Design of wireless gas monitoring system based on ZigBee

　　The traditional gas monitoring system is a data acquisition system composed of wires and cables as the basic transmission medium. At present, the commonly used solution is to place sensors at key locations that need to be monitored at the monitoring site, and transmit the signals collected by each sensor to the central acquisition station through independent cables. The central acquisition station centrally processes all connected signals and sends them to the host computer for real-time data acquisition. If the area to be monitored is large, many sensors are required, and the corresponding monitoring points are scattered, this traditional wired method will have problems such as complex line layout, cumbersome wiring, high installation cost, and high cable maintenance cost in the later stage. Due to the limitations of the wired gas monitoring system itself, network coverage and network support in many special environments are still a problem. For example, in some industrial sites, some industrial environments prohibit or restrict the use of cables, while in other industrial environments, cables are required to be completely shielded to highly prevent interference from machines or other radio control equipment in most industrial facilities. Some high-speed rotating equipment cannot transmit data information through cables. This traditional serial centralized monitoring system greatly affects the processing speed, reliability and flexibility of the system.

　　In view of the above situation, the design of gas monitoring system tends to wireless monitoring, using distributed wireless technology to replace traditional centralized wired technology to achieve direct communication between sensors and data acquisition systems. The emergence and development of wireless sensor networks provides a new solution to this problem. After comparing several communication protocols, this paper selects the low-cost, low-power and low-rate ZigBee protocol and designs a wireless gas monitoring system based on ZigBee.

　　1 Overall system design

　　1.1 System composition and structure

　　The entire monitoring system consists of a monitoring host and a ZigBee wireless sensor network, as shown in Figure 1. This is a hierarchical network structure, with the sensor terminal node (i.e., wireless gas detection transmitter) at the bottom, followed by routers, coordinators, and monitoring hosts. The monitoring host runs data management software, which provides users with a data access interface in the gas monitoring network, and can access the measurement data of the terminal node through the coordinator, and save the data, providing users with data query, analysis, or report generation services. The ZigBee network is responsible for data collection, and it consists of a coordinator, a router, a terminal node, and an alarm. The terminal node transmits the monitored data to the router; the router integrates and processes the information uploaded by the terminal device, and then sends the data to the coordinator; the coordinator uploads the collected data to the monitoring host or sends the command of the monitoring host in the network; the alarm is responsible for receiving the data sent by the router, and sends an alarm signal if the data exceeds the limit. The transmission of data in the entire network follows the principle of proximity. If the coordinator is closer than the router, the terminal node will directly transmit data to the coordinator.

　　Figure 1 System overall design

　　1.2 System Features

　　The wireless gas monitoring system has the following characteristics:

　　(1) Using ZigBee technology as a wireless communication method

　　Currently, the commonly used wireless communication technologies include Wi-Fi, ultra-wideband communication UWB, Bluetooth, infrared data communication IrDA, ZigBee, etc. After comprehensive comparison of the above wireless communication methods, ZigBee technology has the advantages of power saving, reliable communication, low cost, large network capacity, and strong white organization ability. Therefore, ZigBee technology can be well applied to wireless gas monitoring systems.

　　(2) Low power consumption

　　The terminal nodes of the system - wireless gas detection transmitters are numerous and powered by batteries. They are often placed in unattended places, which makes it inconvenient to replace the batteries of the nodes. Therefore, achieving low power consumption of the nodes is not only an important indicator for improving node performance, but also an important step to extend the life cycle of the entire system. The wireless gas monitoring transmitter of this system is powered by disposable No. 2 lithium batteries, and its service life under normal working conditions is not less than 20,000 hours.
 

　　2 Design of wireless gas detection transmitter

　　2.1 Hardware Design of Wireless Gas Detection Transmitter

　　The wireless gas detection transmitter is the front-end acquisition device of the entire system and the core component of the system. The transmitter consists of a central processing unit, measurement circuit, A/D conversion, digital display, RF wireless communication and other unit circuits. The signal generated by the sensitive element is linearly amplified, converted by the A/D converter, and directly sent to the microcontroller for data processing to complete the display, communication and other functions of the transmitter. Its principle block diagram is shown in Figure 2.

　　Figure 2 Schematic diagram of wireless gas measurement transmitter

　　The central processing unit uses MICROCHI P's 64-pin 8-bit CMOS flash microcontroller PIC16F946 with LCD driver and nanoWatt technology. This microcontroller can control the power management mode by selecting the clock through software, that is, turning on the CPU and peripherals when running, turning on the peripherals but turning off the CPU when idle, and turning off the CPU and peripherals when sleeping; the A/D conversion unit uses TI's 12-bit AD with a sampling speed of 200kSPS ADS7866. In this A/D conversion unit, the operating voltage of ADS7866 is kept at 1.3 V, achieving the purpose of saving power.

　　The operational amplifier uses LTc1495, the operating current is only 1.5 uA, and the ±2.5V voltage regulator chip also uses a low-power chip, which minimizes power consumption while meeting performance requirements. The preamplifier circuit is shown in Figure 3.

　　Figure 3 Wireless gas detection transmitter fire circuit diagram

　　The wireless communication module adopts AT86RF212, the world's first wireless transceiver chip launched by Atmel for the Chinese wireless market, which works in the 780MHz frequency band and complies with the IEEE802.15.4 standard. China's wireless personal area network standard has been approved by the Radio Commission and opened the 780MHz frequency band. In this frequency band, the space loss is smaller than that of the 2.4Ghz frequency band, and better transmission performance can be obtained.

　　2.2 Wireless Gas Detection Transmitter Software Design

　　The components of each module of transmitter software design are shown in Figure 4.

　　The main program processing module is used to call other modules to complete the functions that need to be implemented; the sampling processing and compensation module is responsible for collecting and processing the data after analog-to-digital conversion and making corresponding linear compensation; the key query processing module is responsible for key action management; the display module is used to realize LCD display; the parameter management module is responsible for parameter storage and processing; the white detection module is responsible for completing initialization; the wireless communication module is responsible for sending detection data.

　　Figure 4: Software design module for wireless gas detection transmitter

　　The main function of the wireless gas detection transmitter is to collect data from the object being measured through the gas sensor and send it to the router (or coordinator) through the wireless transmission module. Under normal working conditions, data is sent every 15 seconds, and under alarm conditions, data is sent every 8 seconds. The program flow chart is shown in Figure 5.

　　Figure 5 Wireless gas sensor transmitter program flow chart

　　3 Design of Wireless Coordinator

　　3.1 Hardware Design of Wireless Coordinator

　　The wireless coordinator is mainly responsible for establishing the WSN network, receiving the measurement data sent by all wireless detection nodes through the wireless communication module, and transmitting the measurement data to the PC. Its structure is relatively simple, mainly composed of a power module, an EMI processing module, a microcontroller module, a serial communication module and a line communication module, and its composition block diagram is shown in Figure 6.

　　Figure 6 Block diagram of the wireless coordinator

　　3.2 Software Design of Wireless Coordinator

　　The wireless coordinator is the initiator of the wireless gas monitoring network. After the coordinator is powered on, it completes the initialization and establishes the network. When it receives a request from other nodes to join the network, it assigns a network address to the node, receives data transmitted by the router or the terminal node close to it, and forwards the data to the host computer. The program flow chart is shown in Figure 7.

　　Figure 7 Wireless Coordinator Program Flowchart

　　4 Conclusion

　　The ZigBee-based wireless gas monitoring system has the advantages of low cost, low power consumption, dynamic routing, automatic networking, free protocol, and simple application. It solves the problem of limited mobility of field equipment and flexible changes in network structure when cables are connected, making data transmission more flexible and easy to implement. Therefore, it is very suitable for use in industrial sites and will gain more and more development and application space.