As one of the core technologies of wireless sensor network (WSN), ZigBee technology has the following characteristics: 1) low energy consumption and battery power supply, which is suitable for the field where the conventional power supply mode is not available. 2) Reliable communication, network self-organization, self-healing, without manual intervention. Each communication node automatically finds and joins the network according to the determined routing rules. When the nodes break away, fail and move, the network can repair itself and form a new topology structure. 3) Low cost, low complexity of equipment and large network capacity. ZigBee protocol is free, Ti and other companies have also launched the relevant free protocol stack, and it works in the 2.4GHz global unlicensed frequency band, which makes its production cost very low. 4) Data security. ZigBee supports data integrity and authentication engineering, AES-128 encryption algorithm. The new version of ZigBee Pro supports more advanced security algorithms and can be flexibly configured according to different application scenarios.
The working principle of the system is as follows:After collecting data, the terminal sensor node sends packets to the routing node, and the routing node summarizes the packets of each node and forwards them to the coordinator node. After receiving the data packet, the coordinator node continues to forward it to the serial port to network port device. The serial network equipment repackages the received serial data packet into RJ45 data packet and forwards it to the on-site monitoring PC through the wireless router. At the same time, the wireless IP cameras distributed in the greenhouse also transmit the data to the on-site monitoring PC through the router. The handheld embedded gateway can also directly access the router to display the environment data in real time. The on-site monitoring PC is equipped with a service system. As a server, the remote PC can realize remote measurement and control by directly accessing the browser and IP address. The above is mainly the uplink of the sensor data, and the downlink is similar. The upper computer sends the control instructions, which are transmitted to the corresponding control node to execute the control instructions through the router, the serial transmission network equipment and the coordinator.
Design of sensor control module
The sensor and control module is the basis of the system. The air temperature and humidity sensor adopts SHT11 digital temperature and humidity sensor from Sensirion of Switzerland, IIC interface, the output resolution of humidity value is 14 bits, the output resolution of temperature value is 12 bits, and it can be programmed to 12 bits and 8 bits. The power supply voltage ranges from 2.4 V to 5.5 v. Current consumption: 550 when measuring μ A. The average is 28 μ A. 3 in dormancy μ A［4， 5］。 The light sensor uses a photosensitive probe bpw34s , which is input to ad after op amp processing. In the sensor control module, NXP IIC bus is used to control GPIO expansion chip pca9554, which provides 8-bit parallel I / O port. The processor directly reads SHT11 temperature and humidity data through IIC, and indirectly reads illumination data through ad. The schematic diagram of the sensor module is shown in Figure 3.
The control module mainly adopts relay and Da analog voltage output. The schematic diagram of relay module is shown in Fig. After the upper computer gives the control command, pca9554 drives the relevant I / O port to make the triode turn on or off, and the drive relay turn on or off.
Da programmable analog voltage output module uses ti’s Da chip dac5573 and buffer amplifier uses tlv2372. The maximum output voltage of dac5573 is 3.0V, and the output voltage is about 10V after 3.3 times amplification by operational amplifier tlv2372. The schematic diagram of voltage output module is shown in Fig.
Design of node power supply module
The node power module is designed with two DC / DC power conversion chips tps63001drc (3.3V) and tps63002drc (5.0V) from TI company to provide stable 3.3V and 5.0V voltage for the system. The system is designed as two dry batteries or USB power supply mode, with lithium battery interface reserved. Seamless switching between power supply. The schematic diagram of power supply module is shown in Fig.
The system applies ZigBee technology, one of the most popular core technologies of Internet of things, to the greenhouse monitoring system, realizes the acquisition and control of the basic environmental factors of greenhouse, and realizes the remote visual operation of monitoring, with high reliability and strong anti-interference ability. It has high practical significance and promotion value to design and solve the problems of crisscross cables, inconvenient layout and high maintenance cost in traditional greenhouse.