Turbo Stress

Turbines that are housed in aircraft engines are subjected to pretty tough conditions. They must perform at speeds of 30 thousand rpm in temperatures greater than 800ºC for hours at a time.

 

The engine manufacturers fully understand that even small surface defects can reduce performance, increase maintenance costs, and reduce the useful life of an aircraft engine. They need to inspect turbine blades very carefully to maintain the efficiency and reliability that the air transport industry requires.

 

One particular North American manufacturer inspected its blades by hand and human eye. The highly-trained inspectors measured hundreds of features and checked for surface defects at depths in the order of thousandths of an inch. Manual inspection was not only costly in terms of time and labor, but subjective as well. Results were variable and even differed between inspectors. Finally, because manual inspection was so time consuming, there was no systematic inspection of every blade; only a sampling of blades were inspected. Clearly, the manufacturer required an approach that would allow systematic inspections of the blades, save time, and yield consistent and repeatable results.

 

They approached Orus Integration Inc. (Laval, Quebec) to design a turbine inspection system. Project Manager Louis Dicaire says that early in the project, the development team learned that flexibility, repeatability, and precision were absolutely necessary for success. During development, the Orus engineering team relied on their previous experience - they designed vision-based metrology systems for the Canadian military and aerospace industry. They also worked closely with Genik Automation for part handling and mechanical engineering of the machine.

 

The INL-1900x2T has three inspection roles to fill: verify several hundred metrology features of the blade, inspect both sides of the turbine blade and other critical surfaces for defects, and validate the part's character markings. The entire inspection procedure takes 15 seconds per part.

 

To perform a batch inspection, an operator first scans the barcode on the job sheet with a barcode scanner and loads the pocket wheel with the carousel that holds the parts. Then the wheel indexes the first part while a height detector validates its Y position to ensure the part was properly loaded. The robot picks up the part by its blade section and carries it to the metrology station, which is illuminated by the two collimated lights. With the camera's telecentric lenses, and the 4-inch slab of granite to absorb heat and vibrations, the INL-1900x2T has a very stable optical system. "Under these conditions, the contrast of the round sections of really shiny objects appear super sharp," explains Dicaire.

 

 

Orus has used Matrox Imaging Library (MIL) for almost nine years. "This is probably the project where we dug deepest inside the library," says Dicaire. One of the implemented algorithms features an adaptive threshold: the algorithm dynamically locates bright spots in dark areas and dark spots in bright areas. For other operations, the developers at Orus used the GUI interfaces for several MIL modules: OCR, Edge Finder, Geometric Model Finder, and of course Metrology. The metrology software was almost exclusively developed with the GUI so the system could learn the parts. Indeed, the INL-1900x2T's flexibility means the system can accommodate several different parts, even new parts that are still under development. "With the GUIs," explains Dicaire, "we can build a system that lets our clients be more autonomous with the final product. We find that, with the level of complexity of our projects, MIL's ease-of-use, performance, flexibility, and the great support from [Matrox Imaging's] Vision Squad team are the reasons we continue to develop with MIL."

 

 

Dicaire says that the challenge of designing metrology machines are always the same: repeatability, precision, and linearity. To get the system to return predictable, repeatable results, the software must exhibit fine sub-pixel accuracy; the machine shows +/-3 sigma under 5 microns. Of course, an image is only as good as its lighting, and Dicaire notes that the system requires a stable and high-performance optical system. To achieve the required precision, Orus used a military-grade calibration target to calibrate both cameras at the same time.

 

Though the INL-1900x2T saves thousands of hours of labor, its main advantage is its ability to perform very complex analyses, while offering a simple interface and a very easy-to-use concept for the operators. "This project uses an array of field-proven technology: robot, axis, vision library. All of this enables the machine to grow and adapt to the client's future needs. Except for the mechanical design, almost all components are off-the-shelf."

 

"This machine is a mix of technologies that are all dedicated to make the vision system work in optimal conditions," says Dicaire. "It's a great example of what we can do."