Measuring dynamic motion in three axes using multiple cameras
Systems and methods are provided to perform measurements of vibratory motion in three axes using two or more cameras. The cameras are mounted in a fixed spatial relationship to each other and are calibrated such that internal camera distortion and external camera coordinate transformations are determined. Data from each pair of cameras can be processed independently to determine the z-axis motion from changes in the x-axis and y-axis motion measured by the cameras. Measurement noise can impact the small differences in values from camera pairs and multiple techniques have been identified to reduce noise and make reliable measurements of the z-axis vibratory motion. If more than two cameras are utilized, then the data from each pair can be averaged to improve the measured values.
1 . A system for measuring vibratory motion along three spatial axes of an object undergoing movement, from video recordings comprising a plurality of video images of the object as it undergoes movement, the video images being divisible into individual video image frames with each individual video image frame being divisible into a plurality of pixels, said system comprising:
two or more video acquisition devices operably connected to a processor, that have a fixed spatial relationship and matching lens that are calibrated to determine a depth or series of position measurements in a direction parallel to the optical axis of one of the video acquisition devices serving as a reference camera to a point on the object undergoing motion;
a computer program operating in said processor to:
adjust a frame rate of the two or more video acquisition devices to provide for a sampling rate to capture a plurality of frequencies present in the vibratory motion and acquire the individual frames on all video acquisition devices simultaneously;
calculate a displacement of the object along two axes of the vibratory motion representing the focal plane of each camera pair, wherein the two axes are a first axis referred to as an x-axis and a second axis referred to as a y-axis, respectively; and
determine the vibratory motion in a third axis, referred to as a z-axis, wherein the third axis is parallel to the optical axis of the reference camera.
2 . The system of claim 1 , wherein when the video acquisition devices are horizontally-aligned, the vibratory motion is derived based on differences in the first axis motion waveform constructed from the video images from the two or more video acquisition devices, and when the two or more video acquisition devices are vertically-aligned, the vibratory motion is derived based on differences in the second axis motion waveform constructed from the video images from the two or more video acquisition devices.
3 . The system of claim 1 , wherein when the two or more video acquisition devices are horizontally-aligned or vertically-aligned, the vibratory motion is derived based on differences between matching frequency peaks in the first axis motion frequency spectra constructed from sampled data from the two or more video acquisition devices, and when the two or more video acquisition devices are vertically-aligned, the vibratory motion is derived based on differences between matching frequency peaks in the second axis motion frequency spectra constructed from sampled data from the two or more video acquisition devices.
4 . The system of claim 3 , wherein the computer program operates to increase a measurement accuracy of the third axis vibratory motion by constructing averaged spectra from at least one of sequential overlapped averaging on a longer data set or by averaging data sets from multiple, independent sequential recordings.
5 . The system of claim 1 , wherein the computer program further operates to calculate additional measurements of the third axis vibratory motion from at least one additional pair of calibrated video acquisition devices to increase a measurement accuracy of the third axis vibratory motion.
6 . The system of claim 1 , wherein the computer program further operates to identify at least one feature on the object as a site for evaluating the vibratory motion of the object.
7 . The system of claim 1 , wherein the computer program further operates to identify at least one of a fiducial mark or a target mounted in or on the object.
8 . The system of claim 1 , wherein the computer program further operates to track multiple features in a selected ROI to increase a measurement accuracy of the third axis vibratory motion.