Technology update
Laser measuring system designed for airfoil metrology
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Laser Measurement International's PACS system is specifically designed for precision measurement of gas turbine airfoils for aircraft engines.
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Measuring compressor and turbine blades can be challenging because of the complex three-dimensional compound curves on their surfaces, and critical small radii on their leading and trailing edges. Hundreds of points make up each curve, making inspection an even bigger challenge. Laser Measurement International (LMI) has developed a laser-based, computer-controlled measuring system specifically designed for airfoil metrology. The system, called PACS (Programmable Automatic Contour System), uses fast and accurate noncontact laser-based sensors to profile surface points. The sensor, which is based on dual-viewing laser triangulation principles, provides surface measurement accuracy up to 0.00005 in.
Because of the many different processes performed and the surface protection coatings applied to the airfoils during manufacture, surface reflectivity may vary significantly, making measurement difficult for conventional laser systems. The LMI sensor overcomes these difficulties with its gain control that automatically compensates sensor operation for any surface reflectivity changes in the range of 100,000 to 1. The effective size of the laser beam (0.001 in.) also benefits measurement by eliminating point of tangency errors so that the small edge radii on compressor blades can be accurately quantified.
Within the PACS system, the sensor is mounted on specially designed, high accuracy positioning tables configured in a cylindrical design (three linear and one rotary axis). Axes are split for maximum accuracy. The PACS controller uses software designed to provide inspection path planning, measuring control, and data analysis display.
CAD models can be downloaded by PACS for part actual to nominal comparisons, inspection of parts, or reverse engineering of unknown parts. Point-to-point, on-the-fly scanning, and manual modes of operation are provided by the system. Using on-the-fly scanning, concave and convex surfaces can be scanned at rates up to 20 points per second. Reporting software provides the ability to plot contours, compare actual to nominal, compare surfaces to tolerance bands, and determine calculated parameters such as lean, twist, bow, and displacement at various locations.
Frank Bokulich
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