High accuracy low-cost videogrammetric system: an application to 6DOF estimation of ship models

The article reports the development of an off-line low-cost videogrammetric system for measuring six degrees of freedom (6DOF) of scaled models in ship model basin. Sub-millimeter accuracy is required to measure the floating rigid body movements. To meet this requirement, in depth analyses, exposed in this paper, are performed to choose the most appropriate number of cameras, their configuration and a proper technique for camera synchronization. The proposed system, composed of three consumer-grade High Definition (Full HD) video cameras, is used to record interlaced video sequences at a frequency of 50 frames per second. A special device which emits simultaneously sounds at known frequency and flashes a LED is used to introduce a common event used for an automatic a-posteriori synchronization of video sequences up to 1 msec. The video sequences are synchronized using matching procedures based on cross correlation between audio signals recorded by camcorders. The ship model is targeted with retro illuminated (LEDs) targets whose positions in the ship reference frame are also measured with photogrammetry. The 6DOF of the ship model are estimated on the basis of rigid transformations computed through the image sequences with the tracked active targets. An error analysis is performed with the assumption of the rigid body using the target coordinates known with photogrammetry. The measured synchronization error is used to correct the image trajectories of tracked points. An improvement of the accuracy of a factor 5 was observed for the trial with highest velocity of tracked points (up to 0.35 m/s).