental parameters and major production parameters such as equipment start and stop in the mine. After installing sensors at some important locations, some environmental parameters and equipment start and stop status information can be directly reflected on the ground central station and management network workstation, reducing the workload of relevant inspection and duty personnel in the mine, helping leaders and dispatchers to grasp the safety production situation in a timely manner, and providing data support for command and dispatch.

    The basic tasks of the ventilation system are: to supply sufficient fresh air underground; to remove or dilute toxic and harmful gases and dust in the mine; to adjust the climate conditions in the mine to create a good working environment; and to improve the mine's ability to resist disasters.

    The power supply monitoring subsystem is used to display and query the bus voltage frequency of each underground substation, the current power provided to the power-consuming equipment and various protection equipment, and other parameters. The functions completed include: real-time display of the monitoring data of the central substation, the ventilator power distribution system, and the main inclined shaft power distribution system room; query historical monitoring data according to time, room, measurement point and other conditions; and the system can also generate some user reports.

    The bundle pipe monitoring system can enable leaders at all levels to query and browse the monitoring data of the bundle pipe system. The trend of the monitoring data can be intuitively displayed in a graphical manner. There is a fixed report format for the data items monitored every day, which can be queried. Gas samples will be drawn at least three times a day, and eight types of gases will be drawn at the same time each time. Through the analysis of the monitoring data, the hazard analysis of the gas can be carried out through the "Triangle Explosion-Proof Zone Theory" and the results can be obtained.

    The GIS system is responsible for representing the mine safety geographic information system with two-dimensional graphics. It mainly includes the graphic data system and the attribute data system. The former mainly inputs, stores, and queries graphic data (such as: points, lines, surfaces and their topological relationships, etc.), and also has functions such as graphic analysis and output. The attribute data system mainly completes the attribute data input and management of graphic entities, the output of various attribute data values, etc. However, the two are not isolated. The graphic data and attribute data are dynamically connected through the data interface and database server . The system mainly includes data input, graphic editing, graphic setting, graphic display, professional operation, data output, retrieval query, model analysis and professional legend library. At the same time, it has various functions of the general GIS system, such as: zoom in, zoom out, modify, delete, move, rotate, copy, etc.

    The functions finally realized by the whole system include:

    1) Collection of various sensor parameters: including environmental parameters (such as methane, carbon monoxide, temperature and negative pressure, etc.) and industrial and mining parameters.

    2) Display of various sensor parameters: The various parameters collected by the collector are displayed on the collector's LCD screen and computer respectively.

    3) Over-limit alarm: When it is detected that the parameters of a certain sensor are out of limit, the collector promptly controls the buzzer to sound an alarm, and the computer gives both sound and visual alarms.

    4) Equipment power-off control: When a device needs to be powered off, the collector can operate the device's breaker to achieve control of the device.

    5) Long-distance transmission of data: The parameters collected by each mine collector need to be transmitted to the ground central station for monitoring in a timely manner.

    6) Recording and playback of various sensor parameters: The computer software stores various parameters sent from the lower computer in the hard disk space, and the staff can choose any parameter to record and playback.

    3.2 System Module Design

    System management: It is divided into three parts: user management, log management, and code maintenance. User management is responsible for managing user information, including adding, deleting, modifying user information, and assigning corresponding roles to users. Role assignment is responsible for assigning corresponding roles to existing users in the system to limit and assign the operating permissions of staff with different functions. Log management includes alarm log management and security log management, and the corresponding alarm log and security log can be queried by time.

    The five systems of safety monitoring, ventilation monitoring, power supply monitoring, coal flow metering, and main coal flow monitoring all have high requirements for real-time performance, and the data types reflected are basically similar. Therefore, the system designs these five modules into unified functions and display styles. Each module has five functional points: "real-time display", "real-time curve", "data query", "historical curve", and "report". "Real-time display" mainly completes the display function of real-time data of important parameters closely related to safety or equipment status underground. "Real-time curve" mainly realizes the dynamic curve drawing of real-time data of more important monitoring points or parameters, so that the staff can see the trend of data changes very intuitively, so as to facilitate their analysis of production conditions. "Data query" is the process of viewing some required data, including alarm data query and historical data query. Alarm data query can see some important alarm data, and major alarm historical data can be printed for reporting or self-analysis and processing. "Historical curve" mainly realizes the curve drawing of historical data of more important monitoring parameters over a period of time, so that the staff can see the trend of historical data changes very intuitively, so as to facilitate their analysis of production conditions. "Reports" are designed to organize and compile key data based on the needs of the enterprise, forming intuitive written documents for easy storage and printing, and to provide convenience for enterprise work.

    Since bundle pipe monitoring does not require real-time performance, it is different from the previous system functions and is divided into four functions: "data query", "curve analysis", "report query", and "triangle explosion-proof zone". "Data query" is to query the concentration content of all gases in a table by measuring point location and selecting the start and end dates. "Curve analysis" is to query the concentration percentage curve of each sampled gas at a fixed time by date and measuring point. "Report query" is to generate the "Yangchangwan Coal Mine Bundle Pipe Monitoring Analysis Daily" every day, and can also be queried by date. "Triangle explosion-proof zone" can query all gas sampling records in the explosion-hazardous area, and can display the analysis chart and analysis results of each sampling record, and locate the results on the analysis chart.

    The “GIS system” is divided into six functional modules from the overall function, namely: map browsing, layer control, layer editing, search and positioning, real-time alarm, and information query. The entire system function is shown in Figure 3.

Figure 3 Functional module diagram of coal mine safety geographic information system 

    3.3 System database design

    The database of the "coal mine safety monitoring system" should include the main systems for underground monitoring: power supply, drainage, main coal flow, ventilation, bundle pipe, and security inspection. At the same time, it is also necessary to add the database table information that can be developed by GIS (geographic information system) and the database table information for system management. When designing the table structure, considering the characteristics of this monitoring system, the monitoring data of the security inspection, ventilation, alarm and other modules are separately created.

4 Conclusion

    This paper studies the composition and architecture of the wireless sensor network coal mine safety monitoring system and the software and hardware design methods. The system combines ZigBee wireless network technology with the traditional coal mine safety monitoring system, making up for the limitation of underground monitoring network relying on wired transmission, so that the safety monitoring system can effectively cover the places with potential accidents as much as possible, and provides technical support for the mobile equipment connection of continuous mining working face. Using this technology, it is very convenient to realize the seamless interface between wired network and wireless network, which provides an important reference for the realization of hybrid network.