Water-saving irrigation control system based on wireless sensor network

Agricultural irrigation is a major water user in my country, accounting for about 70% of the total water consumption. According to statistics, the average annual area of ​​grain affected by drought in my country is 20 million hectares, and the loss of grain accounts for 50% of the national grain production reduction due to disasters. For a long time, due to the backward technology and maintenance level, the waste of irrigation water is very serious, and the utilization rate of agricultural irrigation water is only 40%. If the irrigation time and water volume are controlled in time based on the monitoring of soil moisture information, the water use efficiency can be effectively improved. However, the manual regular measurement of soil moisture not only consumes a lot of manpower, but also cannot achieve timely monitoring; the use of wired measurement and control systems requires high wiring costs, is not easy to expand, and brings inconvenience to farmland cultivation. Therefore, a water-saving irrigation control system based on wireless sensor network is designed. The system is mainly composed of low-power wireless sensor network nodes through ZigBee self-organizing network mode, thus avoiding the inconvenience of wiring and the defects of poor sensitivity, realizing continuous online monitoring of soil moisture and automatic control of farmland water-saving irrigation, which not only improves the utilization rate of irrigation water, alleviates the contradiction of increasingly tense 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 this water-saving irrigation control system, and its core 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 consists of application layer, network layer, medium access control layer and physical layer. The devices in ZigBee network are divided into full function devices (Full Function Device, FFD) and reduced function devices (RedUCe Function Device, RFD). ZigBee network supports three topological structures: star network, tree network and mesh network. This system adopts a hybrid network, with multiple ZigBee monitoring networks at the bottom layer, responsible for the collection of monitoring data. Each ZigBee monitoring network has a gateway node and several soil temperature and humidity data collection 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 dual functions. One is to act as a network coordinator, responsible for the automatic establishment and maintenance of the network and data aggregation; the other is to serve as an interface between the monitoring network and the monitoring center, and transmit information to the monitoring center. This system has an automatic networking function. The wireless gateway is always in a monitoring state. The newly added wireless sensor node will be automatically discovered by the network. At this time, the wireless router will send the node information to the wireless gateway, and the wireless gateway will address and calculate its routing information, update the data transfer and configuration association table, etc.
1.2 System Architecture
The system uses a single-chip microcomputer as the control center and consists of four major parts: wireless sensor nodes (RFD), wireless routing nodes (FFD), wireless gateways (FFD), and monitoring centers. Through ZigBee self-organizing networks, the monitoring center and wireless gateways transmit soil moisture and control information through GPRS. Each sensor node automatically collects soil moisture information through temperature and humidity sensors, and analyzes it in combination with the preset upper and lower limits of humidity to determine whether irrigation is needed and when to stop. Each node is powered by solar cells, and the battery voltage is monitored at any time. Once the voltage is too low, the node will send a low voltage alarm signal. After the signal is sent successfully, the node enters sleep mode until the power is sufficient. The wireless gateway connects the ZigBee wireless network and the GPRS network. It is the core part of the water-saving irrigation control system based on the wireless sensor network and is responsible for the maintenance of wireless sensor nodes. The sensor nodes and routing nodes autonomously form a multi-hop network. Temperature and humidity sensors are distributed in the monitoring area, and the collected data is sent to the nearest wireless routing node. The routing node selects the best route according to the routing algorithm and establishes a corresponding routing list, which contains its own information and the information of the neighboring gateway. The data is transmitted to the remote monitoring center through the gateway, 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 basis and guarantee for water-saving irrigation and optimal regulation of farmland, and plays a vital role in the effective implementation of water-saving technology. The hardware structure of the sensor node of this system is shown in Figure 2.

The system uses TDR-3A soil temperature and humidity sensor, which integrates temperature and humidity measurement. It is sealed, waterproof and has high precision. It is an ideal instrument for measuring soil temperature and humidity. The temperature range is -40~+80℃, with an accuracy of ±0.2℃; the humidity range is 0~100%, with an accuracy of ±2% in the range of 0~50%. The output signal of the temperature and humidity sensor is a standard current loop of 4~20 mA. It is first converted into I/U in the main controller circuit, and then converted into a digital signal by A/D and transmitted through the radio frequency antenna. The current converter uses RCV420JP chip, which integrates a resistor network, an operational amplifier and a standard 10 V reference voltage source, and can convert the 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, which has a sampling frequency of 12 bits and an 8-channel multiplexer inside. It can latch the decoded signal according to the address code and only select one of the 8 analog input signals for A/D conversion.

2.2 Wireless communication module
The communication system of the 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 working frequency bands: 2.4 GHz (global), 915 MHz (USA) and 868 MHz (Europe). This system uses the global universal frequency band 2.4 GHz, which is currently the preferred choice for sensor networks, with a transmission rate of 250 KB/s. This frequency band does not require a license in most countries.
The communication modules of wireless sensor nodes (RFD), wireless routing nodes (FFD) and wireless gateways (FFD) all use CC2530 chips, and have certain 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 processing of information. It connects the sensor network with the GPRS network, completes the conversion of the two communication protocols, announces the functions of the monitoring terminal, and transmits the collected data to the remote monitoring center through the GPRS network. The structural block diagram is shown in Figure 4.

The gateway uses Huawei's GPRS communication module GTM900C and TI's ZigBee RF chip module CC2530. The GTM900C GPRS module supports GSM900/1800 dual-band, provides power interface, analog audio interface, standard SIM card interface and UART interface, and supports voice service, short message service, GPRS data service and circuit data service. CC2530 is a new generation of ZigBee SoC chip, with up to 256 B flash memory, allowing wireless chip download, supporting system programming, providing 101 dB link quality, excellent receiver sensitivity and strong anti-interference. In addition, CC2530 combines a fully integrated, high-performance RF transceiver with an 8051 microprocessor, 8 KB RAM, 32/64/128/256 KB flash memory, and a wide range of peripheral sets - including 2 USARTs, 12-bit ADC and 21 general-purpose GPIOs (General Purpose Input Output). The PC software of the remote monitoring center uses Delphi to design the maintenance interface, establish the corresponding database, complete the query, maintenance, printing of soil moisture, and transmit control commands and soil temperature and humidity information through the GPRS network.
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, initializes, establishes a link and enters the sleep state. When the wireless gateway receives an interrupt request, it triggers an interrupt, activates the sensor node through the routing node, sends or receives information packets, and continues to enter the sleep state after the processing is completed, waiting for a request to activate again. Only two nodes can communicate in the same channel, and the channel is acquired through a contention mechanism. Each node sleeps and monitors the channel periodically. If the channel is idle, it automatically seizes the channel. If the channel is busy, it retreats for a period of time according to the backoff algorithm and then listens to the channel again. In the program design, the method of collecting interrupts is mainly used 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 avoid 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 to implement precise irrigation according to soil moisture and crop water use sequence. It can not only effectively deal with the problem of low utilization rate of agricultural irrigation water and alleviate the contradiction of increasingly tense water resources, but also provide a better growth environment for crops, give full play to the role of existing water-saving equipment, optimize scheduling, improve efficiency, make irrigation more scientific and simple, and improve the maintenance level. This system also supports manual correction and remote control of relevant parameters. It is suitable for a variety of crops, can increase the yield of crops, reduce the irrigation cost of agricultural products, and improve the quality of irrigation. It has great promotion value. In addition, with different sensors, the system can form a monitoring network with different functions.