Development of a Laser Speckle Interferometer to Measure Two-dimensional In-plane Vibration in Real-time Based on Fourier Transform with Electronic Circuit

为了研制一种能实时、高精度测量多维位移的高性能激光散斑干涉仪，提出了一种基于四孔径法的激光散斑干涉仪，并对该干涉仪进行了理论和实验研究。设计了二维傅里叶变换的专用电路。所得结果总结如下。(1)A建立了一台四孔径法激光散斑干涉仪样机。并且通过用相关方法检测点阵型干涉图像的二维位移，可以测量目标的二维位移。（2）从理论上推导了点阵型干涉像的二维傅里叶变换。结果表明，在两倍于干涉图像基频的空间频率处，目标位移与相位谱成正比，与相关法相比，计算量显著减少。（3）进行了二维静态位移的测量实验，并对引起测量误差的因素进行了讨论。结果表明，随着干涉图像的平均可见度的提高和图像中相位变化的减小，测量误差减小。(4)By选择激光照射点，使得干涉图像的平均可见度高并且图像中的相位变化小，并且通过对图像应用25点中值滤波器，减小测量误差。(5)An设计了一种模拟电路，可根据二维傅里叶变换实时计算C-MOS图像传感器获得的干涉图像的基频分量的相位。

Aiming to develop a high-performance vibrometer which can measure multi-dimensional displacement with high accuracy in real-time, a new laser speckle interferometer based on the quad aperture method with a phase detector of interference image using two-dimensional Fourier transform was proposed and was theoretically and experimentally examined. Analogue electronic circuit for two-dimensional Fourier transform was then designed. The results obtained are summarized as follows.(1)A prototype of a laser speckle interferometer with the quad aperture method was built. And by detecting a two-dimensional displacement of lattice type interference image with correlation method, a two-dimensional displacement of a target can be measured.(2)Two-dimensional Fourier transform of the lattice type interference image was theoretically derived. It was then revealed that displacement of the target was in proportion to the phase spectrum at the spatial frequency twice as high as the fundamental spatial frequency of interference image and that the calculation cost could be remarkably reduced in compare with that of the correlation method.(3)Experiments to measure the two-dimensional static displacement were carried out and then the factors to cause measuring error were discussed. It was revealed that the measuring error became smaller as the average visibility of the interference image was higher and as the phase variation in the image was smaller.(4)By selecting laser irradiation point so that the average visibility of the interference image is high and that the phase variation in the image is small, and by applying the 25 points median filter to the image, the measuring error becomes reduced.(5)An analogue electronic circuit which can calculate the phase of the fundamental frequency component of the interference image obtained with C-MOS image sensor according to two-dimensional Fourier Transform in real-time was designed.