Application of wireless sensor networks in vegetable greenhouses

0 Introduction
Driven by the Internet of Things technology, wireless sensor network technology has achieved many practical and effective applications in all aspects of society, saving a lot of manpower and material resources compared to the use of traditional technologies. The wireless sensor network designed in this paper is a wireless monitoring system based on ZigBee technology, which can collect temperature and humidity in vegetable greenhouses in real time, so that growers can make appropriate adjustments according to the detected vegetable growth environment, increase production and obtain greater benefits. It is suitable for large-scale vegetable planting bases.
The complete Zigbee protocol stack consists of the physical layer, media access control layer, network layer, security layer and high-level application specifications. The core part of the ZigBee protocol stack is in the network layer. The network layer mainly implements functions such as nodes joining or leaving the network, receiving or abandoning other nodes, route search and data transmission, supports multiple routing algorithms such as Cluster-Tree, AODVjr, Cluster-Tree+AODVir, and supports multiple topological structures such as star, tree, and mesh. The application layer is developed according to its own needs.

1 Overall system design
The wireless sensor network coordination node designed in this paper is responsible for establishing the network using ZigBee, and completing the data exchange function with the PC through the USB interface; the sensor node is responsible for collecting the temperature and humidity values ​​in the vegetable greenhouse and sending and forwarding the data to the coordination node, and vice versa, it can also receive and forward the command information sent by the coordination node; the communication of the entire system is based on ZigBee technology, and the interaction of data and control information between the sensor node and the coordination node in the wireless sensor network is realized on this stable network. Figure 1 System block diagram.

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2 Sensor Nodes
The sensor node collects the temperature and humidity values ​​in the vegetable greenhouse through the sensor SHT11, and transmits them through the 2.4G antenna after being processed by CC2533; on the contrary, the antenna receives the control command and hands it to the CC2533 chip, which makes the corresponding operation.
SHT11 is a new temperature and humidity sensor based on CMOSensTM technology launched by Swiss Sensirion. This sensor combines CMOS chip technology with sensor technology to play their powerful complementary role. The main features of the temperature and humidity sensor are as follows: 1) The temperature and humidity sensor, signal amplification, A/D conversion, and I2C bus interface are all integrated into one chip (CMOSensTM technology); 2) It can give fully calibrated relative humidity and temperature output values; 3) It has the function of calculating and outputting dew point values; 4) It has excellent long-term stability; 5) The output resolution of the humidity value is 14 bits, the output resolution of the temperature value is 12 bits, and it can be programmed to 12 bits and 8 bits; 6) It is small in size (7.65×5.08×23.5mm) and can be surface mounted; 7) It has a reliable CRC data transmission verification function; 8) The calibration coefficients loaded on the chip can ensure 100% interchangeability; 9) The current consumption is 550μA during measurement, 28μA on average, and 3μA in sleep mode.

The CC2533 chip is a 2.4GHz RF system-on-chip produced by TI, which integrates the ZigBee RF front-end, memory and microcontroller. Its main features are as follows: high-performance and low-power 8051 microcontroller core; integrated 2.4GHz RF radio transceiver that complies with the IEEE 802.15.4 standard; excellent wireless reception sensitivity and robustness; only 1 μA current consumption in sleep mode; hardware support for CSMA/CA function; I2C interface; digital RSSI (received signal strength indication)/LQI (link quality indication) support and powerful 5-channel DMA (direct memory access) function; battery monitoring function; integrated AES (Advanced Encryption Standard) security coprocessor; with 2 powerful USARTs (universal asynchronous synchronous receivers and transmitters) that support several groups of protocols, and 1 MAC (media access control layer) timer that complies with the IEEE802.15.4 specification, 1 conventional 16-bit timer and 2 8-bit timers; 23 programmable I/O pins.

3 Coordination Node
The coordination node of the wireless sensor network needs to take on the work of wireless network formation, uplink data and downlink command forwarding. In the wireless sensor network, the coordinator mainly handles five aspects of work: 1) Add terminal devices to the wireless sensor network; 2) Receive and process the data sent by the terminal device, and then send it to the computer through the USB port for processing or display; 3) Receive the command information sent by the computer through the USB port, and send it to the corresponding terminal device after processing; 4) Remove the terminal device from the wireless sensor network; 5) Display the working status of the terminal device and automatically report repairs.
The CC2531 chip is a 2.4GHz RF system-on-chip produced by TI, which integrates the ZigBee RF front end, memory and microcontroller. Its main features are as follows: high-performance and low-power 8051 microcontroller core; integrated 2.4GHz RF radio transceiver compliant with IEEE 802.15.4 standard; excellent wireless reception sensitivity and robustness; only 1μA current consumption in sleep mode; hardware support for CSMA/CA function; USB2.0 certified full-speed device (12Mbps); digital RSSI/LQI support and powerful 5-channel DMA (direct memory access) function; battery monitoring and temperature sensing functions; integrated 12-bit ADC; integrated AES (Advanced Encryption Standard) security coprocessor; with 2 powerful USARTs (Universal Asynchronous Synchronous Receiver/Transmitter) supporting several groups of protocols, as well as 1 MAC timer compliant with IEEE802.15.4 specification, 1 conventional 16-bit timer and 2 8-bit timers; 21 programmable I/O pins. Figure 3 Coordination node hardware block diagram.

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4 Software
According to the needs of this system, the application layer development uses IAR Embedded Workbench software to modify the Z-Stack protocol stack, compile, download and debug. The coordination node is connected to the computer through the USB port, and the USB driver is developed in C++; the computer and the coordination node are the upper and lower computers, and the cache area is divided in the RAM of the coordination node CC2531. Both the computer and the coordination node can read and write data from the cache area; VB is used to write the management display interface, as shown in Figure 4. Each vegetable greenhouse is a group, which contains several sensor nodes. Groups and nodes can be added, deleted, matched and commented; the time can be set by the sensor node to collect and send back temperature and humidity data, and real-time operation can also be performed.

5 Conclusion
The wireless sensor network designed in this paper can meet the basic needs of growers to monitor the temperature and humidity in the vegetable greenhouse in real time. The system networking is simple and easy to use. In order to make the system more perfect, it is planned to focus on improving the following two functions in the next development process: optimizing the routing algorithm for communication between nodes and node energy control. This paper has a certain reference value for the design of other wireless monitoring networks, such as forest fire early warning systems, mine earthquake monitoring systems, etc.