Pelton turbines are used in hydroelectric power plants with high water pressure and a low amount of water (fig. 2). As these water turbines run at a great speed, the rotor's geometry and a steady rotation are of great importance. 3D scanning of the rotor starts with photogrammetric shots (TRITOP) using a professional digital camera (fig. 3). The TRITOP software then processes these images and calculates the exact position of the reference points applied to the turbine's rotor and blades. The ATOS system projects a dense fringe pattern on the surface of the blades and scans the visible part by means of two calibrated stereo mounted digital cameras. During this process, the system also exactly measures the reference points visible in the measuring area. Based on the recorded reference points, the scan data are automatically transformed into the mesh of the predefined reference points (object coordinate system). Each scan takes about 2 seconds and - depending on the ATOS system used - generates up to 4 million 3D measuring points. Thus, the shape of the scanned blade segment can be determined precisely. The scan process was repeated until the turbine blades were recorded from all sides. The common coordinate system of the individual scans is defined by the TRITOP measurement but can be adapted to current requirements at any time. Fig. 4 shows the 3D measuring data of the rotor digitizing. The resulting data file consists of millions of 3D points describing in detail the shape of each blade. The normal point density for such measurements is 10 points per mm, corresponding to a measuring point distance of approx. 0,3 mm. If required, the ATOS system can easily be adjusted to scan much more or fewer measuring points.
Such a detailed 3D model detects the slightest deviations from the ideal geometry of the rotor. The data of the scanned turbine blades can be compared with the CAD model (if available), with the data of other blades or with the mirrored data of the same blade (for symmetry checks), see fig. 5. The deviations in the shape are represented in different colors corresponding to the scale on the right side in the picture. Areas in which the shape of the turbine blades deviates up to 2 mm are clearly visible (red and blue). It is also possible to display and export the result with fewer measuring points (thinned data set) or as section data so that, depending on the task to be performed, the measuring data can be loaded into less powerful CAD systems (fig. 6).