Single High-Speed Camera Based 3D Deflection Reconstruction in Frequency Domain
High-speed camera systems are a well-established alternative to traditional vibration measurement techniques, particularly in cases when the region of interest on the observed object is near-planar. With the introduction of 3D digital image correlation so
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Single High-Speed Camera Based 3D Deflection Reconstruction in Frequency Domain Janko Slaviˇc, Domen Gorjup, and Miha Boltežar
Abstract High-speed camera systems are a well-established alternative to traditional vibration measurement techniques, particularly in cases when the region of interest on the observed object is near-planar. With the introduction of 3D digital image correlation some of the traditional limitations of 2D imaging systems are eliminated, but the limited field of view of stereo camera pairs remain problematic in some applications. In this paper the possibility of extending the use of high-speed camera systems to vibration measurement of arbitrarily shaped structures by applying methods, commonly used in multi-view computer vision is explored. A single high speed camera is used to record the vibrating structure from multiple points of view. By utilizing properties of linear, time-invariant mechanical systems, multi-view triangulation is then performed in frequency domain on displacement data, extracted from these image sequences using optical flow or digital image correlation. The acquired 3D spectra are finally used in full-field deflection reconstruction. Keywords High-speed camera · Vibration measurement · Frequency domain · Multiview geometry · Optical flow
2.1 Introduction The use of high-speed cameras in vibration measurement is best suited to near-planar structures due to an inherent limitation of 2D imaging systems. Depth information, lost in the imaging process, can be recovered by using the well-established 3D DIC technique [1]. Its field of view is, however, usually limited to a single face of the object, observed by the stereo pair. In recent years, various methods have been proposed that extend the use of digital cameras for displacement measurements to objects of arbitrary shapes and dimensions. These methods employ the principles of multiview geometry and triangulation [2] to extract spatial information from simultaneously acquired image sequences of the observed mechanical process [3– 5]. Multiple digital cameras used in the measurement process can be arranged in various configurations around [6]. Data acquired by a moving stereo-pair of high-speed cameras can also be used to extend the field of view of 3D DIC in a process called surface stitching [7, 8].
2.2 Measurement Setup A concave steel object, composed of three 1 mm thick 200 × 200 mm sheet metal planes, bent and welded at one seam (Fig. 2.1) was placed on a LDS V555 electrodynamic shaker and excited with a constant profile of 3 g in the 25 Hz–2000 Hz frequency range. The object was mounted to the shaker in such a way that the excitation force vector formed an equal angle with all of its three planes. Six separate video sequences were acquired using a single Photron Fastcam SA-Z monochrome high-speed camera operating at 20,000 fps and a resolution of 640 × 640 pixels. Each video contained a sequence of 20,000 images for a sampling period of 1 s. The camera was stationary during the measurement process, but the object was
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