Hardware Design of Gas Wireless Monitoring System Based on ZigBee

　　With the development of computer and wireless communication technology, computer and wireless communication technology have begun to be applied to mine safety and production monitoring, widely involving computer application technology, wireless communication technology, sensor technology, information transmission technology, electrical explosion-proof technology, applied chemical technology, control technology, optical fiber technology and embedded technology. ZigBee is a short-range, low-complexity, low-power, low-data-rate, low-cost two-way wireless communication technology. The complete protocol stack is only 32KB, which can be embedded in various devices and supports geolocation function. These characteristics determine that ZigBee technology is very suitable for application in wireless sensor networks. Compared with various existing wireless communication technologies, ZigBee technology has relatively low power consumption and cost, and has great advantages. The coal mine gas wireless monitoring system designed in this paper is a wireless sensor network based on ZigBee technology, which is used to monitor the gas concentration in the mine air in real time, realize the reliability, easy installation, low cost, energy saving and practical feasibility of the system, so as to provide certain guarantees for the smooth progress of coal mine production work, and play a certain role in disaster prevention, disaster reduction and improving production efficiency in mines.

　　Overall design of coal mine gas wireless monitoring system

　　This system is composed of multiple self-powered ZigBee nodes. Each ZigBee node can collect data about the surrounding environment, perform simple calculations, and communicate with other nodes and the outside world, so that many sensors can work together to perform high-quality sensing and form a collection system with good fault tolerance. This system mainly includes ZigBee nodes and ZigBee base station nodes. The overall system structure is shown in Figure 1. The ZigBee base station node is mainly used to combine the data obtained from each sensor node and is responsible for communicating with the outside world. The node is based on an embedded system. This system adopts a partial mesh topology. Its main advantage is that the range of each node is multiplied, and there is no maximum communication distance limit because all its nodes are used as repeaters or routers. In Figure 1, node A transmits signals to the ZigBee base station node through the following path: A-B-ZigBee base station node. Another alternative path is: A-E-F-ZigBee base station node. There are also several other redundant paths.

　　1 ZigBee Node Design

　　The main function of the ZigBee node is to collect gas concentration data and send the data to the base station of each ZigBee node group. The ZigBee node is mainly composed of five parts: sensor module, MSP430F149 module, CC2420 transceiver module, storage module and power management module, including power module indicator/switch, sensor module interface, USB interface, antenna interface, etc.

　　The sensor module of the ZigBee node is responsible for the collection and data conversion of information in the monitoring area. In this design, the sensor module uses a gas sensor and a temperature and humidity sensor; the MSP430F149 module is responsible for controlling the processing operations, routing protocols, synchronous positioning, power consumption management, task management, etc. of the entire node. The MSP430F149 processor itself has an A/D function, and the analog signal obtained from the sensor can be directly sent to the MSP430F149 for analog-to-digital conversion with low power consumption; the CC2420 transceiver module is responsible for wireless communication with other nodes, exchanging control messages and sending and receiving collected data, providing an SPI interface to communicate with the MSP430F149, and the MSP430F149 is connected to human-computer interaction interfaces such as keyboards and displays, sensors, controllers, etc.; the storage module is responsible for storing the collected data; the power management module provides energy and selects the sensors used, and is composed of two 1.5V alkaline batteries.

　　①MSP430F149 module

　　The MSP430F149 module, i.e. the MCU module, uses the MSP430F149 of TI, as shown in Figure 2. TI's MSP430 series microcontroller is an ultra-low power mixed signal controller that can work at ultra-low power consumption at low voltage; its controller has powerful processing capabilities and rich on-chip peripherals; the microcontroller with FLASH memory can also be conveniently and efficiently online for simulation and programming. The MSP430F149 is the most powerful microcontroller in the MSP430X1XX series, and the operating environment temperature range of the MSP430F149 is -40℃～+85℃, which can adapt to various harsh environments.

　　②CC2420 transceiver module

　　CC2420 is the first RF transceiver introduced by Chipcon that complies with the 2.4GHz IEEE 802.15.4 standard. This device is the first RF device suitable for Zigbee products. It is based on Chipcon's SmartRF03 technology and is made in 0.18μm CMOS process. It requires very few external components, has stable performance and extremely low power consumption. The selectivity and sensitivity index of CC2420 exceeds the requirements of the IEEE 802.15.4 standard, which can ensure the effectiveness and reliability of short-distance communication. The wireless communication equipment developed using this chip supports a data transmission rate of up to 250Kb/s, and can achieve multi-point to multi-point rapid networking. Its peripheral circuits include three parts: crystal oscillator clock circuit, RF input/output matching circuit and microcontroller interface circuit. The chip local oscillator signal can be provided by an external active crystal or by an internal circuit. When provided by the internal circuit, an external crystal oscillator and two load capacitors are required. The size of the capacitor depends on the crystal frequency and input capacitance and other parameters. The peripheral circuit of the CC2420 transceiver module is shown in Figure 3.

　　③ Storage module

　　The storage module uses STMicroelectronics' 1Mb serial EEPROM chip M24M01. M24M01 uses a micro SO8N package with a package shell width of only 150mil (3.8mm). It has a built-in I2C two-wire serial port, which is an ideal choice for storing massive data whose parameters are frequently modified. The storage density ranges from 1Kb to 1Mb. M24M01 supports 100kHz and 400kHz clock frequencies, is fully compatible with I2C specifications, and has a wide operating power supply voltage of 1.8 to 5.5V. The 128Kb×8 storage structure supports byte and page write modes, and the write operation is extremely fast, with a 256B write operation of less than 5ms. The data storage capacity exceeds 40 years, the erase and write endurance is not less than 1 million erase and write cycles, and the operating temperature range is -40℃ to +85℃. The storage module is shown in Figure 4. [page]

　　④Sensor module

　　There are many types of sensors that can detect physical quantities such as temperature and humidity, light, noise, vibration, magnetic field, acceleration, gas concentration, etc. The sensor module used in this system uses a gas sensor and a temperature and humidity sensor.

　　The temperature and humidity sensor adopts the SHTxx series, which supports low power consumption mode and automatically enters sleep mode after collecting data. The current is less than 1μA. This design uses the SHT11 temperature and humidity sensor, which is a digital temperature and humidity sensor chip launched by Scnsirion of Switzerland. This chip is widely used in HVAC, automobile, consumer electronics, automatic control and other fields. The SHT11 temperature and humidity sensor adopts SMD (LCC) surface mount package, and the interface is very simple, as shown in Figure 5.

　　The gas sensor uses the KGS-20 low-power gas sensor produced by Beijing Dongfang Jihua Technology Co., Ltd., as shown in Figure 6. KGS-20 uses tin dioxide as the basic sensitive material and is a semiconductor gas sensor specifically used for detecting the concentration of combustible gas. Its basic characteristics are: extremely high sensitivity, extremely fast response speed and low power consumption. The KGS-20 combustible gas sensor is suitable for detecting the concentration of combustible gases such as gas, and is used for gas alarms, combustible gas alarms, gas detectors, etc. The sensor is small in size, low in power consumption, and has a simple application circuit. The alarm concentration is methane ≥1%, the response time is ≤20s, the recovery time is ≤30s, the operating temperature range is -15℃～+50℃, the humidity is ≤97%RH, the static power consumption is 150mW, the alarm state power consumption is 300mW, and the power supply voltage is 3～5VDC. The change in the resistance RS of the sensor sensitive component is manifested as a voltage change on the load resistance RL. The values ​​of the drive voltage VH, the load resistance RL and the detection voltage VD refer to the typical data provided by the company to keep the sensor in the best working state.

　　The design of the sensor power supply circuit is very important for the energy consumption of the sensor module. For sensors that work with low current (hundreds of μA), they can be directly driven by the processor I/O port; when the sensor is not used, the I/O port is set to input mode. In this way, the external sensor has no energy input and no energy consumption.

　　⑤Power management module

　　There are many types of batteries. The amount of energy stored in a battery is related to factors such as shape, diffusion rate of active ions, and selection of electrode materials. The batteries in wireless sensor network nodes are generally not easy to replace, so battery selection is very important, and the efficiency of the DCDC module is also crucial. This design uses 2 AA batteries for power supply.

　　2 ZigBee Base Station Node Design

　　The ZigBee base station node is mainly composed of an ARM module, a storage module (NAND Flash, 64MB-1GB optional), a CC2420 transceiver module, a sensor module, and a power management module. The ARM module uses the STR710FZ2T6 microprocessor produced by ST. The STR710FZ2T6 is an ARM processor based on the ARM7 instruction set. It has 256KB of Flash and 64KB of SRAM on the chip, and the processor integrates many peripheral interfaces, including USB2.0 and Ethernet interfaces. The design block diagram of the ZigBee base station node is shown in Figure 7.

　　Coal Mine Gas Wireless Monitoring System Architecture

　　Generally, the length of a coal mine is more than 10km, and more than 100 ZigBee nodes need to be deployed. These nodes are powered by batteries and perform networking, perception, sampling and preliminary data processing tasks. Each node is required to communicate with each other and have strong coordination capabilities. By forwarding information between nodes, a multi-hop communication network is formed between nodes. The network designed in this project adopts a MESH mesh network structure to ensure the reliability of data transmission, and a ZigBee base station node (network terminal) is set up in one area, and the data is uploaded to the control center through the network through the ZigBee base station node.

　　in conclusion

　　The design of this system combines the current status of coal mine safety production and the development status of ZigBee technology. It uses the low-power microprocessor MSP430F149 and the corresponding peripheral circuit to design the ZigBee node, which can collect temperature, humidity and gas test data and transmit them to the host computer through the ZigBee base station node. As a short-distance, low-complexity, low-power, low-data-rate, low-cost wireless network technology, ZigBee technology will become the most ideal application solution and development trend for realizing wireless monitoring systems with the further development of technology.

References:

[1]. MSP430F149 datasheet http://www.dzsc.com/datasheet/MSP430F149_.
[2]. CC2420 datasheet http://www.dzsc.com/datasheet/CC2420_213269.html.
[3]. MSP430 datasheet http: //www.dzsc.com/datasheet/MSP430_490166.html.
[4]. M24M01 datasheet http://www.dzsc.com/datasheet/M24M01_62922.html.
[5]. SO8N datasheet http://www.dzsc. com/datasheet/SO8N_1182496.html.
[6]. SHT11 datasheet http://www.dzsc.com/datasheet/SHT11_607252.html.
[7]. 30s datasheet http://www.dzsc.com/datasheet/30s_2233621.html.
[8]. STR710FZ2T6 datasheet http://www.dzsc.com/datasheet/STR710FZ2T6_.