Volume 8, Number 4     July/August 2000

Acoustic Emission Diagnostics Improved

Acoustic emissions have been used to detect crack development for more than 20 years, but a Huntsville, Alabama-based company has put a new spin on the process. Traditional methods entail listening for acoustic emissions, or “hits,” for possible crack initiation and crack growth. However, the Acoustic Emission Bearing Fault Diagnostics System (AEBDS) developed by AI Signal Research focuses on the periodicity of these hits.

In a Small Business Technology Transfer (STTR) effort with NASA Marshall Space Flight Center in Huntsville and Georgia Tech University in Atlanta, Georgia, AI Signal is employing high-frequency sensors to monitor periodic hit rates and patterns for incipient fault detection of rolling element bearings. With the AEBDS, such competing noises as rotor dynamics, hydrodynamics and engine combustion are taken out of the equation, enabling a much cleaner signature. Innovative signal processing techniques with real-time results run an order of magnitude less in computational intensity.

As part of Phase II of the STTR effort, AI Signal performed AEBDS laboratory testing using a Bridgeport vertical mill as a bearing test rig. Bearing signatures for both good bearings and bearings with known seeded faults were tested. The vast majority of ball bearing defects tested fell into one of three categories: inner race defect, outer race defect, and ball defect. Digitizing conditioned analog electrical outputs of four instrumentation transducers produced the raw data for testing. Each 100-second test produced a 0.667GB data file that was post-processed using PC-SIGNAL™ software.

Bearing test data with seeded light (4-mill-wide) inner race defect: (a) Raw PSD of accelerometer output; (b) Envelope PSD of the accelerometer; (c) PPSA PSD of an acoustic emission transducer. (Photo provided by Marshall Space Flight Center.)

PC-SIGNAL general purpose vibration analysis software, also developed by AI Signal, is tightly integrated into the AEBDS system. A compilation of signal processing code that runs totally autonomous on personal computers, PC-SIGNAL is slated for release later this year. Because much of the software incorporated into PC-SIGNAL was developed in SBIR efforts to support the Space Shuttle main engine and other propulsion programs, the software possesses advanced capabilities to diagnose very subtle information. After all, bearing failure in a high-speed turbopump could have potentially catastrophic consequences.

PC-SIGNAL has already been used by NASA engineers to analyze high-frequency data for the Fastrac engine and the X-33. For the U.S. Army Redstone Technical Test Center, the easy-to-use software design has significantly reduced the analysis time for large volumes of test data.

The AEBDS has proven its effectiveness in detecting incipient bearing degradation. As a safeguard against costly plant downtime, the AEBDS has commercial applications for the aircraft/helicopter, transportation and nuclear power industries, as well as for rocket engine manufacturers. PC-SIGNAL has applications for all types of machinery.