Study On The Soil Wind Erosion Monitoring System Based On Wireless Sensor Network

Soil wind erosion is the main reason for ecosystem degradation of arid and semi-arid areas in northern China. The study around soil wind erosion such as field observation and wind tunnel tests has achieved an unprecedented development. Because the physical mechanism of soil erosion is extremely complex, and abruptness in time, no boundary in space and difficult to repeat, these characters lead to greater error of erosion data, as well as clearly differences between the various research results and poor comparability. Therefore, employing the wireless sensor network technology on the study of soil erosion, developing a set of soil erosion monitoring equipment that is stable and reliable also could acquire the erosion data in real-time, so as to establish an soil erosion monitoring system with efficient and professional, improve the continuity, scientific and reliability of the observed data, satisfy the needs of the complex field erosion monitoring. All of the above are the urgent issues need to be resolved of the study of soil erosion, also the hot and difficult ones in recently.For the existing shortcomings and lack of currently soil erosion test or detection methods, for example labor-intensive, unstable of the monitored erosion data, unable to collect real-time data of wind erosion in the cross-regional scale, etc. integrating the wireless sensor network technology, sensor technology, electronics and information technology, computer technology and other modern theory and technology, soil erosion monitoring system was established which was based on wireless sensor networks, on the basis of developing multi-channel wind speed profilers and sand collection device, real-time data acquisition, processing and transmission of ambient temperature, humidity, atmospheric pressure, wind profile near surface and sand flow structure soil erosion area have been achieved. The research has designed the wireless sensor network layered architecture; meanwhile optimization deployment model of sensor nodes and sink node has been established and the stream-based application-layer low-power data access strategy has been studied, so it could realize collection and transmission of the data with low power consumption and high reliability. The monitoring system could observe the indicators of wind erosion for a long-term, by laying a plurality of observation points orderly in the study area, in order to provide the theoretical and technical basis for achieving the cross-regional and large-scale real-time monitoring of soil erosion.The main conclusions are as follows:1. Constant temperature measurement principle was being exploited to develop the wind speed sensor with small size, low power consumption, high intensity and high accuracy of measurement on the basis of the FS5 thermal probes. and research the temperature compensation circuit of the hardware, and the signal processing algorithms of the sensor as well as the L-shaped probe outer casing that has less effect on the flow field quality. Problems such as accumulation of errors and lack of precision due to defects in the circuit have been resolved by testing and optimizing the hardware independently; Sensor group was calibrated by using wind tunnel test method defining the output characteristic curve of the sensor; Least squares method was applied to polynomial fit the calibration data for five time, and curve fitting of 8-channal sensor were all above 0.999. Experimental results showed that the sensor could work for more than 7.2h with continuous and stable, and the time during on power to output stable was less than 8s, as well as the response time of wind speed was 2-3s; The measurement accuracy under the wind speed of 0-6m/s was not less than 0.05m/s, and the maximum output voltage deviation was lower than 10mv; While the measurement accuracy under the wind speed of 6-16m/s was not less than 0.13m/s, and the maximum output voltage deviation was lower than 30mv.2. A low-cost wireless data terminal which integrated a temperature and humidity sensor, a air pressure sensor,8 wind speed sensors, a single chip microcomputer system and a wireless transmission module and a low-power multi-channel wind speed profiler which has the function of wireless ad-hoc network have been designed, realizing the automatic acquisition, processing and wireless transmission of the environmental temperature, humidity, atmospheric pressure and the wind speeds at 2、4、8、16、23、 32、45 and 64cm high. Synchronous acquisition processing algorithm of wind speed data is studied, and experiments in the wind tunnel have been done to test the performances of the wind speed profiler under 4、6、8、10、12 and 14m/s, respectively. After the calibration, the wind speed profile in height is consistent with the exponential distribution, and can accurately reflect the near-surface velocity of vertical distribution.3. According to the similarity theory, a multi-channel automatic sand sampler which has the functions of resisting strong wind disturbance,wireless ad-hoc network, air bypass hedge and multi-stage expansion speed reduction has been developed. At the same time,a signal conditioning circuit for the high-precision weighing sensor is designed to make the sand collection unit realize the function of automatic weighing. Combining the average method and the threshold limit method, the sand collection unit can effectively eliminate the disturbances caused by external noise signal and environmental wind speed mutations to the weighing system. This instrument can real-time collect and transmit the weights of the soil wind erosion at 2、8、19、32、36、 49、53 of 66cm high. The results showed that:the weighing sensor group has excellent linearity with the measuring range from 0 to 180g and accuracy at 0.01 g. its cumulative error for 10 hours is not more than ±0.04g; With the wind speed from 0 to 15 m/s, the maximum wind speed at the separator export is less than 0.56 m/s, and the deceleration ratio is above 96%. When the working environment temperature is-19.4℃ and 4.2℃ respectively, wireless transmission distance of the sand sampler can reach 280m and communication distance with zero packet loss rate is around 210m. The power performance of the eight sand collect units are more than 91.9% and 91.3% with the wind speed in the central of wind tunnel at 6m/s and 14m/s. The average sand collection efficience of the sand sampler is 91.7% under 6、9、12、15 and 18m/s.4. The soil erosion monitoring system has been built based on wireless sensor network, and the optimized deployment model of sensor node and sink node is researched and designed in a star network topology structure. Using 433MHz wireless module and microcontroller technology, the underlying ad-hoc network data acquisition terminal equipment has been developed. After the study of the hierarchical wireless sensor network architecture and ad-hoc network instruction system, the underlying ad-hoc network communication protocol has been written. The data packet structure of the network nodes, the wireless data communication mechanism, the whole network coverage solution, independent data processing mechanism and the low-power application layer access strategy are developed to make function of low power consumption, high reliability data acquisition, processing and wireless transmission come true. Using "synchronous acquisition, asynchronous transfer" and "polling data access" approaches to solve problems of the data synchronization acquisition of sensor nodes and the receive bottleneck of sink node. Finally, the GPRS wireless communication technology is also used to connect a remote server to achieve data transmission and monitoring between the underlying network and remote client terminals.5. Using the modular programming ideas and base on the LabVIEW programming language, a multifunctional client data processing software which is integrated with ad-hoc network protocol module, Internet data analysis module, data processing module, graphic display module and database module is accomplished. By using a USB wireless module, functions of wireless ad-hoc network data collection and processing can be achieved directly on the client PC. UDP data communication nethod can allow the software on the client access the database on the remote server,realizing data access, read, download and real-time processing, and other functions. At the same time, the systerm can not only store the processed data, but also realize real-time calculating and drawing of the wind speed profile and the sand flow structure curve.6. Multiple monitoring areas can establish the data transmission with server at the same time, and the software can realize the real-time data processing of the monitoring area according to the GPRS device identification code. Synchronous monitoring of multiple areas can be easily realized by running more than one data processing software on the client. Field test results showed that:With the wireless air transmission rate at 2、 10、25 and 50kbps, the maximum effective distance of the underlying communication network can reach 260、210、125 and 65m, respectively. For a network composed of six sensor nodes, the setup time of the ad-hoc network system is not more than 5s, and the sink node can finish one time node traversal and data gathering within 3s. The single maximum working hours of wind speed profile node and sand sampler node is not less than 6h and 22h. The system can meet the large scale, cross-regional and real-time soil wind erosion monitoring requirements.