Node design of coal mine comprehensive monitoring system based on ZigBee

O Introduction
The coal mine integrated monitoring system based on wireless sensor network consists of underground sensor nodes, router nodes, coordinators and surface monitoring computers, data centers, and network servers. Sensor nodes can be fixed underground or worn by personnel. The system collects environmental information such as methane, hydrogen sulfide, carbon monoxide, temperature, and humidity from fixed nodes, and transmits environmental information data to the surface monitoring computer through the wireless sensor network composed of sensor nodes, router nodes, and coordinators; at the same time, personnel positioning can be achieved based on the location information of the nodes worn by personnel, and the identity and physical condition of the corresponding personnel can be obtained through the nodes. This paper mainly introduces the design of sensor nodes.

1 Functional requirements and overall structure of sensor nodes
1.1 Functional requirements
The system does not design dedicated routing nodes, so the sensor nodes not only collect sensor signals, but also act as routers. The sensor signals mainly include methane concentration, hydrogen sulfide concentration, oxygen concentration, temperature, humidity, wind speed, pressure, etc.; personnel information includes basic information such as height, weight, blood type, and name.
1.2 Overall structure
The circuit structure of the sensor node is shown in Figure 1. It mainly includes sensor conditioning circuit, A/D conversion circuit, alarm circuit, general control output circuit and power supply circuit.

2 Hardware Circuit Design
2. 1 Processor and Main Component Selection
The sensor node processor selected is the JN5121 wireless module from Jennic, UK. It is the industry's first low-power, low-cost wireless microcontroller compatible with IEEE 802.15.4. The module has a built-in 32-bit RISC processor, a 2.4 GHz band IEEE 802.15.4 standard RF transceiver, 64 KB ROM, and 96 KB RAM, providing a complete solution for wireless sensor network applications. At the same time, the highly integrated design simplifies the total system cost. The built-in ROM storage of JN5121 integrates the complete protocol stack of point-to-point communication and mesh network communication; the built-in RAM storage of JN5121 can support network routing and control functions without the need for external expansion of any storage space. The built-in hardware MAC address and highly secure AES encryption algorithm accelerator of JN5121 reduce the system power consumption and processor load. JN5121 can be applied to various ZigBee wireless sensor network nodes running in the 2.4 GHz frequency band, including coordinators, routers and terminal devices.
The A/D converter uses the AD7708 of ADI, which is a configurable 10-channel 16-bit A/D converter device developed by ADI in the United States with the advantages of low noise, high resolution, high reliability and good linearity. It uses ∑-△ conversion technology. Its flexible serial interface enables AD7718 to be easily connected to a microprocessor or shift register, and can use the SPI bus to complete communication with the microprocessor. It can be widely used in industrial process control, measuring instruments, portable test instruments, intelligent transmitters, strain measurement and other fields.
2.2 Methane measurement circuit design
The methane concentration measurement uses a domestically produced coal mine methane detection carrier catalytic element. The catalytic element works according to the principle of catalytic combustion effect. The detection element and the compensation element are paired to form the two arms of the bridge. When encountering combustible gas, the resistance of the detection element increases, and the output voltage of the bridge changes. The voltage variable increases in direct proportion to the increase in gas concentration. The compensation element plays a reference and temperature and humidity compensation role. The measurement circuit is shown in Figure 2.

2.3 Design of carbon monoxide and hydrogen sulfide measurement circuit
Domestic electrochemical gas sensors are used for the measurement of carbon monoxide and hydrogen sulfide concentrations. Electrochemical elements work according to the principle of electrochemistry. The electrochemical oxidation process of the gas to be measured on the working electrode in the electrolytic cell is used to keep the working electrode and the reference electrode of the electrolytic cell constant at an appropriate potential through electronic circuits. At this potential, electrochemical oxidation of the gas to be measured can occur. Since the Faraday current generated by oxygen during oxidation and reduction reactions is very small and can be ignored, the current generated by the electrochemical reaction of the gas to be measured is proportional to its concentration and follows Faraday's law. In this way, the concentration of the gas to be measured can be determined by measuring the magnitude of the current. The measurement circuit is shown in Figure 3.

2.4 Selection of other main components
Motorola's air pressure sensor MPX5100 is used for negative pressure measurement, and the integrated digital temperature sensor DSl8B20 is used for temperature measurement.
2.5 Power supply design
For fixed nodes, the node power supply is powered by batteries, and for wearable nodes, it is directly powered by mining lamps. Since it is powered by batteries, its conversion efficiency must be considered when designing the power supply. The power supply design of this system uses three components: CS51412, CS51411, and MAX660. Among them, CS51411 and CS51412 chips are new compensation regulator series products launched by ON Semiconductor. They have high accuracy, excellent switching frequency performance, and complete functions. They are specially used for cellular base stations and wireless communication infrastructure. Their input voltage range is between 4.5 and 40 V. The MAX660 chip is a power conversion chip launched by MAXIM, which can realize the conversion of +3 V to -3 V power supply. The 12 V to 5 V circuit is shown in Figure 4.

3 Node software design
3.1 Development of communication protocol
The design of communication protocol should fully reduce the coupling between the two communicating parties, so that the increase or decrease of nodes does not affect the normal operation of the monitoring computer software (that is, the host computer software does not need to be modified due to changes in the number and type of sensors). At the same time, the host computer monitoring software can automatically parse the relevant information of the sensor node according to the protocol regulations through communication with the node. In combination with the characteristics of the system, a set of application layer communication protocols dedicated to the monitoring system is developed. This protocol runs on ZigBee and is used to standardize the data exchange of the application layer.

The coal mine monitoring application system transmits data in the form of frames, and a frame is a transmission unit. The transmission frame is the data frame actually transmitted in the wireless network, and its structure is: leading + UU encoding packet. The leading byte, data range: 0x61 ~ 0xff, its meaning represents different operation commands in the wireless network. According to different operation commands, the data in the UU encoding packet in the node system can be divided into device description packet, environment data packet, control instruction packet, time synchronization packet, and response packet; followed by the UU encoding packet, all the data to be sent, that is, the actual data, is encoded in the UuEncode mode to obtain the UU encoding packet, of which the maximum is 80 bytes, and the data range of each byte is 0x20 ~ 0x5f printing characters. The frame header can be searched by comparing whether the characters in the received data are greater than 0x61. The hexadecimal data actually to be sent in the wireless network is called the actual frame here, and its structure is shown in Table 1. These data must first be encoded before being sent, and then filled into the UU encoding packet of the sending frame after encoding. Every 3 bytes in the actual frame are converted into 4 bytes in the UU encoded packet, so the maximum capacity of the actual frame is 60 B.

3.2 A/D acquisition program design
A/D acquisition program consists of two parts: initialization configuration of AD7708 and AD interrupt data reading. A/D initialization flow chart is shown in Figure 5.

3.3 Design of node main program
The node main program mainly includes several parts: node initialization, sending device description package to apply for joining the network, reading A/D data, and sending data packets. Its flow chart is shown in Figure 6.

4 Conclusion
The terminal sensor node of the coal mine comprehensive monitoring system based on ZigBee can monitor various information of coal mine underground production in real time and comprehensively, and can timely discover safety hazards and send them to the monitoring computer above the well in time, so that protective measures can be taken in time and effectively. The node is easy to place, data communication is reliable, and it has the ability to automatically enter the network; nodes can be added and removed at will, and networking is convenient; it solves many disadvantages such as troublesome wired network wiring and inflexible node placement. The design of this node provides a good solution for establishing a comprehensive coal mine safety monitoring system.