Design of Gas Monitoring System Based on ZigBee

1 Introduction

Real-time knowledge of underground gas concentration is an important factor in coal mine safety production. Due to the increase in coal mining depth and scale, it is difficult for various wired detection equipment to follow up in time, resulting in the difficulty of timely transmission of real-time environmental data underground to the ground monitoring center. Especially in the event of a sudden disaster, various wired communication equipment is almost paralyzed, which brings great difficulties to the rescue work. Therefore, it is particularly important to find a method to collect underground environmental information in time at any time. Here, the design of the gas collection terminal and the wireless communication module CC2420 is discussed.

2 System overall structure

Figure 1 shows the overall structure of the gas monitoring system. It consists of a ground monitoring center, an underground ZigBee transmission network, and a gas collection terminal. The design concept is to use different gas collection terminals to collect gas at each collection point, relay and transmit the data through the established Mesh wireless communication network, and finally reach the ground monitoring center through step-by-step routing to achieve dynamic display, analysis and other processing.

In accordance with the need for reliable data transmission, the system adopts ZigBee's unique Mesh network mode, which automatically links network repeaters for data transmission in a step-by-step routing manner. When the optimal communication path in the network fails, the Mesh network will reselect the most suitable path for data communication from other redundant paths. Therefore, the Mesh network effectively shortens the delay of information transmission and improves the reliability of network communication. In addition to sending data from the node itself, the FFD routing node based on Zigbee technology is also responsible for forwarding data from other nodes to the central node, thus forming a wireless communication network.

3. Working of Gas Monitoring System

The gas monitoring system is arranged at the test site. Its main tasks include: multiple sets of data acquisition. The system converts the analog signals transmitted by the sensor into digital signals through A/D converters at a high sampling rate; data processing. The system can analyze the collected multi-channel sensor data in real time, make decisions on the results and plan the execution sequence; emergency processing. The analysis results can control the alarm system to alarm if there are dangers such as methane exceeding the safety range or other fault phenomena; data communication. The gas monitoring system has a high baud rate and stable wireless communication function, and maintains real-time communication with the remote host computer of the ground command and monitoring center for underground data collection.

3.1 Gas collection terminal design

The gas sensor used in the gas collection terminal is a thermal catalytic element, also known as a combustion carrier catalytic element. Its detection principle uses a catalytic element, a compensation element and a bridge arm resistor to form a Wheatstone bridge. Since the skeleton of the thermal catalytic element is a platinum wire material, a constant voltage is added to the bridge, and the current is heated when it flows through, so that the temperature reaches up to 500°C. Therefore, when encountering gas, the gas contacts the surface of the catalytic element to produce an oxidation reaction, that is, "flameless combustion", which generates a large amount of heat, causing the temperature of the catalytic element to rise, the resistance value to increase, and the bridge outputs an unbalanced voltage, which reflects the concentration change of the measured gas. The detection circuit of the catalytic gas sensor is shown in Figure 2.

After signal proportional amplification and filtering, u0 will perform two tasks: one is to enter the A/D conversion and calculation processing inside the MCU; the other is to compare the amplified voltage, i.e. the A/D input value Vadc, with the dangerous reference signal VH obtained from the locator R13 through the comparator. If Vadc>VH, the output terminal PB01 outputs a high level of 5 V, and the MCU generates an alarm control signal, which means that the gas concentration has reached a dangerous value and a dangerous alarm signal needs to be triggered; on the contrary, if Vadc

In coal mine safety regulations, gas concentration is expressed as a percentage, and accidents are more likely to occur between 5% and 16%. It is necessary to establish an approximate linear relationship between Vadc and the concentration percentage so that the final expression value is also the corresponding percentage. The calibrated gas concentration percentage obtained through experiments is:

In the formula, 0.001 6 is the correction value. It is stipulated in the design process that when the gas concentration reaches 6%, the MCU issues a pre-alarm signal; when the gas concentration reaches 16%, Vadc>VH, that is, 2.85 V, the MCU issues a danger alarm signal. Considering the unexpected accidents, the gas concentration detection range of the whole system is determined to be 0% to 50.5%.