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Environmental temperature, humidity and other parameters are very important data in industrial and agricultural production and scientific research, especially in the grain warehouse, greenhouse, greenhouse and other environments, temperature and humidity measurement and control is more important. With the development of science and technology, traditional temperature and humidity monitoring systems based on wired communication systems require a lot of manpower and material resources to lay lines, and the construction volume is large and the installation time is long. Therefore, an environmental parameter monitoring system based on wireless transmission technology is imperative. . This article describes an environment temperature and humidity monitoring system based on ZigBee wireless sensor network. It can monitor grain warehouses or other locations that require strict environmental parameters in real time. Administrators can keep abreast of site information in the control room. When the parameter exceeds the set value, an audible and visual alarm is immediately performed, and the location where the abnormality occurs is displayed on the monitoring host. The staff can immediately take measures to automate and intelligentize the management of grain storage and greenhouses.
1ZigBee technology
ZigBee is a global unified standard and open wireless sensor network using mature wireless communication technology. Based on the IEEE 802.15.4 protocol, it aims to implement a low-complexity, low-cost, low-power, and low-rate short-range two-way wireless communication protocol suitable for use in fixed, portable, or mobile devices. ZigBee can work in the international free license 2.4GHz, European 868MHz and the United States 915MHz 3 free frequency bands, work in the 2.4GHz band with the highest data rate of 250kbps, the typical application of the reliable transmission distance of 10 ~ 75m, the actual transmission distance according to Transmission power size and application environment. Compared with existing wireless communication technologies, ZigBee technology is best suited as a sensor network standard due to its low power consumption and low speed.
Devices in a ZigBee network are classified into FFD (Full Function Device) and RFD (Simplified Function Device). Among them, FFD equipment can also be used as a coordinator. FFD is a network node with routing and relaying functions. It can communicate with RFD nodes or with other FFD nodes. RFD nodes, as network termination nodes, cannot directly communicate with each other and can only send and receive information through FFD nodes. No routing and relaying capabilities. The hardware structure of RFD and FFD is exactly the same, but the network layer is not the same. The coordinator is the network organizer and is responsible for network formation and information routing.
2 system hardware design 2.1 system network structure
ZigBee wireless sensor network consists of a large number of tiny ZigBee sensor nodes (ie ZigBeeRFD nodes), ZigBee FFD nodes and network coordinators deployed in the monitoring area, and a multi-hop self-organizing network system formed through wireless communication.
The network coordinator is installed in the central monitoring room and is responsible for establishing the network and managing the network. It also displays the current status of the entire network and sends the received data to the computer. The RFD is installed in the location where the data needs to be monitored, and is responsible for collecting the temperature and humidity values, and then sending the data to the network coordinator periodically or when there is an interruption. The monitoring personnel can monitor the environmental parameters through the display in the control room without having to reach the site.
In order to ensure the reliability of the entire system, part of the transmitter is an RFD node, and part of it is an FFD node with routing and relay functions. The sensor data is converted into a ZigBee communication protocol packet in the ZigBeeRFD or FFD module and passed to the nearest FFD module. The data packet is transmitted to the ZigBee coordinator in multi-hop communication. After receiving the data packet, the ZigBee coordinator returns the received acknowledgment information according to the original path on one hand, to arrive at the RFD or FFD module that sends the data, implements the handshake signal, and completes a complete ZigBee wireless communication; on the other hand, the Zig2Bee coordinator receives The data is available for display and management.
2.2 signal acquisition circuit
The temperature sensor adopts DS18B20, a one-line digital temperature sensor introduced by DALLAS. The pin of this chip is simple and it can measure temperature conveniently without any peripheral hardware. Only one I/0 port line and multiple DS18B20 are needed to exchange information with the SCM. It can be connected in parallel to three or two wires. The CPU can communicate with many DS18B20s with only one port line, and fewer ports occupy the microprocessor, which can save a large number of leads and logic circuits. Temperature measurement range is -55 ~ 125 °C, can be programmed to 9 ~ l2 A / D conversion accuracy, the measured temperature is extended with a sign of the l6-bit digital output. Because the DS18B20 temperature sensor chip size is small, can be welded directly with the cable, the external heat-shrinkable tube fastening and forming a temperature measurement cable, the cable's external sleeve on the mouse bite. To monitor the site where temperature measurement is required, directly route the cable to the temperature measurement point [2].
Humidity sensor uses HS1101 capacitive sensor. The HS1101 capacitive sensor accurately converts the capacitance variation into a signal that is easily accepted by the microcontroller. The humidity sensitive capacitor can be placed in an oscillating circuit to form a multivibrator circuit that converts the change in capacitance value to a voltage signal that is inversely proportional to it. , can be directly collected by the microcontroller counter [3]. This circuit can be easily connected with the microcontroller, the output signal is frequency type, high sensitivity, good linearity, with excellent repeatability, resolution and stability. In order to improve the sensitivity and linearity of the system, considering the factors such as reducing the cost of the acquisition circuit, the pulsed oscillation circuit was used to design the humidity acquisition circuit.
2.3 Data Transmission Unit
In the data transmission unit, the wireless transmission controller selects Microchip's SPI interface low-power PIC18F4620 chip, and the RF chip selects the CC2420 produced by Chipcon to meet the 2.4GHz ZigBee product requirements. These two chips are the most critical in the design of the system. The two parts. The PIC18F4620 microcontroller has a rich on-chip memory function, with 64kB Flash and 3968 bytes of RAM, and the microcontroller has a variety of power-saving modes to choose from. The chip has 13 10-bit A/D converters and multiple I/O data lines that can be easily programmed and simulated in software. These interfaces can also be used as interfaces to sensing units.
The CC2420 uses Chipcon's SmartRF03 technology, which is fabricated on a 0.18 μm CMOS process. It requires very few external components and has stable performance and extremely low power consumption. The CC2420's selectivity and sensitivity index exceeds the requirements of the IEEE 802.15.4 standard, ensuring the effectiveness and reliability of short-range communications. Wireless communication devices developed using this chip support data transmission rates of up to 250kbps, enabling rapid multipoint-to-multipoint networking. The CC2420 can set the operating mode of the chip through the 4-Wire SPI bus (SI, SO, SCK, CSn) and implement read/write buffer data, read/write status registers, and so on. The transmit/receive buffer can be set by controlling the state of the FIFO and FIFOP pin interfaces. Most of the address and data transmissions on the SPI bus interface are MSB first. There are 33 16Bit status setting registers in the CC2420 chip. In the read/write cycle of each register, there are 24Bit data on the SI bus, which are 1Bit RAM/register select bits (0: register, 1: RAM), 1Bit read/write. Control bits (0: write, 1: read), 6Bit address select bits, 16 Bit data bits. CSn must always be kept low during data transmission. In addition, the CCA pin status setting can control the clear channel estimation, and the input of the clock/timing information can be controlled through the setting of the SFD pin status. These interfaces must be connected to the corresponding pins of the microprocessor to control and manage the RF function of the system.
3 System Software Design 3.1 Monitoring Center Software Design
The monitoring center software acts as a full-function system and is responsible for network coordination and human-machine dialogue. The use of the monitoring system is ultimately to face the user, so the development of monitoring software will determine whether the system can be easily and efficiently operated. Because the Windows operating system is popular and easy to operate, the data display and processing of the system uses the Windows operating system and the Visual Basic development platform. VB supports object-oriented programming, has a structured event-driven programming model, and can easily make a good man-machine interface. The software design of the monitoring center includes modules such as system self-test, user management, data read display, alarm parameter setting, and historical data analysis.
3.2 monitoring node software design
The monitoring node software acts as a simple functional system, waiting for host commands and transmitting local point data. Monitoring node data transmission flow chart, as shown in Figure 4. Among them, the most critical part is the data acquisition and judgment: if the data is not abnormal, then send the normal information; if there is a data exceeding the standard, it will send out the corresponding alarm signal; at the same time, it can also monitor the process of information change and transmit the acquisition in real time. The data can be controlled by the specific instructions of the control center.
4 Conclusion
ZigBee is a new short-range, low-cost and low-power wireless network technology. It is applied to environmental temperature and humidity monitoring systems such as grain storage and greenhouses. It has a simple networking, low system cost, easy expansion network, and communication. The advantages of stability, in practice have a good application value.
references:
[1] Lu Zhaoquan, Huang Meichu, Du Zheng, et al. Monitoring system of greenhouse planting temperature based on ZigBee wireless network[J].Journal of Anhui Agricultural Sciences,2008,36(13):5682-5684.
[2] Ye Yiming, Yao Bowei. Design and implementation of multi-point temperature monitoring system for grain warehouses [J]. China Measurement Technology, 2005(5): 63-64.
[3]JIA Shao-rui, LI Xiao-dong, ZHAO Xin. Design of Multi-parameter Wireless Monitoring System in Intelligent Greenhouse[J].Journal of Agricultural Mechanization Research,2008(5):99-101.
Temperature and humidity monitoring system based on wireless communication technology
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