Agricultural irrigation is a large water user in China, and its water consumption accounts for about 70% of the total water consumption. According to statistics, due to the drought, the average grain-affected area of ​​our country reaches 20 million hectares per year, and the food consumption accounts for 50% and 50% of the grain reduction in the country due to disasters. Since the endurance, due to the backward technology and maintenance level, the cost of irrigation water is very serious, and the use rate of agricultural irrigation water is only 40% to 40%. It is assumed that based on monitoring soil moisture information, timely control of irrigation opportunities and water volume can effectively improve water use efficiency. The manual measurement of moisture content not only consumes a lot of manpower, but also cannot be monitored in time; the use of a wired measurement and control system requires higher wiring costs, is not easy to expand, and brings inconvenience to farming. Therefore, a water-saving irrigation control system based on a wireless sensor network is designed. The system is mainly composed of a low-power wireless sensor network node and a ZigBee self-organizing network, thereby preventing the inconvenience of wiring and the poor sensitivity. Complete the continuous online monitoring of soil moisture and the automatic control of water-saving irrigation in farmland, which not only improves the use rate of irrigation water, relieves the growing tension of water resources in China, but also provides a good growth environment for crop growth.
1 System architecture
1.1 Wireless Sensor Network
Wireless sensor network technology is used in the water-saving irrigation control system, and its central technology is ZigBee self-organizing network technology. ZigBee is a two-way wireless communication technology with low complexity, low power consumption, low data rate, low cost, high reliability, and large network capacity. It is composed of use layer, network layer, media access control layer and physical layer. The equipment in the ZigBee network is divided into two types: full function equipment (Full FuncTIon Device, FFD) and simplified function equipment (RedUCe FuncTIon Device, RFD). ZigBee network supports three topological structures: star network, tree network and mesh network. This system uses a hybrid network, and the bottom layer is multiple ZigBee monitoring networks, which are used to collect monitoring data. Each ZigBee monitoring network has a gateway node and several soil temperature and humidity data acquisition nodes. The monitoring network adopts a star structure, and the gateway node serves as the base station of each monitoring network. The gateway node has a dual function. First, it acts as a network coordinator, serving as the automatic establishment and maintenance of the network, and data aggregation. Second, it serves as the interface between the monitoring network and the monitoring center, and transmits information with the monitoring center. This system has the function of automatic networking. The wireless gateway is constantly in the monitoring mode. The newly added wireless sensor nodes will be automatically detected by the network. At this time, the wireless router will send the information of the node to the wireless gateway, and the wireless gateway will address and calculate its Routing information, updated data transfer announcements, and associated tables.
1.2 System architecture
The system uses a single-chip microcomputer as the control center, which is composed of four parts: wireless sensor node (RFD), wireless routing node (FFD), wireless gateway (FFD), and monitoring center. It is decided that GPRS should hold the transmission of moisture and control information. Each sensor node determines the temperature and humidity sensor, automatically collects moisture information, and analyzes the combination of the preset upper and lower humidity limits to determine whether it is necessary to irrigate and when to stop. Each node decides that the solar battery is powered, and the battery voltage is monitored at any time. Once the voltage is too low, the node will send out an alarm signal that the voltage is too low. After successful transmission, the node enters sleep mode until the power is sufficient. Among them, the wireless gateway connects the ZigBee wireless network and the GPRS network, which is the central part of the water-saving irrigation control system based on the wireless sensor network and serves as the maintenance of the wireless sensor node. Sensor nodes and routing nodes autonomously form a multi-hop network. The temperature and humidity sensors are scattered in the monitoring area and send the collected data to the nearest wireless routing node. The routing node selects the best route based on the routing algorithm and establishes the corresponding routing list, which contains its own information and neighbor gateway information. . The resolution gateway transmits the data to the remote monitoring center, which is convenient for users to remotely monitor and maintain. The block diagram of the water-saving irrigation control system based on wireless sensor network designed in this paper is shown in Figure 1.
2 Hardware design
2.1 Sensor node module
Soil moisture is the primary limiting factor for crop growth. Accurate collection of soil moisture information is the foundation and guarantee of holding water-saving irrigation and optimal regulation of farmland. The effective implementation of water-saving technologies has a primary role. The hardware structure of the sensor node of this system is shown in Figure 2.
The system uses the TDR-3A soil temperature and humidity sensor, which integrates temperature and humidity measurement in one, has the characteristics of sealing, waterproofing, and high accuracy. It is an ideal instrument for measuring soil temperature and humidity. The temperature range is -40 ～ + 80 ℃, and the accuracy is ± 0.2 ℃; the humidity range is 0 ～ 100%, and the accuracy is ± 2% in the range of 0 ～ 50%. The output signal of the temperature and humidity sensor is a standard current loop of 4 to 20 mA. I / U conversion is first performed on the main controller circuit, then A / D is converted into a digital signal, and then the RF antenna is transmitted. The current converter uses the RCV420JP chip, which integrates a resistor network, an operational amplifier, and a standard 10 V reference voltage source, which can convert a 4-20 mA current loop into a 0-5 V voltage output.
The signal conditioning circuit is shown in Figure 3. The A / D converter uses the ADC converter inside the low-power RF integrated circuit CC2530. Its sampling frequency is 12 bits, and there is an 8-channel multi-way switch inside. It can latch the decoded signal according to the address code. One of the 8 analog input signals is used for A / D conversion.
2.2 Wireless communication module
The communication system of water-saving irrigation control system based on wireless sensor network is based on ZigBee wireless communication technology and GPRS. ZigBee is a highly reliable wireless data transmission network with three mission frequency bands: 2.4 GHz (global), 915 MHz (United States) and 868 MHz (Europe). This system adopts the global frequency band currently selected by the sensor network as priority-2.4 GHz, with a transmission rate of 250 KB / s. This frequency band does not require permission in most countries.
The communication modules of the wireless sensor node (RFD), wireless routing node (FFD), and wireless gateway (FFD) all use the CC2530 chip, which also has the necessary consistency in structure. Here, we only introduce the hardware structure of the wireless gateway in detail. The gateway is responsible for the control and maintenance of the wireless sensor network and completes the fusion of information. He connects the sensor network and the GPRS network to complete the conversion of the two communication protocols. At the same time, he announces the role of the monitoring terminal and resolves the collected data to GPRS network transmission To the remote monitoring center, the structure diagram is shown in Figure 4.
The gateway uses Huawei's GPRS communication module GTM900C and TI's ZigBee RF chip module CC2530. GTM900CGPRS module supports GSM900 / 1800 dual frequency, provides power interface, analog audio interface, standardized SIM card interface and UART interface, supports voice services, short audio services, GPRS data services and circuit data services. CC2530 is a new generation SoC chip of ZigBee, with up to 256 B of flash memory, allowing wireless download of the chip, supporting system programming, providing 101 dB link quality, excellent receiver sensitivity and robust anti-jamming. In addition, CC2530 combines a fully integrated, high-performance RF transceiver with an 8051 micro-processor, 8 KB of RAM, 32/64/128/256 KB flash memory, and a common set of peripherals --- including 2 USARTs, 12-bit ADC and 21 general purpose GPIO (General Purpose Input Output, general purpose input output). The PC terminal software of the remote monitoring center uses Delphi to design and maintain the interface, establish the corresponding database, complete the query, maintenance, printing of soil moisture, and pass the GPRS network to transmit control commands and soil temperature and humidity information.
3 Software design
In this water-saving irrigation control system, monitoring data and control commands are transmitted between wireless sensor nodes, wireless routing nodes, wireless gateways, and monitoring centers. The sensor node turns on the power, enters the dormant state after being original, establishing the link. When the wireless gateway receives the infix request, it triggers the infix, goes through the routing node to activate the sensor node, sends or accepts the information packet, and continues to enter the sleep mode after the end of the processing, and activates again when there is an request. Only two nodes can communicate in the same channel, and the competition mechanism is decided to obtain the channel. Each node sleeps and monitors the channel periodically. If the channel is idle, the channel is automatically preempted. If the channel is busy, the channel shape is re-monitored after a period of back-off based on the back-off algorithm. In the sequential design, the first way to collect infixes is to complete the reception and transmission of information.
4 Conclusion
The water-saving irrigation control system based on wireless sensor network designed in this paper uses low-cost and low-power ZigBee wireless communication technology to prevent the inconvenience of wiring and improve the sensitivity of the water-saving irrigation control system. The system uses high-precision soil temperature and humidity sensors, and implements precise irrigation based on soil moisture and crop water usage order. It not only can effectively deal with the question of low agricultural irrigation water usage, but also alleviate the growing tension of water resources, and also provide better crops. Growing environment, fully promoting the role of existing water-saving equipment, optimizing scheduling, improving efficiency, making irrigation more superstitious and simple, and improving maintenance levels. The system also supports manual correction and remote control of relevant parameters, and is suitable for a variety of crops. It can add the output of crops, reduce the irrigation cost of agricultural products, and improve the quality of irrigation. It has great implementation value. In addition, with different sensors, the system can form a monitoring network with different functions.
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