By John Rhea
PLYMOUTH, Minn. — The effects of vibration in limiting mean time between failure (MTBF) of airborne electronic systems have long been known empirically, but the increasing availability of computer-aided design (CAD) tools is enabling system designers to simulate these effects and design around them.
That is the approach that John Starr, a structural consultant at CirVibe Inc. of Plymouth, Minn., is taking with a software package that complements trial-and-error testing with predictive analyses. Airborne assemblies are particularly prone to vibration, he says. High-G loads on wing-mounted systems can cause solder joints to break.
Starr estimates a typical airborne radar experiences forces of 6 Gs while such other systems as a laser system to measure the amount of oxygen in the fuel of the engine of the F-22 fighter can experience forces as high as 44 Gs.
To use commercial off-the-shelf (COTS) electronics with confidence, he continues, someone in the development team — and preferably all the way through design, development, production, and test — should understand vibration capabilities. This requires understanding at the component level, Starr says.
"Understanding vibration means being able to list the few components that might be at risk of failure during a vibration test, and list the many components that would survive if tested at that level for 100 years," he says.
The benefit of positioning components for shock and vibration loading depends on the life fraction consumed by this portion of life cycle loading. If the life fraction consumed by vibration and shock is 1 percent, there is no gain from positioning. If the vibration life fraction is 95 percent, it may be possible to increase MTBF by a factor of 20.
One of the users of the CirVibe software package is Ed Abner, senior engineer at CPI Satcom of Palo Alto, Calif. Abner reports that he was able to eliminate costly and time-consuming highly accelerated life testing (HALT) and highly accelerated stress screening (HASS) on the hub mount traveling wave tube low power amplifier for a Ku-band satellite ground station.
Abner estimates the HALT and HASS physical tests would have taken three or four days and cost $2,500 a day. The use of a computerized alternative cut non-recurring engineering costs by at least $10,000, he says. It also increased confidence that the terminal would survive in severe weather conditions in both fixed and mobile applications.
The CirVibe software package runs on Windows 95. When interfaced with a CAD system running with Pentium 4 GHz microprocessors, such as Mentor Graphics, it can create detailed models in a few minutes. The package costs $9,500. For more information contact Starr at CirVibe 763-559-5166, by post at P.O. Box 47394, Plymouth, Minn. 55447, or on the World Wide Web at http://www.cirvibe.com/.