From ignition of an automotive safety airbag to full deployment takes just 150 ms. The visible part of the process has been well documented; however, the first 10 to 20 ms during which the airbag is ignited and begins to unfold are essentially hidden from view with only isolated photos of this critical phase.
Philip Helberg and his colleagues at the Ernst Mach Institute of the Fraunhofer Institute for High-Speed Dynamics (Efringen-Kirchen, Germany; www.emi.fraunhofer.de) have developed a procedure that makes it possible to render the entire process visible. Using an extension of flash x-ray technology, he creates image sequences with intervals of no less than 10 μs at up to 100,000 frames per second. Exposure time per image is 20 ns so motion blurring is insignificant.
"A resolution of 1000 images per second is sufficient for examining the airbag-ignition process," says Helberg. The sequence begins about 5 ms after the airbag is activated and finishes eight images and 8 ms later. Multiple x-ray flashes irradiate the object under examination from one direction. The unabsorbed radiation strikes a fluorescent screen on the opposite side and illuminates it for 1.6 ìs. The images are registered by a CCD camera or an array of cameras positioned behind the screen. With the help of these exposures, airbag manufacturers should be able to determine the best way to stow airbags in their housings so that they unfold in the optimum manner.
