Even the slightest fault in aircraft engines could pose a safety risk. Until now, inspectors have relied solely on their well-trained eye to unmask defects on blade-integrated disks, known as blisks. In the future, they will be supported by an automatic control system that is the first to unite geometric measurement and surface inspection. Researchers will introduce the AMI4BLISK system at Control 2015, the international trade fair for quality assurance in Stuttgart on May 5 – 8. (Hall 1, Booth 1502)
Day after day, an aircraft hurtles down the runway, slowly lifts from the ground and climbs towards the clouds. To do this for years at a time and still remain safely in the air, jet engine turbines must be precision engineered without even the slightest flaw. That’s why, before installation, inspectors examine every single blisk – the turbine disk and its 30 to 60 blades – very carefully for six to eight hours. In this process, they rely exclusively on their practiced eye to spot any problems.
Capturing the geometry and analyzing the surface
In the future, inspectors will receive technical support via the AMI4BLISK system (Automated Geometrical Measurement and Visual Inspection for Blisks): it measures the blisk’s geometry and also automatically examines it for defects. Researchers at the Fraunhofer Institute for Industrial Mathematics ITWM in Kaiserslautern developed the system in close cooperation with Hexagon Metrology GmbH and the Hexagon Technology Center GmbH. The EU project is part of the Clean Sky joint technology initiative. “With our system, inspectors can examine the blisks almost twice as fast as before – and with the same precision,” says Markus Rauhut, head of department at ITWM. “What’s more, we always have the same recognition rate: the system never gets tired and even night shifts cause no complaint.”
ust as before, geometric measurement is performed using a coordinate measuring machine. A cantilever arm with a probe touches several hundred points on the blisk. In the future, the machine will also examine surface quality. Instead of just a measuring probe, the cantilever arm will be outfitted with two cameras and a light to ferret out scratches, dents and pressure marks. The corresponding software collects details in a list about the type and exact position of each flaw. An optical sensor developed by Hexagon in turn uses the data to measure the individual defects in more detail. Although it can inspect small areas only of up to one square millimeter in size, the sensor measures more precisely than a camera.
Objective test reports
Once these investigations have been completed, the human inspector steps in and takes a closer look at any defects detected, using the report generated by the system and a 3D representation of the blisk. Color coding reveals which areas of the turbine the system analyzed automatically. If the camera was unable to identify certain spots – inside a drilled hole, for instance – those areas are marked red on the computer display. “The system doesn’t just deliver an objective test report, but also confirms that the entire blisk was examined,” explains Rauhut.
One of the challenges for the camera system is determining exactly how far away the camera is from the respective blisk part. This is important because the software can’t calculate how long or how deep scratches or dents are if the distance information isn’t accurate. Otherwise, areas that are further away appear smaller than they actually are. The CAD data for each blisk offers the solution: the software takes the information about the blisk’s exact geometry and turns the virtual CAD turbine so that its position in space perfectly corresponds to the test subject’s real position. Then for each individual pixel, it calculates the distance from the camera to the point on the blisk. This means the optical sensor‘s tool can provide exact information about where a defect is located, as well as how long and wide a scratch is.
The core system is already complete. Researchers will introduce the AMI4BLISK system at Control 2015, the international trade fair for quality assurance in Stuttgart on May 5–8. You can find it at the Fraunhofer shared exhibition stand, which is coordinated by the Fraunhofer Vision Alliance (Hall 1, Booth 1502). This year, the booth focuses on Industry 4.0. In the next step of the development process, the scientists will aim to have the AMI4BLISK system ready for the market in 2016.
Filed Under: Aerospace + defense