A high-speed imaging system has been devised as a noninvasive means of collecting data on the kinematics of working models of developmental underwater or aerial vehicles that would utilize flapping fins or flapping wings for propulsion. The system includes two high-speed digital electronic cameras aimed along orthogonal axes that acquire snapshots of a model simultaneously in rapid succession. The data from successive images are postprocessed to obtain three-dimensional coordinates of points of interest on the model as functions of time. In the case of a flapping appendage, the points of interest are tips on the appendage, and the temporal evolution of the tip coordinates through multiple flapping cycles is utilized, in conjunction with computational fluid dynamics and other analytical tools, in an iterative process of testing and design directed toward improving the swimming or flying performance of the model. The system can, of course, be used as a noninvasive means of kinematic testing of models other than those of vehicles utilizing flapping appendages.
The cameras are positioned outside a tank wherein the model is mounted (see figure). The cameras are triggered to acquire images simultaneously, and are capable of acquiring as many as 10,000 frames per second. For the original flapping-fin application, a frame rate of about 50 times the flapping frequency is sufficient to capture the required kinematic information. The camera outputs are stored in a computer hard drive.
During postprocessing, the images acquired by the two cameras are displayed side by side, and in a manual procedure, the user selects the point(s) of interest in the two images. Then the system software implements a least-squares-best-fit linear transformation that converts the two-dimensional pixel coordinates of the selected points to three-dimensional coordinates. The transformation requires calibration data, which must be obtained in a separate prior set of similar observations of still three-dimensional targets (e.g., assemblies of blocks) that provide a sufficiently dense array of points having known three-dimensional coordinates distributed throughout the volume of interest.
The cameras are mounted on pivots on a framework of slider bars, so that their positions and viewing angles can be adjusted for different tests, subject to an overriding requirement to keep their lines of sight orthogonal. A new calibration must be performed whenever the cameras are moved to new positions.
This work was done by Jason Geder, William C. Sandberg, and Ravi Ramamurti of the Naval Research Laboratory.
This Brief includes a Technical Support Package (TSP).
Two-Camera Imaging System for Kinematic Measurements
(reference NRL-0027) is currently available for download from the TSP library.
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