Full-Field 3D Experimental Modal Analysis from Dynamic Point Clouds Measured Using a Time-of-Flight Imager

Abstract

The ability to measure static, high-resolution 3D point cloud data has existed for multiple decades and has been used to great benefit in both civil and mechanical engineering applications. Recently, time-of-flight imagers have emerged that are capable of measuring 3D dynamic point clouds at rates as high as 30 point cloud captures per second with resolutions approaching the millimeter scale. Conventional modal analysis utilizes contact measurements that are captured in the Lagrangian (i.e., material) coordinate system. Imager measurements such as used for DIC are captured in what is approximately an Eulerian frame of reference. However, oftentimes the imager measurements are captured in a small-motion, sub-pixel regime and can be assumed to be captured in a Lagrangian reference frame. As a result, most experimental modal identification algorithms are designed to operate on data captured in a Lagrangian reference frame. Measurements of 3D point clouds of vibrating structures do not necessarily fit into either an Eulerian or Lagrangian framework, particularly in the case where motion of the structure is large. An additional feature of these measurements is that the number of points measured on the structure can change significantly through time as a result of occlusions, the change in angle of the structure, or simply noise in the measurement. This feature of point clouds is significantly different from imagers and contact sensors in which the dimensionality of the measurements does not change through time. In this work we present the first known technique for extracting structural dynamics information from dynamic point clouds.