摘要
精确的大气风场观测对提高全球长期天气预报和风暴预报的准确性、检测大气污染、改进气候研究模型、提高航天航空的安全性等具有重大意义。传统的多普勒激光测风雷达基于多普勒效应来测量风速。为克服多普勒激光测风仪中反射回波相位畸变,对激光器的波长、单模工作模式和频率稳定度要求高、使用维护不便等不足,本文提出并研制了一种基于激光后向散射的多光束风场位移测量法的激光测风仪。该系统通过采集激光光束传播路径上气溶胶团散射回来的光信号强度,确定气溶胶微粒通过多束平行激光光束的时间来计算风速。由于该系统测量的物理量是回波信号的强度,因此对激光源的单色性和频率稳定度无要求,系统可靠性高,结构简单,适用于低空风力测量,具有很好的工程实用性。本文所做的主要工作为:
1.分析脉冲激光在风场中的传输特性,完善了基于小扰动理论的多光束位移测风理论,即根据激光后向散射回波信号间接测量气溶胶微粒经过等间距的多光束的时间来对风速进行测量;
2.研制了多光束风场位移测风仪系统,该系统主要由激光发射系统、回波检测系统、数据处理模块和工作维护系统等组成。激光发射系统采用了20 Hz的YAG固体激光器作为激光源。为提高测量精度,利用光束角控制器和分束器后将激光脉冲分成三束。回波检测系统主要包括四象限APD探测器、对数增益放大器、基于FPGA的数字化仪、增益控制电路等。探测器和放大器直接影响着整个系统的测量精度。本系统采用低噪声、宽带宽、可变增益放大器芯片AD603。同时,本设计采取对数增益放大器进行级联的方式对弱信号有效地放大;
3.设计了基于现场可编程门阵列(FPGA)的高速数据采集和缓存电路,该电路主要由模数转换芯片ADC08200、FPGA芯片EP2C8Q208C8和单片机系统C8051F120组成;其中,设计了实现高速数据缓存和传输控制的FPGA芯片,主要包括PLL时钟管理模块、前级FIFO缓冲模块、后级双口RAM存储模块以及FIFO和RAM的读写控制模块等,很好地完成了数据的缓存和异步读取,并且极大简化了A/D芯片接口电路结构和印制电路板设计的复杂性。并在开发环境Quartus II 6.1中对各个模块成功完成了综合及仿真。
4.设计了针对低信噪比弱散射回波信号的信号处理模块。首先,为滤除接收机的热噪声、ADC转换器的量化噪声以及环境噪声等对激光回波信号的干扰,本系统采用中位值滤波器和FIR滤波器对采样回波信号进行滤波。其次,采用多脉冲互相关检测法进一步滤除噪声信号。理论研究结果表明,经过N个信号进行平均互相关后,信噪比将提高N1/2倍。
5.采用光学互相关方法对探测得到的回波信号进行分析和计算得到风速和风向。其中包括峰值延时法、斜率法、频率法、Briggs法和协方差法。最后进行实地风场测量实验,包括固定距离处的风速实时测量,不同距离处的风速测量,以及系统在旋转一定角度时的风速测量,从而全面检测系统的测量精度。
6计算了系统的响应时间和测量风场的占空比。另外,通过变化环境温度,得出了系统在40度以下均能够保持很高的精度。当系统工作于连续工作或者间隔工作状态时,只要能够对系统进行有效的降温处理,系统就能够长时间维持高精度测量状态。
总体而言,本文详尽研究了基于多光束激光后向散射风场位移测量相关检测系统。该系统成功地克服了大气后向散射对高灵敏度探测的影响,解决了目标回波探测、低信噪比弱信号提取、空间滤波、精密延时和精确波门控制等技术,成功研制收发一体化系统。本系统具有距离分辨率为0.75米,测风误差有效控制在5%以内,并实现了对不同距离风场的实时准确监测。
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.
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