Using Light to Reverse-Engineer a Steam Turbine
- Saturday, 01 May 2010
Generating this number of points with a CMM would take months, as the most that can be captured in a week is probably somewhere in the tens of thousands. Of course, it is possible to intelligently select and capture only the most critical areas, but you are often left approximating contours and can never be totally sure that you aren’t missing important points. Like a CMM, the MAXOS follows a pre-programmed path, but the MAXOS can capture cross-sections at a rate of 100 points per second, many times faster than a CMM. Another problem with the CMM is that there is no way to determine the angle at which the contact probe is contacting the surface of the part under measurement. This leads to a phenomenon known as cosine error in which the measurement is off by the cosine of the probe radius.
Point of Light
The MAXOS makes measurements with a single point of concentrated light rather than a touch probe. The light is measured using triangulation by a camera like a conventional laser scanner. However, the MAXOS provides higher accuracy than any laser scanner and provides accurate measurements on shiny surfaces because it reads the very center of the point of light. The MAXOS can inspect turbine blades without having to apply a matt coating like other non-contact scanning systems, which introduces dimensional inaccuracy.
NVision and the OEM worked jointly to determine the areas that required measurement, such as the roots and leading and trailing edges of the turbine blades, and then NVision technicians wrote a program to capture just those points. The MAXOS generally provides accuracy of 2 to 10 microns depending on the surface finish of the part. The accuracy of the system is ±0.0004" on polished and machined services and ±0.0001" on matt finished surfaces. On the other hand, the accuracy of the laser scanner is typically in the 1 to 5 thousandths range depending on the length of the arm. The MAXOS system eliminates the inaccuracies inherent in contact probe measurement on small radii or sharp edges and can measure radii down to 0.004". The MAXOS is used in-house by turbine OEMs including Toshiba, GE, Alstom, and Siemens.
The NVision HandHeld scanner, used in this case to inspect the larger components, is a portable device that is capable of capturing 3D geometry from components of virtually any size. It is attached to a mechanical arm that moves about the object, freeing the user to capture data rapidly and with a high degree of resolution. An optional tripod provides complete portability in the field. Intuitive software allows full model editing, polygon reduction, and data output to all standard 3D packages.
The technicians visited the OEM’s site and scanned millions of critical points in only three weeks. They scanned all of the turbine components including the case, inlet chamber, blades, rotors and diaphragm using the NVision Hand-Held scanner and touch probe. They scanned the smaller blades up to 12" in size using the MAXOS non-contact measurement system.
Next, the technicians used software to convert the points to STL file format, which consists of a triangulated mesh. The STL model was converted to a fully parametric CAD model, which took another three weeks. NVision engineers then edited the resulting CAD models by hand to correct machining inaccuracies in the as-built parts. The CAD models were used by the turbine manufacturer as the basis for CFD simulations, the results of which helped to design new blades and diaphragms. The new designs saved considerable amounts of energy by dramatically improving the efficiency of hundreds of existing turbines.