May 19 2006
Take a close-up look at an aircraft and you soon realize what holds it together – rivets. Several hundred thousand metal rivets go into every passenger aircraft. The riveted joints are subjected to enormous forces. A new quality shows defects during production.
Riveted joints used to be inspected manually. Test engineers would make sure the rivet head was not scratched and run their thumb over the rivets to check how far each rivet protrudes. Critical joints were also checked using a dial gauge. Yet this manual check is time-consuming. Depending on the size of the aircraft, the test engineers need anything between an hour and a couple of hours to inspect the riveted joints on the fuselage. This downstream quality check has another disadvantage: Say a chip caught in the drill causes a defect – then up to 150 defective metal rivets might be fitted before the problem comes to light.
Engineers from the Fraunhofer Institute for Factory Operation and Automation IFF are working together with Airbus Deutschland GmbH on an automated inspection system. Their aim is to integrate the system into the riveting machines so that defects can be uncovered during production. "Since a joint is riveted every 4.5 seconds, one of the main tasks when developing the system was to ensure data was recorded quickly to monitor quality. In such circumstances, optical inspection techniques were the only viable option", explains Dirk Berndt of the IFF in Magdeburg. Other requirements meant that the system had to carry out a full inspection, provide reliable feedback on existing defects, be easy to use and interface with existing process control systems. And all that without adding substantially to the riveting cycle.
To meet these varied requirements, the scientists have come up with a sophisticated inspection concept, “OptoInspect” for three-dimensional rivet measurement. Once a joint has been riveted, a pattern of 18 strips of light is projected onto the rivet head and recorded by a camera. A three-dimensional cloud of measuring points is calculated from the stripe pattern. The rivet head protrusion and the tilt angle can then be determined from this data. If a riveted joint is defective, the riveting machine operator has all the necessary information to decide whether it is a one-off defect or whether the equipment needs to be shut down immediately, to remove a chip caught in the drill or replace a worn tool. Again the measurement process takes just a second. The inspection process is currently being tested at the Airbus plant in Nordenham. If the system manages to withstand the rigors of daily series production, it will be integrated into the existing riveting machines.
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