Photogrammetric measurement of deformations of horse hoof horn capsules

摘要

In a coorperation of the Department of Veterinary Surgery at the University of Zurich and the Institute of Geodesy and Photogrammetry at ETH Zurich, a system for the measurement of three-dimensional deformations of horse hooves under different load conditions has been developed. The system consists of a force sensor panel and three Sony XC75CE CCD cameras mounted on a trolley which can be moved on a circular rail around the hoof. To achieve a reliable photogrammetric network and to ensure the full coverage of the hoof, triplets of images are acquired at three different positions of the trolley. The nine images are processed separately for each single experiment to determine discrete marked points mounted on the horn capsule of the hoof. Prior to the experiment, the horse was sedated to reduce its movement to a minimum. The force sensor panel is used to assure a more or less constant load condition of the hoof during the image acquisition. To increase the accuracy of the object point determination, a calibration of the three cameras is performed before the experiment. For the definition of the reference system, 84 points were milled into the anodized surface of an aluminium plate. From the specifications of the controlled manufacturing machine, the coordinates of the reference points can be expected to be milled with a precision in the order of a few micrometers. The aluminium plate has a size of 0.5x0.5 meters and in its center a notch for the force sensor panel upon which the hoof is placed. For a good visibility of the reference points a diameter of 8 mm was chosen and to improve the contrast on the images the milled position was covered with retro-reflective sheeting. The circular rail for the camera trolley has a diameter of 75 cm and 6 mm objectives are used to get a suitable field of view. Using nine images distributed regularly around the hoof, each marked object point is visible and can be measured on three images. The reference points on the aluminium plate might even appear on more images and provide for a well-defined photogrammetric network. To analyse the deformation of the horn capsule, another set of nine images is acquired after changing the load conditions by remaining the horseshoe or placing a wedge pad under the hoof. After image acquisition the data is processed with the commercial software PhotoModeler® from EOS Systems Inc. to determine the 3-D coordinates of the marked points on the hoof. The estimated precision of the 3-D points positions is in the order of 0.1 - 0.2 mm, which is sufficient for this application. The two resulting 3-D point clouds can then be used for a deformation analysis. The paper describes the basic design of the system, discusses a calibration strategy and presents first results.