48 B. Weekes and D. Ewins Notional inspection distance, z0 qy z x y True measurement direction Assumed geometry SLDV Assumed node location <x,y,z>, generated by projecting SLDV beam at mirror angles <θx,θy> onto plane z = z0 True geometry True node Assumed measurement direction Fig. 5.3 Schematic of assumptions in a typical SLDV test geometry without range-finding EMA test-point FE node Search limit 1 Search limit 2 Search limit 1 Node-point pairing (closest FE node within search envelope) a b Fig. 5.4 Schematic showing (a) typical FEA/EMA node-point pairing, (b) a simple, improved FEA/EMA node-point pairing method for SLDV data by the pixels in the video feed is the only geometry information available, and is often converted into units of measurement by simple scaling. Since the geometry lacks a depth dimension, the geometry is limited to a plane (Fig. 5.3). Further, the velocity measurement is typically considered to be perpendicular to the plane, rather than along the laser beam axis. This places limitations on the test design since the assumption that the velocity is out of plane could lead to significant inaccuracy in a model correlation. An accelerometer array geometry is usually fitted to the FE model most easily by simple minimum distance (Fig. 5.4a). There are often small errors in the matching process due to mismatches between the FE and EMA geometries, which are typically corrected more easily for accelerometers simply because there are usually fewer accelerometer measurement points.
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