Study On Multi-laser Beams Displacement Measurement System Of Wind Field And Correlation Detection Of Weak Signals Based On Backscatter Signals Of Lasers

Accurate observation of atmospheric wind field has great significance in improving long-term global weather forecasting and storm prediction accuracy, detection of air pollution, ameliorating climate research model and improving the safety of aerospace, etc. Traditional Doppler laser radars are based on the Doppler effect to measure wind speed. To overcome the instinct inadequatenesses such as phase distortion of reflected signals, rigorous demand of laser wavelength, single working mode and frequency stability, and inconvenient to use and mainten, a novel multi-laser beams displacement measurement (MLBM) system for high precision detection of flow fields based on laser backscatter is proposed in this work. Our system can collect signal strength of back-scattering light from the aerosol passing through propagation path of multi-laser beams. Then, we extract the time interval of the aerosol passing through parallel laser beams to calculate wind velocity. As the physical quantity of measurement of our system is the strength of echo signal, MLBM system has no limitation on monochrome and frequency stability of laser emitter. Therefore, MLBM system presents high reliability, simple structure, adaptation in wind measurement at low altitude, and good engineering practicability. Main contents of this articale are as follws.1. The transmission characteristics of impulse lasers in atmospheric is analysed, and the measuring theory of multi-laser beams based on small perturbation theory is consummated. According to the time interval of the aerosol passing through parallel and equidistant laser beams, wind velocity can be calculated;2. A novel multi-laser beams displacement measurement system is successfully designed and completed. MLBM system is mainly consisted of four subsystems: laser launch system, laser detection system, data-processing unit, and working security system. In the laser launch system, YAG solid-state laser was adopted as laser source. In order to improve measuring accuracy, output laser were divided into three beams by beam-angle controller and beam splitter. The laser detection system majorly embraces four-quadrant APD detectors, logarithmic gain amplifiers, digitizer based on FPGA, gain control circuit, et al. The APD detectors and amplifiers directly affect the accuracy of the system. Low-noise, wide bandwidth, variable gain amplifier chip AD603 was adopted as a gain amplifier. Meanwhile, two gain amplifiers were designed in a cascade structure to effectively amplify weak signals;3. High-speed data acquisition and buffer circuits are well designed, and they are mainly consisted of analog-digital conversion chip ADC08200, FPGA chip EP2C8Q208C8 and SCM System C8051F120. In this paper, the FPGA chips was well designed to achieve cache and transmission control of high-speed data to meet the actual demand, including PLL clock management module, the preceding stage of FIFO buffer module, the rear stage of dual-port RAM memory modules and the reading and writing control module of FIFO and RAM. It not only fulfilled caching and asynchronous data reading, but also greatly simplified the interface circuit structures of A/D chip and the design of printed circuit board. Synthesis and simulation of all modules was carried out using the program Quartus II6.1;4. Signal processing module for weak backscattering signals was successfully designed and completed. Firstly, to decrease the interference to echo signals by filtering thermal noise of signal receivers, quantization noise of ADC converter, and environmental noises, median filter and FIR filter were adopted to consult backscattering sampling signals. Then, multi-pulse cross-correlation detection method was used to further filter out noise. Theoretically, after consulted by N-pulse cross-correlation method, the SNR of backscattering signals was increased by N1/2 times;5. Optical cross-correlation method was adopted to analyze backscattering signals and calculate wind speed and wind direction, which embracing the peak delay method, slope method, frequency method, Briggs method and covariance method. Finally, experiments were carried out to measuring the wind field, including real-time measurement at a fixed distance, measurement of wind speeds at different distances, and conforming measurement by rotating the system to different direction. Thus, measuring accuracy of detection system can be comprehensively determined.6 The response time and duty cirle of wind measuring have been calculated. Moreover, our system can maintain a high measuring accuracy at the 40 celsius or lowerer by varing the surouding temperature. When the system is working at the continuous duty or the interval duty situation, a high measuring accuracy can also achieved in a long time by suitably cooling system.Overall, the MLBM system has been studied in detail. The system has successfully overcome the atmospheric backscattering effects of high-sensitivity detection, detected the target echo, extracted weak signals with low signal to noise ratio, achieved spatial filtering, precision delay and exact wave gate control technologies, and successfully developed transceiver system. The system has a range resolution of 0.75 meters, and the wind error is less than 5%. As a result, we successful realize monitoring the wind field at different distances in real time.