考虑相机运动补偿的视觉模态测试方法

Vision-based structural modal analysis in presence of camera motions

基于计算机视觉的结构模态测试方法因其非接触式的特点和全场测量的优势而备受关注,但此类方法对相机运动敏感,通常要求相机固定以保证测量精度。本文提出一种考虑相机运动补偿的视觉模态测试方法,采用基于特征匹配的单应变换对原抖动视频进行校正,补偿相机的六自由度运动以得到稳定视频;在结构表面设置“虚拟视觉传感器”,利用欧拉视角的稠密光流法估计结构的振动响应;通过随机子空间法识别结构的模态参数。为验证所提方法的有效性,通过手持智能手机拍摄的方式采集模拟抖动的视频,并开展悬臂梁的单频振动试验和模态辨识试验。将视觉模态测试结果与扫描式激光多普勒测振仪的测试结果进行比较,前五阶模态频率和阻尼比的平均误差分别低于0.4%和11.5%,模态振型的MAC矩阵对角值高于98%。试验结果表明,所提方法有效地消除了相机运动的干扰,并获得了良好的结构模态参数估计精度。

Vision-based modal analysis techniques have gained attention due to their non-contact,full-field measurement capabili-ties,making them particularly suitable for the dynamic testing of large-scale or thin-walled structures.However,these techniques often require cameras to be fixed to the ground to avoid coupling with the vibrations of the test structure,a requirement that can be too restrictive in real-world applications.This paper proposes a method to compensate for camera motion using homography trans-formation,followed by the extraction of the test structure’s movement by applying the dense optical flow method to the stabilized video.The procedure involves transforming the video captured by a moving camera using feature matching algorithms,where a ho-mography matrix compensates for six degrees of camera motions.Several"virtual vision sensors"are selected on the edges of the structure,and their vibrations are estimated using optical flow methods.Structural modal parameters are then extracted from the output-only data using stochastic subspace identification algorithms.The proposed procedure was applied to videos recorded using a moving smartphone to conduct an operating modal analysis of a 2 m cantilevered beam.To validate the procedure,the vision-based analysis results were compared with measurements taken with a Scanning Laser Doppler Vibrometer.The results show an average discrepancy of 0.4%and 11.5%for the first five natural frequencies and damping ratios of the beam,respectively.The mode shapes also show strong correlation between the two measurement techniques,as indicated by the diagonal MAC values greater than 98%.Therefore,the proposed procedure effectively cancels out camera motions and achieves accurate estimation of structural modal parameters.