Laser Doppler velocimeter using a single longitudinal mode solid-state laser source

An experimental demonstration of using a single longitudinal mode solid-state laser source in laser Doppler velocimeter (LDV) is presented. The technology of frequency spectrum correction is used in processing Doppler signal. The results of the experiments show that: the magnitude and signal-to-noise ratio (SNR) of Doppler signal are both enhanced by the solid-state laser; the measurement accuracy of LDV is improved by the technology of frequency spectrum correction, and the variance of the measured Doppler frequency is larger than the Cramer-Rao low bound (CRLB) of Doppler frequency about one order of magnitude.     

基于Labview的激光多普勒测速仪的频谱校正

在激光多普勒测速仪中,为了减小由信号处理算法带来的测量误差,提出运用频谱校正算法对多普勒信号的频谱进行校正,详细阐述了能量重心法、比值法和相位差法3种离散频谱校正算法的基本原理,运用这3种算法对理想正弦信号进行仿真,并运用比值法对信号施加不同频率和信噪比的噪音下的正弦信号和实测的多普勒信号进行仿真及实验研究。结果表明:3种算法都可以使信号的频率更接近于真实值,其中能量重心法校正精度相对较低,比值法和相位差法精度较高,尤其是比值校正算法处理速度更快,校正的精度基本不受信噪比的影响。将比值法运用于激光多普勒测速系统,与光接收计数法测量结果进行比较,结果显示:校正后比较正前激光多普勒测速仪测量相对精度提高了2～3倍。     

频谱分析型激光多普勒测速仪信号处理技术

为了提高激光多普勒测速仪的测量精度,提出对激光多普勒信号先进行跟踪滤波,再进行频谱细化和频谱校正的信号处理方法,并利用信号处理系统对实测的多普勒信号进行了实验研究。实验结果表明:跟踪滤波器在滤除基底信号及系统的部分噪声的基础上,实时跟踪多普勒信号的频率,信噪比明显提高,所得结果与多普勒频率的真实值相一致;频谱细化技术大大提高了频谱分辨力,频谱校正技术使校正后的频率更接近真实值。     

Signal processing of double-beam and double-scattering laser Doppler velocimeter

The common method of laser Doppler velocimeter (LDV) signal processing is fast Fourier transform (FFT). Because of the restraint of spectral interference, fence effect and spectrum leakage, the precision of FFT is not high. In order to improve the accuracy of double-beam and double-scattering laser Doppler velocimeter signal processing, an algorithm based on spectral refinement and interpolation is put forward. First, high-pass filter is adopted to remove the baseplate of LDV signal. Second, FFT is adopted to get the estimate of Doppler frequency and then Zoom-FFT is used to refine the local range of Doppler frequency. Third, four-term fifth derivative Nuttall windowed interpolation is used to correct the refined spectrum. Then the accurate estimate of Doppler frequency can be obtained. Simulations and experiments based on the LDV system built in laboratory show that Zoom-FFT can improve the resolution of spectrum and four-term fifth derivative Nuttall windowed interpolation can further increase the precision of signal processing.     

小波分析在激光多普勒信号处理中的应用

 提出将激光多普勒测速仪(LDV)应用于车载惯性导航系统中,阐述了激光多普勒自身速度仪的基本原理和小波变换的相关理论,并运用小波变换对多普勒信号进行检测、去噪及提取多普勒频率,仿真及实验结果表明：信号进行小波分解后,对每一级小波进行阈值处理,得到了较好的去噪效果;在小波降噪常见的阈值原则中,无偏似然估计阈值和极值阈值不容易丢失信号中的有用成分,而启发式阈值原则和固定阈值原则可以更有效地去除噪声;对于提取多普勒频率而言,小波变换与快速傅里叶变换所得的结果是一致的,而小波变换不但可以求出系统自身的运动速度,而且还可以求出对应速度发生的时刻。     

Measurement of acoustic particle velocities in enclosed sound field: Assessment of two Laser Doppler Velocimetry measuring systems

The performances of two Laser Doppler Velocimetry (LDV) systems adapted for measuring the acoustic particle velocities are assessed in enclosed sound field. This assessment is performed by comparing the acoustic velocities measured by means of LDV to reference acoustic velocities estimated from sound pressure measurements. The two LDV systems are based on a single optical bench which delivers an optical signal called Doppler signal. The Doppler signal, which is frequency modulated, is analyzed by means of two signal processing systems, the BSA (Burst Spectrum Analyser from Dantec) on the one hand, and a system specifically developed for the estimation of the acoustic velocity on the other hand. Once the experimental setup has been optimized for minimizing the errors made on the reference velocities, the assessment is performed and shows that both systems can measure the acoustic velocity in enclosed field in two the frequency ranges [0-4 kHz] and [0-2 kHz] respectively for acoustic velocity amplitudes of 10 mm/s and 1 mm/s.     

频移分离型三维激光多普勒测速仪研究

研究一种采用频移分离方法的新型三维激光多普勒测速仪（３ＤＬＤＶ）。该仪器以单色Ｈｅ－Ｎｅ激光器和单只光电探测器并配合简单的光学发射和接收系统即实现了三维速度分量的同时测量。三个分量的速度信号由声光调制器引入的三种光学频移区分，通过电子滤波器完成信号的相互分离。在光学发射系统设计中采用独特的单轴四光束结构，通过光束组合形成三双光束差动模式的激光多普勒测速仪光。本文中还给出了该仪器的系统设计参数并分析     

参考光束型激光多普勒测速仪的误差分析

阐述了参考光束型激光多普勒测速仪的基本原理,从理论上分析了原理公式近似、高斯光束干涉、激光线宽、探测器孔径、有限渡越时间、信号处理算法、空气折射率变化及角度测量等诸多因素对激光多普勒测速仪测量结果造成的误差,研究并给出了每一种影响因素所引起误差的理论公式,在每一种影响因素中具体讨论了各种光学参数对测量误差的影响并对部分参数进行实验研究。理论分析和实验表明:在实验中需要选择线宽较窄的激光器及孔径较小的光电探测器,而在选择光束光斑半径和信号光中心光线的方向与粒子运动方向的夹角时,既要考虑激光的质量和多普勒信号的强度,同时也需要考虑原理公式近似、探测器孔径的尺寸、有限渡越时间及高斯光束干涉等多种因素对测量结果的影响。     

Signal processing of laser Doppler self-velocimeter

A frequency spectrum refinement and correction algorithm has been put forward to improve the accuracy of measurements, and the Cramer–Rao lower bound (CRLB) of Gaussian group Doppler signal was given based on the introduction of acceleration. Results of simulations and experiments showed that the Goertzel refinement algorithm could improve the resolution of the spectrum of the Doppler signal; the ratio correction algorithm made the results closer to the real value. The CRLBs of the estimated parameters were related to the sampling data, the signal to noise ratio (SNR) and the broadness of Gaussian group, and it can be decreased by increasing the length of the sampling data or improving the SNR; the gap between the variances of the measuring results and the CRLBs narrowed when the SNR of the signal was improved, and was almost eliminated when the SNR was higher than 6 dB.     

Application of Hilbert–Huang transform to laser Doppler velocimeter

A new method based on Hilbert–Huang transform is proposed to analyze the laser Doppler signal with a large acceleration. The Doppler signal is decomposed into several Intrinsic Mode Functions (IMFs) via empirical mode decomposition (EMD). And the Hilbert transform is used to compute the instantaneous frequency. The vehicle velocity parameter is estimated by taking linear fitting on the instantaneous frequency of the relevant IMF. The simulation results show that the HHT-based method is quite useful for the LDV that offers velocity parameter to the vehicle self-contained navigation system when the vehicle moves at a large acceleration.     

Solid state and fibre optic laser doppler velocimeters

Laser Doppler velocimetry (LDV) has become established as an important technique for the measurement of velocities of macroscopic objects and fluids: the dynamic range is large (～10^-6-10⁵ ms^-1) and the measurement is absolute and non-invasive. However, the size and cost of LDV has restricted its use in some areas. This paper presents two separate approaches to reduce these problems: we describe a compact LDV system incorporating a solid state laser diode and also an investigation of the feasibility of a fibre optic LDV system in which the conventional optical components are replaced by fibre optics.The experimental arrangement used for the solid state LDV system was of the Doppler difference type; i.e. a system of parallel interference fringes is focused in the measurement volume, so that a particle passing through this volume produces a scattered light signal which is intensity modulated. In its simplest form, the technique cannot determine the direction of motion of the particle, but this difficulty may be overcome by causing the fringes to ‘move’ within the measurement volume with known velocity. In the present experiments, the laser output frequency was modulated by modulating its drive current; since the path lengths of the two beams interfering in the measurement volume were unequal, fringe motion was achieved.The fibre optic LDV experiment was also of the Doppler difference type, and it was demonstrated that the necessary stabilised interference fringe system could be projected using a fibre optic system. An electronic servo was devised to compensate for the random differential thermal drifts in the fibres which would otherwise have produced unacceptable drifts in the fringe pattern.     

油中悬浮粒子激光多普勒测速瞬时速度的分析

二维频移激光多普勒测量油中悬浮粒子速度时,根据悬浮粒子通过针孔光阑时存在的四种不同情况,对原有激光多普勒测量悬浮粒子瞬时速度的方法进行了改进,提出一种悬浮粒子瞬时速度分析处理方法,利用该方法,对水平方管内的油中悬浮粒子的瞬时速度进行了测量分析。结果表明:改进方法获取的悬浮粒子瞬时速度的稳定性较好,能够表征在一组粒子通过激光多普勒的针孔光阑的时间段内粒子组随着时间的变化趋势,提高了频移激光多普勒采样数据的利用效率,有利于进一步准确表征悬浮粒子在油液中二维瞬时速度的分布特征和运动轨迹,为激光多普勒实验测量中悬浮粒子瞬时速度的表征提供理论支撑。     

Measurements of velocity distributions in the deformation zone in cold rolling by a scanning LDV

Experimental determination of the velocity distribution in the deformation zone is of significant importance to investigate the metal flow in both conventional and asymmetrical rolling processes. In this paper, a scanning laser Doppler velocimetry (LDV) system, designed for measuring the velocity distributions in the deformation zone in plate rolling is reported. The LDV used for the rolling process is briefly described and then the scanning mechanism based on beam displacement by a rotating transparent plate is introduced. The relationship among the scanning distance, the beam-cross angle of the LDV system and the rotating angle of the plate is established. The scanning LDV was first tested with a rotating disk and then applied in the rolling process. The test results have demonstrated the feasibility of the scanning LDV.     

自适应滤波技术在激光多普勒测速仪中的应用

激光多普勒测速仪检测系统提取的光电信号中存在较大的噪声信号。为了消除这些噪声干扰, 提高激光多普勒测速仪的测量精度,提出一种新的信号处理方法,将最小均方差自适应滤波技术应用于激光多普勒测量中,利用多普勒信号和噪声信号的统计特性,以最小均方误差估计为准则,最大程度地滤除噪声信号。阐述了最小均方差自适应滤波算法的基本原理,在MATLAB平台上将其应用于理想正弦信号进行仿真,并将其应用于实测多普勒信号的处理中。仿真和实验均表明,该技术可以有效抑制激光多普勒测量中的多频率噪声的干扰,大大提高多普勒信号的信噪比和测量精度,为设计高精度的激光多普勒测速仪创造了条件。     

Laser Doppler vibrometry based on self-mixing effect

In this paper, a simple, compact and low cost laser Doppler vibrometer is presented. It is simply composed of a laser diode (9 mm in diameter, 2 g of weight, emitting at 825 nm) and by a two-lens optical system designed to focus the laser radiation on the vibrating target. This sensor can measure target velocity. The working principle is based on the self-mixing effect that occurs in a semiconductor laser diode when the emitted radiation is back reflected toward the cavity and then re-introduced inside. The target velocity is calculated measuring the frequency of the peak of the spectrum of the signal generated by a photodiode mounted on the back facet of the laser diode. In this article, the design and the realisation of the vibrometer is reported. The self-mixing vibrometer has been calibrated in the range 0–300 mm/s, using a rotating disk covered with white paper. The effect of the angle of measurement and target distance are reported. Finally, the sensor has been compared with a commercial laser Doppler vibrometer using a vibrating surface as a target.     

Acoustic source identification using a scanning laser Doppler vibrometer

This paper shows how a scanning laser Doppler vibrometer (LDV), an instrument designed to measure vibrations of structures or objects, can be used in a non-traditional fashion to identify acoustical sources. This is achieved by measuring the changes in the optical path induced by local fluctuation of the air refraction index to which the LDV is sensitive. The acoustical signal used is sinusoidal and may be recovered by scanning at a uniform rate over a subject area (continuous scan) parallel to the source axis and demodulating this signal. Due to the fact that the measured scan area is in fact a line integral over a measurement volume between the laser head and a rigid object needed to reflect the laser beam, multiple view planes around the axis of the acoustic source are usually measured. These are then passed through a tomographic algorithm, thereby reconstructing the full sound field. In this article however, only one view plane is measured, but the acoustic source is placed on a rotating surface with fixed rotational frequency, thereby imposing a modulation on the measured spectrum. Demodulation will allow reconstruction of the three-dimensional sound field.     

An international review of laser Doppler vibrometry: Making light work of vibration measurement

In 1964, just a few years after the invention of the laser, a fluid velocity measurement based on the frequency shift of scattered light was made and the laser Doppler technique was born. This comprehensive review paper charts advances in the development and applications of laser Doppler vibrometry (LDV) since those first pioneering experiments. Consideration is first given to the challenges that continue to be posed by laser speckle. Scanning LDV is introduced and its significant influence in the field of experimental modal analysis described. Applications in structural health monitoring and MEMS serve to demonstrate LDV's applicability on structures of all sizes. Rotor vibrations and hearing are explored as examples of the classic applications. Applications in acoustics recognise the versatility of LDV as demonstrated by visualisation of sound fields. The paper concludes with thoughts on future developments, using examples of new multi-component and multi-channel instruments.     

车载激光多普勒测速仪参数估计的Cramer-Rao下限

考虑到加速度和高斯型包络对激光多普勒测速仪的影响,在引入加速度分量的基础上,研究了高斯包络型多普勒信号的参数估计.根据数理统计的理论,分析了多普勒圆频率及其一阶变化率估计方差的Cramer-Rao下限(CRLB),给出了参数估计方差的CRLB计算公式,并讨论了各参数带修正的功率谱估计.理论分析及仿真结果表明:参数估计方...     

Laser Doppler velocimeter with variable optical frequency shift

An opto-electronic system that allows a stable accurate frequency shift, variable from 10 kHz to 50 MHz, to be introduced in a laser Doppler velocimeter (LDV) has been developed. The system combines a fixed optical-frequency shift obtained using a Bragg cell with an electronic shift which results from mixing the detector signal with a variable local oscillator frequency phase locked to the Bragg cell driver. This paper discusses the advantages of the system in both a tracker and a counter based LDV.     

激光多普勒测速实验系统的信号处理

本文详细报导了运用集成锁相环路在激光多普勒测速实验教学系统中实现多普勒信号频率跟踪处理的设计原理和具体实施方案。该处理方法将集成度较高的 L M5 6 5锁相环应用于系统 ,使系统具有电路简单、可靠、可实时测量和结果直观、易操作等特点